I came across an article on new / operator new:
The many faces of operator new in C++
I couldn't understand the following example:
int main(int argc, const char* argv[])
{
char mem[sizeof(int)];
int* iptr2 = new (mem) int;
delete iptr2; // Whoops, segmentation fault!
return 0;
}
Here, the memory for int wasn't allocated using new, hence the segfault for delete.
What exactly does delete not like here? Is there some additional structure hidden when an object is initialized with new, that delete looks for?
EDIT:
I'll take out of comments several responses which helped me to understand the situation better:
As #463035818_is_not_a_number and #HolyBlackCat pointed out, mem was allocated on the stack, while delete tries to free memory on the heap. It's a pretty clear cut error and should lead to a segfault on most architectures.
If mem was allocated on the heap without an appropriate new:
The only way to do it I know would be, say, to allocate an int* on a heap, then reinterpret_cast it to an array of chars and give delete a char pointer. On a couple of architectures I tried this on, it actually works, but leads to a memory leak. In general, the C++ standard makes no guarantees in this case, because doing so would make binding assumption on the underlying architecture.
delete deletes objects from the heap. Your object is on the stack, not on the heap.
new (new T, not the placement-new you used) does two things: allocates heap memory (similar to malloc()), then performs initialization (for classes, calls a constructor).
Placement-new (what you used) performs initialization in existing memory, it doesn't allocate its own.
And delete does two things: calls the destructor (for class types), then frees the heap memory (similar to free()).
Since your object is not on the heap, delete can't delete its memory.
There's no "placement-delete" that only calls the destructor. Instead, we have manual destructor calls:
If you had a class type, you'd do iptr2->MyClass::~MyClass(); to call the destructor. And freeing the memory is then unnecessary since stack memory is automatically deallocated when leaving the current scope.
Also note that you forgot alignas(int) on your char array.
What exactly delete doesn't like here?
The fact that it was called for a pointer that did not come from a real new.
Is there some additional structure hidden when an object is initialized
with new, that it looks for?
That is a near certainty for your C++ implementation, but it's completely immaterial. This is undefined behavior, full stop. delete is defined only for pointers to objects that were created with a non-placement new operator. Otherwise this is undefined behavior. This is an important distinction. The "your C++ implementation" part is relevant. It's certainly possible that a different C++ compiler or operating system will produce code that doesn't crash, and does nothing at all. Or it may draw a funny face on your monitor screen. Or play a tune that you hate, on your speakers. This is what "undefined behavior" means. And in this case, "undefined behavior" means a crash, in your case.
You can only delete what has been allocated via new.
As explained in the article, placement new, skips the allocation:
Calling placement new directly skips the first step of object allocation. We don't ask for memory from the OS. Rather, we tell it where there's memory to construct the object in [3]. The following code sample should clarify this:
You cannot delete mem because it has not been allocated via new. mem has automatic storage duration and gets freed when main returns.
Placement new in your code creates an int in already allocated memory. If int had a destructor you would need to call the destructor (but not deallcoate the memory). Placing the int in the memory of mem does not change the fact that mem is allocated on the stack.
Actually the placement new in the code is not that relevant for the issue in the code. Also this code
int main(int argc, const char* argv[])
{
char mem[sizeof(int)];
delete iptr2; // Whoops, Undefined
return 0;
}
Has undefined behavior just as your code has.
There are several flavours of new and delete.
The (non-placement) new operator allocates memory by calling the operator new() function. The delete operator frees memory by calling the operator delete() function. They are a pair.
(Confused yet? Read this, or maybe this).
The new[] operator allocates memory by calling the operator new[]() function. The delete[] operator frees memory by calling the operator delete[]() function. They are a different pair.
The placement new operator does not allocate memory and does not call any kind of operator new()-like function. There is no corresponding delete operator or operator delete()-like function.
You cannot mix new of one flavour with delete of a different flavour, it makes no sense and the behaviour is undefined.
Related
Ignoring programming style and design, is it "safe" to call delete on a variable allocated on the stack?
For example:
int nAmount;
delete &nAmount;
or
class sample
{
public:
sample();
~sample() { delete &nAmount;}
int nAmount;
}
No, it is not safe to call delete on a stack-allocated variable. You should only call delete on things created by new.
For each malloc or calloc, there should be exactly one free.
For each new there should be exactly one delete.
For each new[] there should be exactly one delete[].
For each stack allocation, there should be no explicit freeing or deletion. The destructor is called automatically, where applicable.
In general, you cannot mix and match any of these, e.g. no free-ing or delete[]-ing a new object. Doing so results in undefined behavior.
Well, let's try it:
jeremy#jeremy-desktop:~$ echo 'main() { int a; delete &a; }' > test.cpp
jeremy#jeremy-desktop:~$ g++ -o test test.cpp
jeremy#jