Related
I see in C++ there are multiple ways to allocate and free data and I understand that when you call malloc you should call free and when you use the new operator you should pair with delete and it is a mistake to mix the two (e.g. Calling free() on something that was created with the new operator), but I'm not clear on when I should use malloc/ free and when I should use new/ delete in my real world programs.
If you're a C++ expert, please let me know any rules of thumb or conventions you follow in this regard.
Unless you are forced to use C, you should never use malloc. Always use new.
If you need a big chunk of data just do something like:
char *pBuffer = new char[1024];
Be careful though this is not correct:
//This is incorrect - may delete only one element, may corrupt the heap, or worse...
delete pBuffer;
Instead you should do this when deleting an array of data:
//This deletes all items in the array
delete[] pBuffer;
The new keyword is the C++ way of doing it, and it will ensure that your type will have its constructor called. The new keyword is also more type-safe whereas malloc is not type-safe at all.
The only way I could think that would be beneficial to use malloc would be if you needed to change the size of your buffer of data. The new keyword does not have an analogous way like realloc. The realloc function might be able to extend the size of a chunk of memory for you more efficiently.
It is worth mentioning that you cannot mix new/free and malloc/delete.
Note: Some answers in this question are invalid.
int* p_scalar = new int(5); // Does not create 5 elements, but initializes to 5
int* p_array = new int[5]; // Creates 5 elements
The short answer is: don't use malloc for C++ without a really good reason for doing so. malloc has a number of deficiencies when used with C++, which new was defined to overcome.
Deficiencies fixed by new for C++ code
malloc is not typesafe in any meaningful way. In C++ you are required to cast the return from void*. This potentially introduces a lot of problems:
#include <stdlib.h>
struct foo {
double d[5];
};
int main() {
foo *f1 = malloc(1); // error, no cast
foo *f2 = static_cast<foo*>(malloc(sizeof(foo)));
foo *f3 = static_cast<foo*>(malloc(1)); // No error, bad
}
It's worse than that though. If the type in question is POD (plain old data) then you can semi-sensibly use malloc to allocate memory for it, as f2 does in the first example.
It's not so obvious though if a type is POD. The fact that it's possible for a given type to change from POD to non-POD with no resulting compiler error and potentially very hard to debug problems is a significant factor. For example if someone (possibly another programmer, during maintenance, much later on were to make a change that caused foo to no longer be POD then no obvious error would appear at compile time as you'd hope, e.g.:
struct foo {
double d[5];
virtual ~foo() { }
};
would make the malloc of f2 also become bad, without any obvious diagnostics. The example here is trivial, but it's possible to accidentally introduce non-PODness much further away (e.g. in a base class, by adding a non-POD member). If you have C++11/boost you can use is_pod to check that this assumption is correct and produce an error if it's not:
#include <type_traits>
#include <stdlib.h>
foo *safe_foo_malloc() {
static_assert(std::is_pod<foo>::value, "foo must be POD");
return static_cast<foo*>(malloc(sizeof(foo)));
}
Although boost is unable to determine if a type is POD without C++11 or some other compiler extensions.
malloc returns NULL if allocation fails. new will throw std::bad_alloc. The behaviour of later using a NULL pointer is undefined. An exception has clean semantics when it is thrown and it is thrown from the source of the error. Wrapping malloc with an appropriate test at every call seems tedious and error prone. (You only have to forget once to undo all that good work). An exception can be allowed to propagate to a level where a caller is able to sensibly process it, where as NULL is much harder to pass back meaningfully. We could extend our safe_foo_malloc function to throw an exception or exit the program or call some handler:
#include <type_traits>
#include <stdlib.h>
void my_malloc_failed_handler();
foo *safe_foo_malloc() {
static_assert(std::is_pod<foo>::value, "foo must be POD");
foo *mem = static_cast<foo*>(malloc(sizeof(foo)));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return mem;
}
Fundamentally malloc is a C feature and new is a C++ feature. As a result malloc does not play nicely with constructors, it only looks at allocating a chunk of bytes. We could extend our safe_foo_malloc further to use placement new:
#include <stdlib.h>
#include <new>
void my_malloc_failed_handler();
foo *safe_foo_malloc() {
void *mem = malloc(sizeof(foo));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return new (mem)foo();
}
Our safe_foo_malloc function isn't very generic - ideally we'd want something that can handle any type, not just foo. We can achieve this with templates and variadic templates for non-default constructors:
#include <functional>
#include <new>
#include <stdlib.h>
void my_malloc_failed_handler();
template <typename T>
struct alloc {
template <typename ...Args>
static T *safe_malloc(Args&&... args) {
void *mem = malloc(sizeof(T));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return new (mem)T(std::forward(args)...);
}
};
Now though in fixing all the issues we identified so far we've practically reinvented the default new operator. If you're going to use malloc and placement new then you might as well just use new to begin with!
From the C++ FQA Lite:
[16.4] Why should I use new instead of
trustworthy old malloc()?
FAQ: new/delete call the
constructor/destructor; new is type
safe, malloc is not; new can be
overridden by a class.
FQA: The virtues of new mentioned by
the FAQ are not virtues, because
constructors, destructors, and
operator overloading are garbage (see
what happens when you have no garbage
collection?), and the type safety
issue is really tiny here (normally
you have to cast the void* returned by
malloc to the right pointer type to
assign it to a typed pointer variable,
which may be annoying, but far from
"unsafe").
Oh, and using trustworthy old malloc
makes it possible to use the equally
trustworthy & old realloc. Too bad we
don't have a shiny new operator renew or something.
Still, new is not bad enough to
justify a deviation from the common
style used throughout a language, even
when the language is C++. In
particular, classes with non-trivial
constructors will misbehave in fatal
ways if you simply malloc the objects.
So why not use new throughout the
code? People rarely overload operator
new, so it probably won't get in your
way too much. And if they do overload
new, you can always ask them to stop.
Sorry, I just couldn't resist. :)
Always use new in C++. If you need a block of untyped memory, you can use operator new directly:
void *p = operator new(size);
...
operator delete(p);
new vs malloc()
1) new is an operator, while malloc() is a function.
2) new calls constructors, while malloc() does not.
3) new returns exact data type, while malloc() returns void *.
4) new never returns a NULL (will throw on failure) while malloc() returns NULL
5) Reallocation of memory not handled by new while malloc() can
To answer your question, you should know the difference between malloc and new. The difference is simple:
malloc allocates memory, while new allocates memory AND calls the constructor of the object you're allocating memory for.
So, unless you're restricted to C, you should never use malloc, especially when dealing with C++ objects. That would be a recipe for breaking your program.
Also the difference between free and delete is quite the same. The difference is that delete will call the destructor of your object in addition to freeing memory.
Use malloc and free only for allocating memory that is going to be managed by c-centric libraries and APIs. Use new and delete (and the [] variants) for everything that you control.
There is one big difference between malloc and new. malloc allocates memory. This is fine for C, because in C, a lump of memory is an object.
In C++, if you're not dealing with POD types (which are similar to C types) you must call a constructor on a memory location to actually have an object there. Non-POD types are very common in C++, as many C++ features make an object automatically non-POD.
new allocates memory and creates an object on that memory location. For non-POD types this means calling a constructor.
If you do something like this:
non_pod_type* p = (non_pod_type*) malloc(sizeof *p);
The pointer you obtain cannot be dereferenced because it does not point to an object. You'd need to call a constructor on it before you can use it (and this is done using placement new).
If, on the other hand, you do:
non_pod_type* p = new non_pod_type();
You get a pointer that is always valid, because new created an object.
Even for POD types, there's a significant difference between the two:
pod_type* p = (pod_type*) malloc(sizeof *p);
std::cout << p->foo;
This piece of code would print an unspecified value, because the POD objects created by malloc are not initialised.
With new, you could specify a constructor to call, and thus get a well defined value.
pod_type* p = new pod_type();
std::cout << p->foo; // prints 0
If you really want it, you can use use new to obtain uninitialised POD objects. See this other answer for more information on that.
Another difference is the behaviour upon failure. When it fails to allocate memory, malloc returns a null pointer, while new throws an exception.
The former requires you to test every pointer returned before using it, while the later will always produce valid pointers.
For these reasons, in C++ code you should use new, and not malloc. But even then, you should not use new "in the open", because it acquires resources you need to release later on. When you use new you should pass its result immediately into a resource managing class:
std::unique_ptr<T> p = std::unique_ptr<T>(new T()); // this won't leak
Dynamic allocation is only required when the life-time of the object should be different than the scope it gets created in (This holds as well for making the scope smaller as larger) and you have a specific reason where storing it by value doesn't work.
For example:
std::vector<int> *createVector(); // Bad
std::vector<int> createVector(); // Good
auto v = new std::vector<int>(); // Bad
auto result = calculate(/*optional output = */ v);
auto v = std::vector<int>(); // Good
auto result = calculate(/*optional output = */ &v);
From C++11 on, we have std::unique_ptr for dealing with allocated memory, which contains the ownership of the allocated memory. std::shared_ptr was created for when you have to share ownership. (you'll need this less than you would expect in a good program)
Creating an instance becomes really easy:
auto instance = std::make_unique<Class>(/*args*/); // C++14
auto instance = std::unique_ptr<Class>(new Class(/*args*/)); // C++11
auto instance = std::make_unique<Class[]>(42); // C++14
auto instance = std::unique_ptr<Class[]>(new Class[](42)); // C++11
C++17 also adds std::optional which can prevent you from requiring memory allocations
auto optInstance = std::optional<Class>{};
if (condition)
optInstance = Class{};
As soon as 'instance' goes out of scope, the memory gets cleaned up. Transferring ownership is also easy:
auto vector = std::vector<std::unique_ptr<Interface>>{};
auto instance = std::make_unique<Class>();
vector.push_back(std::move(instance)); // std::move -> transfer (most of the time)
So when do you still need new? Almost never from C++11 on. Most of the you use std::make_unique until you get to a point where you hit an API that transfers ownership via raw pointers.
auto instance = std::make_unique<Class>();
legacyFunction(instance.release()); // Ownership being transferred
auto instance = std::unique_ptr<Class>{legacyFunction()}; // Ownership being captured in unique_ptr
In C++98/03, you have to do manual memory management. If you are in this case, try upgrading to a more recent version of the standard. If you are stuck:
auto instance = new Class(); // Allocate memory
delete instance; // Deallocate
auto instances = new Class[42](); // Allocate memory
delete[] instances; // Deallocate
Make sure that you track the ownership correctly to not have any memory leaks! Move semantics don't work yet either.
So, when do we need malloc in C++? The only valid reason would be to allocate memory and initialize it later via placement new.
auto instanceBlob = std::malloc(sizeof(Class)); // Allocate memory
auto instance = new(instanceBlob)Class{}; // Initialize via constructor
instance.~Class(); // Destroy via destructor
std::free(instanceBlob); // Deallocate the memory
Even though, the above is valid, this can be done via a new-operator as well. std::vector is a good example for this.
Finally, we still have the elephant in the room: C. If you have to work with a C-library where memory gets allocated in the C++ code and freed in the C code (or the other way around), you are forced to use malloc/free.
If you are in this case, forget about virtual functions, member functions, classes ... Only structs with PODs in it are allowed.
Some exceptions to the rules:
You are writing a standard library with advanced data structures where malloc is appropriate
You have to allocate big amounts of memory (In memory copy of a 10GB file?)
You have tooling preventing you to use certain constructs
You need to store an incomplete type
There are a few things which new does that malloc doesn’t:
new constructs the object by calling the constructor of that object
new doesn’t require typecasting of allocated memory.
It doesn’t require an amount of memory to be allocated, rather it requires a number of
objects to be constructed.
So, if you use malloc, then you need to do above things explicitly, which is not always practical. Additionally, new can be overloaded but malloc can’t be.
If you work with data that doesn't need construction/destruction and requires reallocations (e.g., a large array of ints), then I believe malloc/free is a good choice as it gives you realloc, which is way faster than new-memcpy-delete (it is on my Linux box, but I guess this may be platform dependent). If you work with C++ objects that are not POD and require construction/destruction, then you must use the new and delete operators.
Anyway, I don't see why you shouldn't use both (provided that you free your malloced memory and delete objects allocated with new) if can take advantage of the speed boost (sometimes a significant one, if you're reallocing large arrays of POD) that realloc can give you.
Unless you need it though, you should stick to new/delete in C++.
If you are using C++, try to use new/delete instead of malloc/calloc as they are operators. For malloc/calloc, you need to include another header. Don't mix two different languages in the same code. Their work is similar in every manner, both allocates memory dynamically from heap segment in hash table.
new will initialise the default values of the struct and correctly links the references in it to itself.
E.g.
struct test_s {
int some_strange_name = 1;
int &easy = some_strange_name;
}
So new struct test_s will return an initialised structure with a working reference, while the malloc'ed version has no default values and the intern references aren't initialised.
If you have C code you want to port over to C++, you might leave any malloc() calls in it. For any new C++ code, I'd recommend using new instead.
From a lower perspective, new will initialize all the memory before giving the memory whereas malloc will keep the original content of the memory.
In the following scenario, we can't use new since it calls constructor.
class B {
private:
B *ptr;
int x;
public:
B(int n) {
cout<<"B: ctr"<<endl;
//ptr = new B; //keep calling ctr, result is segmentation fault
ptr = (B *)malloc(sizeof(B));
x = n;
ptr->x = n + 10;
}
~B() {
//delete ptr;
free(ptr);
cout<<"B: dtr"<<endl;
}
};
Rare case to consider using malloc/free instead of new/delete is when you're allocating and then reallocating (simple pod types, not objects) using realloc as there is no similar function to realloc in C++ (although this can be done using a more C++ approach).
I had played before with few C/C++ applications for computer graphics.
After so many time, some things are vanished and I missed them a lot.
The point is, that malloc and new, or free and delete, can work both,
especially for certain basic types, which are the most common.
For instance, a char array, can be allocated both with malloc, or new.
A main difference is, with new you can instantiate a fixed array size.
char* pWord = new char[5]; // allocation of char array of fixed size
You cannot use a variable for the size of the array in this case.
By the contrary, the malloc function could allow a variable size.
int size = 5;
char* pWord = (char*)malloc(size);
In this case, it might be required a conversion cast operator.
For the returned type from malloc it's a pointer to void, not char.
And sometimes the compiler could not know, how to convert this type.
After allocation the memory block, you can set the variable values.
the memset function can be indeed slower for some bigger arrays.
But all the bites must be set first to 0, before assigning a value.
Because the values of an array could have an arbitrary content.
Suppose, the array is assigned with another array of smaller size.
Part of the array element could still have arbitrary content.
And a call to a memset function would be recomended in this case.
memset((void*)pWord, 0, sizeof(pWord) / sizeof(char));
The allocation functions are available for all C packages.
So, these are general functions, that must work for more C types.
And the C++ libraries are extensions of the older C libraries.
Therefore the malloc function returns a generic void* pointer.
The sructures do not have defined a new, or a delete operator.
In this case, a custom variable can be allocated with malloc.
The new and delete keywords are actually some defined C operators.
Maybe a custom union, or class, can have defined these operators.
If new and delete are not defined in a class, these may not work.
But if a class is derived from another, which has these operators,
the new and delete keywords can have the basic class behavior.
About freeing an array, free can be only used in pair with malloc.
Cannot allocate a variable with malloc, and then free with delete.
The simple delete operator references just first item of an array.
Because the pWord array can be also written as:
pWord = &pWord[0]; // or *pWord = pWord[0];
When an array must be deleted, use the delete[] operator instead:
delete[] pWord;
Casts are not bad, they just don't work for all the variable types.
A conversion cast is also an operator function, that must be defined.
If this operator is not defined for a certain type, it may not work.
But not all the errors are because of this conversion cast operator.
Also a cast to a void pointer must be used when using a free call.
This is because the argument of the free function is a void pointer.
free((void*)pWord);
Some errors can arise, because the size of the array is too small.
But this is another story, it is not because of using the cast.
With kind regards, Adrian Brinas
The new and delete operators can operate on classes and structures, whereas malloc and free only work with blocks of memory that need to be cast.
Using new/delete will help to improve your code as you will not need to cast allocated memory to the required data structure.
malloc() is used to dynamically assign memory in C
while the same work is done by new() in c++.
So you cannot mix coding conventions of 2 languages.
It would be good if you asked for difference between calloc and malloc()
I see in C++ there are multiple ways to allocate and free data and I understand that when you call malloc you should call free and when you use the new operator you should pair with delete and it is a mistake to mix the two (e.g. Calling free() on something that was created with the new operator), but I'm not clear on when I should use malloc/ free and when I should use new/ delete in my real world programs.
If you're a C++ expert, please let me know any rules of thumb or conventions you follow in this regard.
Unless you are forced to use C, you should never use malloc. Always use new.
If you need a big chunk of data just do something like:
char *pBuffer = new char[1024];
Be careful though this is not correct:
//This is incorrect - may delete only one element, may corrupt the heap, or worse...
delete pBuffer;
Instead you should do this when deleting an array of data:
//This deletes all items in the array
delete[] pBuffer;
The new keyword is the C++ way of doing it, and it will ensure that your type will have its constructor called. The new keyword is also more type-safe whereas malloc is not type-safe at all.
The only way I could think that would be beneficial to use malloc would be if you needed to change the size of your buffer of data. The new keyword does not have an analogous way like realloc. The realloc function might be able to extend the size of a chunk of memory for you more efficiently.
It is worth mentioning that you cannot mix new/free and malloc/delete.
Note: Some answers in this question are invalid.
int* p_scalar = new int(5); // Does not create 5 elements, but initializes to 5
int* p_array = new int[5]; // Creates 5 elements
The short answer is: don't use malloc for C++ without a really good reason for doing so. malloc has a number of deficiencies when used with C++, which new was defined to overcome.
Deficiencies fixed by new for C++ code
malloc is not typesafe in any meaningful way. In C++ you are required to cast the return from void*. This potentially introduces a lot of problems:
#include <stdlib.h>
struct foo {
double d[5];
};
int main() {
foo *f1 = malloc(1); // error, no cast
foo *f2 = static_cast<foo*>(malloc(sizeof(foo)));
foo *f3 = static_cast<foo*>(malloc(1)); // No error, bad
}
It's worse than that though. If the type in question is POD (plain old data) then you can semi-sensibly use malloc to allocate memory for it, as f2 does in the first example.
It's not so obvious though if a type is POD. The fact that it's possible for a given type to change from POD to non-POD with no resulting compiler error and potentially very hard to debug problems is a significant factor. For example if someone (possibly another programmer, during maintenance, much later on were to make a change that caused foo to no longer be POD then no obvious error would appear at compile time as you'd hope, e.g.:
struct foo {
double d[5];
virtual ~foo() { }
};
would make the malloc of f2 also become bad, without any obvious diagnostics. The example here is trivial, but it's possible to accidentally introduce non-PODness much further away (e.g. in a base class, by adding a non-POD member). If you have C++11/boost you can use is_pod to check that this assumption is correct and produce an error if it's not:
#include <type_traits>
#include <stdlib.h>
foo *safe_foo_malloc() {
static_assert(std::is_pod<foo>::value, "foo must be POD");
return static_cast<foo*>(malloc(sizeof(foo)));
}
Although boost is unable to determine if a type is POD without C++11 or some other compiler extensions.
malloc returns NULL if allocation fails. new will throw std::bad_alloc. The behaviour of later using a NULL pointer is undefined. An exception has clean semantics when it is thrown and it is thrown from the source of the error. Wrapping malloc with an appropriate test at every call seems tedious and error prone. (You only have to forget once to undo all that good work). An exception can be allowed to propagate to a level where a caller is able to sensibly process it, where as NULL is much harder to pass back meaningfully. We could extend our safe_foo_malloc function to throw an exception or exit the program or call some handler:
#include <type_traits>
#include <stdlib.h>
void my_malloc_failed_handler();
foo *safe_foo_malloc() {
static_assert(std::is_pod<foo>::value, "foo must be POD");
foo *mem = static_cast<foo*>(malloc(sizeof(foo)));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return mem;
}
Fundamentally malloc is a C feature and new is a C++ feature. As a result malloc does not play nicely with constructors, it only looks at allocating a chunk of bytes. We could extend our safe_foo_malloc further to use placement new:
#include <stdlib.h>
#include <new>
void my_malloc_failed_handler();
foo *safe_foo_malloc() {
void *mem = malloc(sizeof(foo));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return new (mem)foo();
}
Our safe_foo_malloc function isn't very generic - ideally we'd want something that can handle any type, not just foo. We can achieve this with templates and variadic templates for non-default constructors:
#include <functional>
#include <new>
#include <stdlib.h>
void my_malloc_failed_handler();
template <typename T>
struct alloc {
template <typename ...Args>
static T *safe_malloc(Args&&... args) {
void *mem = malloc(sizeof(T));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return new (mem)T(std::forward(args)...);
}
};
Now though in fixing all the issues we identified so far we've practically reinvented the default new operator. If you're going to use malloc and placement new then you might as well just use new to begin with!
From the C++ FQA Lite:
[16.4] Why should I use new instead of
trustworthy old malloc()?
FAQ: new/delete call the
constructor/destructor; new is type
safe, malloc is not; new can be
overridden by a class.
FQA: The virtues of new mentioned by
the FAQ are not virtues, because
constructors, destructors, and
operator overloading are garbage (see
what happens when you have no garbage
collection?), and the type safety
issue is really tiny here (normally
you have to cast the void* returned by
malloc to the right pointer type to
assign it to a typed pointer variable,
which may be annoying, but far from
"unsafe").
Oh, and using trustworthy old malloc
makes it possible to use the equally
trustworthy & old realloc. Too bad we
don't have a shiny new operator renew or something.
Still, new is not bad enough to
justify a deviation from the common
style used throughout a language, even
when the language is C++. In
particular, classes with non-trivial
constructors will misbehave in fatal
ways if you simply malloc the objects.
So why not use new throughout the
code? People rarely overload operator
new, so it probably won't get in your
way too much. And if they do overload
new, you can always ask them to stop.
Sorry, I just couldn't resist. :)
Always use new in C++. If you need a block of untyped memory, you can use operator new directly:
void *p = operator new(size);
...
operator delete(p);
new vs malloc()
1) new is an operator, while malloc() is a function.
2) new calls constructors, while malloc() does not.
3) new returns exact data type, while malloc() returns void *.
4) new never returns a NULL (will throw on failure) while malloc() returns NULL
5) Reallocation of memory not handled by new while malloc() can
To answer your question, you should know the difference between malloc and new. The difference is simple:
malloc allocates memory, while new allocates memory AND calls the constructor of the object you're allocating memory for.
So, unless you're restricted to C, you should never use malloc, especially when dealing with C++ objects. That would be a recipe for breaking your program.
Also the difference between free and delete is quite the same. The difference is that delete will call the destructor of your object in addition to freeing memory.
Use malloc and free only for allocating memory that is going to be managed by c-centric libraries and APIs. Use new and delete (and the [] variants) for everything that you control.
There is one big difference between malloc and new. malloc allocates memory. This is fine for C, because in C, a lump of memory is an object.
In C++, if you're not dealing with POD types (which are similar to C types) you must call a constructor on a memory location to actually have an object there. Non-POD types are very common in C++, as many C++ features make an object automatically non-POD.
new allocates memory and creates an object on that memory location. For non-POD types this means calling a constructor.
If you do something like this:
non_pod_type* p = (non_pod_type*) malloc(sizeof *p);
The pointer you obtain cannot be dereferenced because it does not point to an object. You'd need to call a constructor on it before you can use it (and this is done using placement new).
If, on the other hand, you do:
non_pod_type* p = new non_pod_type();
You get a pointer that is always valid, because new created an object.
Even for POD types, there's a significant difference between the two:
pod_type* p = (pod_type*) malloc(sizeof *p);
std::cout << p->foo;
This piece of code would print an unspecified value, because the POD objects created by malloc are not initialised.
With new, you could specify a constructor to call, and thus get a well defined value.
pod_type* p = new pod_type();
std::cout << p->foo; // prints 0
If you really want it, you can use use new to obtain uninitialised POD objects. See this other answer for more information on that.
Another difference is the behaviour upon failure. When it fails to allocate memory, malloc returns a null pointer, while new throws an exception.
The former requires you to test every pointer returned before using it, while the later will always produce valid pointers.
For these reasons, in C++ code you should use new, and not malloc. But even then, you should not use new "in the open", because it acquires resources you need to release later on. When you use new you should pass its result immediately into a resource managing class:
std::unique_ptr<T> p = std::unique_ptr<T>(new T()); // this won't leak
Dynamic allocation is only required when the life-time of the object should be different than the scope it gets created in (This holds as well for making the scope smaller as larger) and you have a specific reason where storing it by value doesn't work.
For example:
std::vector<int> *createVector(); // Bad
std::vector<int> createVector(); // Good
auto v = new std::vector<int>(); // Bad
auto result = calculate(/*optional output = */ v);
auto v = std::vector<int>(); // Good
auto result = calculate(/*optional output = */ &v);
From C++11 on, we have std::unique_ptr for dealing with allocated memory, which contains the ownership of the allocated memory. std::shared_ptr was created for when you have to share ownership. (you'll need this less than you would expect in a good program)
Creating an instance becomes really easy:
auto instance = std::make_unique<Class>(/*args*/); // C++14
auto instance = std::unique_ptr<Class>(new Class(/*args*/)); // C++11
auto instance = std::make_unique<Class[]>(42); // C++14
auto instance = std::unique_ptr<Class[]>(new Class[](42)); // C++11
C++17 also adds std::optional which can prevent you from requiring memory allocations
auto optInstance = std::optional<Class>{};
if (condition)
optInstance = Class{};
As soon as 'instance' goes out of scope, the memory gets cleaned up. Transferring ownership is also easy:
auto vector = std::vector<std::unique_ptr<Interface>>{};
auto instance = std::make_unique<Class>();
vector.push_back(std::move(instance)); // std::move -> transfer (most of the time)
So when do you still need new? Almost never from C++11 on. Most of the you use std::make_unique until you get to a point where you hit an API that transfers ownership via raw pointers.
auto instance = std::make_unique<Class>();
legacyFunction(instance.release()); // Ownership being transferred
auto instance = std::unique_ptr<Class>{legacyFunction()}; // Ownership being captured in unique_ptr
In C++98/03, you have to do manual memory management. If you are in this case, try upgrading to a more recent version of the standard. If you are stuck:
auto instance = new Class(); // Allocate memory
delete instance; // Deallocate
auto instances = new Class[42](); // Allocate memory
delete[] instances; // Deallocate
Make sure that you track the ownership correctly to not have any memory leaks! Move semantics don't work yet either.
So, when do we need malloc in C++? The only valid reason would be to allocate memory and initialize it later via placement new.
auto instanceBlob = std::malloc(sizeof(Class)); // Allocate memory
auto instance = new(instanceBlob)Class{}; // Initialize via constructor
instance.~Class(); // Destroy via destructor
std::free(instanceBlob); // Deallocate the memory
Even though, the above is valid, this can be done via a new-operator as well. std::vector is a good example for this.
Finally, we still have the elephant in the room: C. If you have to work with a C-library where memory gets allocated in the C++ code and freed in the C code (or the other way around), you are forced to use malloc/free.
If you are in this case, forget about virtual functions, member functions, classes ... Only structs with PODs in it are allowed.
Some exceptions to the rules:
You are writing a standard library with advanced data structures where malloc is appropriate
You have to allocate big amounts of memory (In memory copy of a 10GB file?)
You have tooling preventing you to use certain constructs
You need to store an incomplete type
There are a few things which new does that malloc doesn’t:
new constructs the object by calling the constructor of that object
new doesn’t require typecasting of allocated memory.
It doesn’t require an amount of memory to be allocated, rather it requires a number of
objects to be constructed.
So, if you use malloc, then you need to do above things explicitly, which is not always practical. Additionally, new can be overloaded but malloc can’t be.
If you work with data that doesn't need construction/destruction and requires reallocations (e.g., a large array of ints), then I believe malloc/free is a good choice as it gives you realloc, which is way faster than new-memcpy-delete (it is on my Linux box, but I guess this may be platform dependent). If you work with C++ objects that are not POD and require construction/destruction, then you must use the new and delete operators.
Anyway, I don't see why you shouldn't use both (provided that you free your malloced memory and delete objects allocated with new) if can take advantage of the speed boost (sometimes a significant one, if you're reallocing large arrays of POD) that realloc can give you.
Unless you need it though, you should stick to new/delete in C++.
If you are using C++, try to use new/delete instead of malloc/calloc as they are operators. For malloc/calloc, you need to include another header. Don't mix two different languages in the same code. Their work is similar in every manner, both allocates memory dynamically from heap segment in hash table.
new will initialise the default values of the struct and correctly links the references in it to itself.
E.g.
struct test_s {
int some_strange_name = 1;
int &easy = some_strange_name;
}
So new struct test_s will return an initialised structure with a working reference, while the malloc'ed version has no default values and the intern references aren't initialised.
If you have C code you want to port over to C++, you might leave any malloc() calls in it. For any new C++ code, I'd recommend using new instead.
From a lower perspective, new will initialize all the memory before giving the memory whereas malloc will keep the original content of the memory.
In the following scenario, we can't use new since it calls constructor.
class B {
private:
B *ptr;
int x;
public:
B(int n) {
cout<<"B: ctr"<<endl;
//ptr = new B; //keep calling ctr, result is segmentation fault
ptr = (B *)malloc(sizeof(B));
x = n;
ptr->x = n + 10;
}
~B() {
//delete ptr;
free(ptr);
cout<<"B: dtr"<<endl;
}
};
Rare case to consider using malloc/free instead of new/delete is when you're allocating and then reallocating (simple pod types, not objects) using realloc as there is no similar function to realloc in C++ (although this can be done using a more C++ approach).
I had played before with few C/C++ applications for computer graphics.
After so many time, some things are vanished and I missed them a lot.
The point is, that malloc and new, or free and delete, can work both,
especially for certain basic types, which are the most common.
For instance, a char array, can be allocated both with malloc, or new.
A main difference is, with new you can instantiate a fixed array size.
char* pWord = new char[5]; // allocation of char array of fixed size
You cannot use a variable for the size of the array in this case.
By the contrary, the malloc function could allow a variable size.
int size = 5;
char* pWord = (char*)malloc(size);
In this case, it might be required a conversion cast operator.
For the returned type from malloc it's a pointer to void, not char.
And sometimes the compiler could not know, how to convert this type.
After allocation the memory block, you can set the variable values.
the memset function can be indeed slower for some bigger arrays.
But all the bites must be set first to 0, before assigning a value.
Because the values of an array could have an arbitrary content.
Suppose, the array is assigned with another array of smaller size.
Part of the array element could still have arbitrary content.
And a call to a memset function would be recomended in this case.
memset((void*)pWord, 0, sizeof(pWord) / sizeof(char));
The allocation functions are available for all C packages.
So, these are general functions, that must work for more C types.
And the C++ libraries are extensions of the older C libraries.
Therefore the malloc function returns a generic void* pointer.
The sructures do not have defined a new, or a delete operator.
In this case, a custom variable can be allocated with malloc.
The new and delete keywords are actually some defined C operators.
Maybe a custom union, or class, can have defined these operators.
If new and delete are not defined in a class, these may not work.
But if a class is derived from another, which has these operators,
the new and delete keywords can have the basic class behavior.
About freeing an array, free can be only used in pair with malloc.
Cannot allocate a variable with malloc, and then free with delete.
The simple delete operator references just first item of an array.
Because the pWord array can be also written as:
pWord = &pWord[0]; // or *pWord = pWord[0];
When an array must be deleted, use the delete[] operator instead:
delete[] pWord;
Casts are not bad, they just don't work for all the variable types.
A conversion cast is also an operator function, that must be defined.
If this operator is not defined for a certain type, it may not work.
But not all the errors are because of this conversion cast operator.
Also a cast to a void pointer must be used when using a free call.
This is because the argument of the free function is a void pointer.
free((void*)pWord);
Some errors can arise, because the size of the array is too small.
But this is another story, it is not because of using the cast.
With kind regards, Adrian Brinas
The new and delete operators can operate on classes and structures, whereas malloc and free only work with blocks of memory that need to be cast.
Using new/delete will help to improve your code as you will not need to cast allocated memory to the required data structure.
malloc() is used to dynamically assign memory in C
while the same work is done by new() in c++.
So you cannot mix coding conventions of 2 languages.
It would be good if you asked for difference between calloc and malloc()
When I create a class, include a pointer in it, and create an object in automatic storage, should I include a destructor in the class? (is it necessary to free the space in the memory?)
Example:
class Node{
char *Name;
int age;
Node(char *n = 0, int a = 0) {
name = strdup(n);
age = a;
}
~Node(){
if (name != 0)
delete(name);
}
}
Node node1("Roger",20);
is it necessary to free the space in the memory?
Yes, it's essential to avoid memory leaks.
Anyway you must use free(name); in your example, since strdup() is a pure C function, and uses the malloc() function to allocate the memory for the copy returned.
Also, you should avoid managing raw pointers yourself in C++. Either use smart pointers, or standard C++ containers, or for your specific case simply std::string.
Raw pointers don't go well with, respectively complicate, implementing the Rule of Three (5/zero).
If you have something allocated from a C-style interface, you can always use a smart pointer, and provide a custom deleter function, that actually uses the appropriate free() call.
If your class has a pointer that points to memory that is allocated with new \ new[] \ malloc() then you need to implement the Rule of Three
That said, instead of using a raw char * and manual memory management, use a std::string instead. you can still get a const char* out of it if you need it for other functions but it fully self managed container. With it you would not need to provide a copy constructor or destructor as the default ones provided by the compiler will work.
Yes, this is mandatory to avoid memory leaks. It doesn't matter if you are using a class or something else. It's important what strdup says. But you must not use delete, instead use free. The memory in strdup is created by using malloc, not new.
http://pubs.opengroup.org/onlinepubs/009695399/functions/strdup.html
strdup - duplicate a string
The returned pointer can be passed to free(). A null pointer is
returned if the new string cannot be created.
P.S. Don't miss const in your declaration of char *n, otherwise the caller might expect the string is modified and a simple-string literal cannot be passed without a warning.
P.S.2: Prefer using nullptr instead of 0. This has been discussed several times on SO.
I started out with Java, so I am a bit confused on what's going on with the stack/heap on the following line:
string *x = new string("Hello");
where x is a local variable. In C++, does ANYTHING happen on the stack at all in regards to that statement? I know from reading it says that the object is on the heap, but what about x? In Java, x would be on the stack just holding the memory address that points to the object, but I haven't found a clear source that says what's happening in C++.
Any object you just created, e.g. x in your example is on the stack. The object x is just a pointer, though, which points to a heap allocated string which you put on the heap using new string("Hello"). Typically, you wouldn't create a string like this in C++, however. Instead you would use
string x("Hello");
This would still allocate x on the stack. Whether the characters representing x's value also live on the stack or rather on the heap, depends on the string implementation. As a reasonable model you should assume that they are on the heap (some std::string implementation put short string into the stack object, avoiding any heap allocations and helping with locality).
yes, x is on the stack : it is a local variable, which are all on the stack.
The new operator provokes the allocation of memory on the heap.
It's the same as in Java. The string or String is on the heap, and the pointer (or reference, in Java) is on the stack.
In Java, all the Objects are on the heap, and the stack is only made up of primitive types and the references themselves. (The stack has other stuff like return addresses and so on, but never mind that).
The main difference between the C++ stack and the Java stack is that, in C++, you can put the entire object directly onto the stack. e.g. string x = string("Hello");
It's also possible, in C++, to put primitive types directly onto the heap. e.g. int * x = new int();. (In other words, "if autoboxing is the solution, then what was the problem?")
In short, Java has rigid distinctions between primitive types and Objects, and the primitives are very much second-class. C++ is much more relaxed.
It depends on where this line is located. If it is located somewhere at file scope (i.e. outside of a function), then x is a global variable which definitely is not on the stack (well, in theory it could be put on the stack before calling main(), but I strongly doubt any compiler does that), but also not on the heap. If, however, the line is part of a function, then indeed x is on the stack.
Since x is of type string* (i.e. pointer to string), it indeed just contains the address of the string object allocated with new. Since that string object is allocated with new, it indeed lives on the heap.
Note however that, unlike in Java, there's no need that built-in types live on the stack and class objects live on the heap. The following is an example of a pointer living on the heap, pointing to an object living in the stack:
int main()
{
std::string str("Hello"); // a string object on the stack
std::string** ptr = new std::string*(&str); // a pointer to string living on the heap, pointing to str
(**ptr) += " world"; // this adds "world" to the string on the stack
delete ptr; // get rid of the pointer on the heap
std::cout << str << std::endl; // prints "Hello world"
} // the compiler automatically destroys str here
I see in C++ there are multiple ways to allocate and free data and I understand that when you call malloc you should call free and when you use the new operator you should pair with delete and it is a mistake to mix the two (e.g. Calling free() on something that was created with the new operator), but I'm not clear on when I should use malloc/ free and when I should use new/ delete in my real world programs.
If you're a C++ expert, please let me know any rules of thumb or conventions you follow in this regard.
Unless you are forced to use C, you should never use malloc. Always use new.
If you need a big chunk of data just do something like:
char *pBuffer = new char[1024];
Be careful though this is not correct:
//This is incorrect - may delete only one element, may corrupt the heap, or worse...
delete pBuffer;
Instead you should do this when deleting an array of data:
//This deletes all items in the array
delete[] pBuffer;
The new keyword is the C++ way of doing it, and it will ensure that your type will have its constructor called. The new keyword is also more type-safe whereas malloc is not type-safe at all.
The only way I could think that would be beneficial to use malloc would be if you needed to change the size of your buffer of data. The new keyword does not have an analogous way like realloc. The realloc function might be able to extend the size of a chunk of memory for you more efficiently.
It is worth mentioning that you cannot mix new/free and malloc/delete.
Note: Some answers in this question are invalid.
int* p_scalar = new int(5); // Does not create 5 elements, but initializes to 5
int* p_array = new int[5]; // Creates 5 elements
The short answer is: don't use malloc for C++ without a really good reason for doing so. malloc has a number of deficiencies when used with C++, which new was defined to overcome.
Deficiencies fixed by new for C++ code
malloc is not typesafe in any meaningful way. In C++ you are required to cast the return from void*. This potentially introduces a lot of problems:
#include <stdlib.h>
struct foo {
double d[5];
};
int main() {
foo *f1 = malloc(1); // error, no cast
foo *f2 = static_cast<foo*>(malloc(sizeof(foo)));
foo *f3 = static_cast<foo*>(malloc(1)); // No error, bad
}
It's worse than that though. If the type in question is POD (plain old data) then you can semi-sensibly use malloc to allocate memory for it, as f2 does in the first example.
It's not so obvious though if a type is POD. The fact that it's possible for a given type to change from POD to non-POD with no resulting compiler error and potentially very hard to debug problems is a significant factor. For example if someone (possibly another programmer, during maintenance, much later on were to make a change that caused foo to no longer be POD then no obvious error would appear at compile time as you'd hope, e.g.:
struct foo {
double d[5];
virtual ~foo() { }
};
would make the malloc of f2 also become bad, without any obvious diagnostics. The example here is trivial, but it's possible to accidentally introduce non-PODness much further away (e.g. in a base class, by adding a non-POD member). If you have C++11/boost you can use is_pod to check that this assumption is correct and produce an error if it's not:
#include <type_traits>
#include <stdlib.h>
foo *safe_foo_malloc() {
static_assert(std::is_pod<foo>::value, "foo must be POD");
return static_cast<foo*>(malloc(sizeof(foo)));
}
Although boost is unable to determine if a type is POD without C++11 or some other compiler extensions.
malloc returns NULL if allocation fails. new will throw std::bad_alloc. The behaviour of later using a NULL pointer is undefined. An exception has clean semantics when it is thrown and it is thrown from the source of the error. Wrapping malloc with an appropriate test at every call seems tedious and error prone. (You only have to forget once to undo all that good work). An exception can be allowed to propagate to a level where a caller is able to sensibly process it, where as NULL is much harder to pass back meaningfully. We could extend our safe_foo_malloc function to throw an exception or exit the program or call some handler:
#include <type_traits>
#include <stdlib.h>
void my_malloc_failed_handler();
foo *safe_foo_malloc() {
static_assert(std::is_pod<foo>::value, "foo must be POD");
foo *mem = static_cast<foo*>(malloc(sizeof(foo)));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return mem;
}
Fundamentally malloc is a C feature and new is a C++ feature. As a result malloc does not play nicely with constructors, it only looks at allocating a chunk of bytes. We could extend our safe_foo_malloc further to use placement new:
#include <stdlib.h>
#include <new>
void my_malloc_failed_handler();
foo *safe_foo_malloc() {
void *mem = malloc(sizeof(foo));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return new (mem)foo();
}
Our safe_foo_malloc function isn't very generic - ideally we'd want something that can handle any type, not just foo. We can achieve this with templates and variadic templates for non-default constructors:
#include <functional>
#include <new>
#include <stdlib.h>
void my_malloc_failed_handler();
template <typename T>
struct alloc {
template <typename ...Args>
static T *safe_malloc(Args&&... args) {
void *mem = malloc(sizeof(T));
if (!mem) {
my_malloc_failed_handler();
// or throw ...
}
return new (mem)T(std::forward(args)...);
}
};
Now though in fixing all the issues we identified so far we've practically reinvented the default new operator. If you're going to use malloc and placement new then you might as well just use new to begin with!
From the C++ FQA Lite:
[16.4] Why should I use new instead of
trustworthy old malloc()?
FAQ: new/delete call the
constructor/destructor; new is type
safe, malloc is not; new can be
overridden by a class.
FQA: The virtues of new mentioned by
the FAQ are not virtues, because
constructors, destructors, and
operator overloading are garbage (see
what happens when you have no garbage
collection?), and the type safety
issue is really tiny here (normally
you have to cast the void* returned by
malloc to the right pointer type to
assign it to a typed pointer variable,
which may be annoying, but far from
"unsafe").
Oh, and using trustworthy old malloc
makes it possible to use the equally
trustworthy & old realloc. Too bad we
don't have a shiny new operator renew or something.
Still, new is not bad enough to
justify a deviation from the common
style used throughout a language, even
when the language is C++. In
particular, classes with non-trivial
constructors will misbehave in fatal
ways if you simply malloc the objects.
So why not use new throughout the
code? People rarely overload operator
new, so it probably won't get in your
way too much. And if they do overload
new, you can always ask them to stop.
Sorry, I just couldn't resist. :)
Always use new in C++. If you need a block of untyped memory, you can use operator new directly:
void *p = operator new(size);
...
operator delete(p);
new vs malloc()
1) new is an operator, while malloc() is a function.
2) new calls constructors, while malloc() does not.
3) new returns exact data type, while malloc() returns void *.
4) new never returns a NULL (will throw on failure) while malloc() returns NULL
5) Reallocation of memory not handled by new while malloc() can
To answer your question, you should know the difference between malloc and new. The difference is simple:
malloc allocates memory, while new allocates memory AND calls the constructor of the object you're allocating memory for.
So, unless you're restricted to C, you should never use malloc, especially when dealing with C++ objects. That would be a recipe for breaking your program.
Also the difference between free and delete is quite the same. The difference is that delete will call the destructor of your object in addition to freeing memory.
Use malloc and free only for allocating memory that is going to be managed by c-centric libraries and APIs. Use new and delete (and the [] variants) for everything that you control.
There is one big difference between malloc and new. malloc allocates memory. This is fine for C, because in C, a lump of memory is an object.
In C++, if you're not dealing with POD types (which are similar to C types) you must call a constructor on a memory location to actually have an object there. Non-POD types are very common in C++, as many C++ features make an object automatically non-POD.
new allocates memory and creates an object on that memory location. For non-POD types this means calling a constructor.
If you do something like this:
non_pod_type* p = (non_pod_type*) malloc(sizeof *p);
The pointer you obtain cannot be dereferenced because it does not point to an object. You'd need to call a constructor on it before you can use it (and this is done using placement new).
If, on the other hand, you do:
non_pod_type* p = new non_pod_type();
You get a pointer that is always valid, because new created an object.
Even for POD types, there's a significant difference between the two:
pod_type* p = (pod_type*) malloc(sizeof *p);
std::cout << p->foo;
This piece of code would print an unspecified value, because the POD objects created by malloc are not initialised.
With new, you could specify a constructor to call, and thus get a well defined value.
pod_type* p = new pod_type();
std::cout << p->foo; // prints 0
If you really want it, you can use use new to obtain uninitialised POD objects. See this other answer for more information on that.
Another difference is the behaviour upon failure. When it fails to allocate memory, malloc returns a null pointer, while new throws an exception.
The former requires you to test every pointer returned before using it, while the later will always produce valid pointers.
For these reasons, in C++ code you should use new, and not malloc. But even then, you should not use new "in the open", because it acquires resources you need to release later on. When you use new you should pass its result immediately into a resource managing class:
std::unique_ptr<T> p = std::unique_ptr<T>(new T()); // this won't leak
Dynamic allocation is only required when the life-time of the object should be different than the scope it gets created in (This holds as well for making the scope smaller as larger) and you have a specific reason where storing it by value doesn't work.
For example:
std::vector<int> *createVector(); // Bad
std::vector<int> createVector(); // Good
auto v = new std::vector<int>(); // Bad
auto result = calculate(/*optional output = */ v);
auto v = std::vector<int>(); // Good
auto result = calculate(/*optional output = */ &v);
From C++11 on, we have std::unique_ptr for dealing with allocated memory, which contains the ownership of the allocated memory. std::shared_ptr was created for when you have to share ownership. (you'll need this less than you would expect in a good program)
Creating an instance becomes really easy:
auto instance = std::make_unique<Class>(/*args*/); // C++14
auto instance = std::unique_ptr<Class>(new Class(/*args*/)); // C++11
auto instance = std::make_unique<Class[]>(42); // C++14
auto instance = std::unique_ptr<Class[]>(new Class[](42)); // C++11
C++17 also adds std::optional which can prevent you from requiring memory allocations
auto optInstance = std::optional<Class>{};
if (condition)
optInstance = Class{};
As soon as 'instance' goes out of scope, the memory gets cleaned up. Transferring ownership is also easy:
auto vector = std::vector<std::unique_ptr<Interface>>{};
auto instance = std::make_unique<Class>();
vector.push_back(std::move(instance)); // std::move -> transfer (most of the time)
So when do you still need new? Almost never from C++11 on. Most of the you use std::make_unique until you get to a point where you hit an API that transfers ownership via raw pointers.
auto instance = std::make_unique<Class>();
legacyFunction(instance.release()); // Ownership being transferred
auto instance = std::unique_ptr<Class>{legacyFunction()}; // Ownership being captured in unique_ptr
In C++98/03, you have to do manual memory management. If you are in this case, try upgrading to a more recent version of the standard. If you are stuck:
auto instance = new Class(); // Allocate memory
delete instance; // Deallocate
auto instances = new Class[42](); // Allocate memory
delete[] instances; // Deallocate
Make sure that you track the ownership correctly to not have any memory leaks! Move semantics don't work yet either.
So, when do we need malloc in C++? The only valid reason would be to allocate memory and initialize it later via placement new.
auto instanceBlob = std::malloc(sizeof(Class)); // Allocate memory
auto instance = new(instanceBlob)Class{}; // Initialize via constructor
instance.~Class(); // Destroy via destructor
std::free(instanceBlob); // Deallocate the memory
Even though, the above is valid, this can be done via a new-operator as well. std::vector is a good example for this.
Finally, we still have the elephant in the room: C. If you have to work with a C-library where memory gets allocated in the C++ code and freed in the C code (or the other way around), you are forced to use malloc/free.
If you are in this case, forget about virtual functions, member functions, classes ... Only structs with PODs in it are allowed.
Some exceptions to the rules:
You are writing a standard library with advanced data structures where malloc is appropriate
You have to allocate big amounts of memory (In memory copy of a 10GB file?)
You have tooling preventing you to use certain constructs
You need to store an incomplete type
There are a few things which new does that malloc doesn’t:
new constructs the object by calling the constructor of that object
new doesn’t require typecasting of allocated memory.
It doesn’t require an amount of memory to be allocated, rather it requires a number of
objects to be constructed.
So, if you use malloc, then you need to do above things explicitly, which is not always practical. Additionally, new can be overloaded but malloc can’t be.
If you work with data that doesn't need construction/destruction and requires reallocations (e.g., a large array of ints), then I believe malloc/free is a good choice as it gives you realloc, which is way faster than new-memcpy-delete (it is on my Linux box, but I guess this may be platform dependent). If you work with C++ objects that are not POD and require construction/destruction, then you must use the new and delete operators.
Anyway, I don't see why you shouldn't use both (provided that you free your malloced memory and delete objects allocated with new) if can take advantage of the speed boost (sometimes a significant one, if you're reallocing large arrays of POD) that realloc can give you.
Unless you need it though, you should stick to new/delete in C++.
If you are using C++, try to use new/delete instead of malloc/calloc as they are operators. For malloc/calloc, you need to include another header. Don't mix two different languages in the same code. Their work is similar in every manner, both allocates memory dynamically from heap segment in hash table.
new will initialise the default values of the struct and correctly links the references in it to itself.
E.g.
struct test_s {
int some_strange_name = 1;
int &easy = some_strange_name;
}
So new struct test_s will return an initialised structure with a working reference, while the malloc'ed version has no default values and the intern references aren't initialised.
If you have C code you want to port over to C++, you might leave any malloc() calls in it. For any new C++ code, I'd recommend using new instead.
From a lower perspective, new will initialize all the memory before giving the memory whereas malloc will keep the original content of the memory.
In the following scenario, we can't use new since it calls constructor.
class B {
private:
B *ptr;
int x;
public:
B(int n) {
cout<<"B: ctr"<<endl;
//ptr = new B; //keep calling ctr, result is segmentation fault
ptr = (B *)malloc(sizeof(B));
x = n;
ptr->x = n + 10;
}
~B() {
//delete ptr;
free(ptr);
cout<<"B: dtr"<<endl;
}
};
Rare case to consider using malloc/free instead of new/delete is when you're allocating and then reallocating (simple pod types, not objects) using realloc as there is no similar function to realloc in C++ (although this can be done using a more C++ approach).
I had played before with few C/C++ applications for computer graphics.
After so many time, some things are vanished and I missed them a lot.
The point is, that malloc and new, or free and delete, can work both,
especially for certain basic types, which are the most common.
For instance, a char array, can be allocated both with malloc, or new.
A main difference is, with new you can instantiate a fixed array size.
char* pWord = new char[5]; // allocation of char array of fixed size
You cannot use a variable for the size of the array in this case.
By the contrary, the malloc function could allow a variable size.
int size = 5;
char* pWord = (char*)malloc(size);
In this case, it might be required a conversion cast operator.
For the returned type from malloc it's a pointer to void, not char.
And sometimes the compiler could not know, how to convert this type.
After allocation the memory block, you can set the variable values.
the memset function can be indeed slower for some bigger arrays.
But all the bites must be set first to 0, before assigning a value.
Because the values of an array could have an arbitrary content.
Suppose, the array is assigned with another array of smaller size.
Part of the array element could still have arbitrary content.
And a call to a memset function would be recomended in this case.
memset((void*)pWord, 0, sizeof(pWord) / sizeof(char));
The allocation functions are available for all C packages.
So, these are general functions, that must work for more C types.
And the C++ libraries are extensions of the older C libraries.
Therefore the malloc function returns a generic void* pointer.
The sructures do not have defined a new, or a delete operator.
In this case, a custom variable can be allocated with malloc.
The new and delete keywords are actually some defined C operators.
Maybe a custom union, or class, can have defined these operators.
If new and delete are not defined in a class, these may not work.
But if a class is derived from another, which has these operators,
the new and delete keywords can have the basic class behavior.
About freeing an array, free can be only used in pair with malloc.
Cannot allocate a variable with malloc, and then free with delete.
The simple delete operator references just first item of an array.
Because the pWord array can be also written as:
pWord = &pWord[0]; // or *pWord = pWord[0];
When an array must be deleted, use the delete[] operator instead:
delete[] pWord;
Casts are not bad, they just don't work for all the variable types.
A conversion cast is also an operator function, that must be defined.
If this operator is not defined for a certain type, it may not work.
But not all the errors are because of this conversion cast operator.
Also a cast to a void pointer must be used when using a free call.
This is because the argument of the free function is a void pointer.
free((void*)pWord);
Some errors can arise, because the size of the array is too small.
But this is another story, it is not because of using the cast.
With kind regards, Adrian Brinas
The new and delete operators can operate on classes and structures, whereas malloc and free only work with blocks of memory that need to be cast.
Using new/delete will help to improve your code as you will not need to cast allocated memory to the required data structure.
malloc() is used to dynamically assign memory in C
while the same work is done by new() in c++.
So you cannot mix coding conventions of 2 languages.
It would be good if you asked for difference between calloc and malloc()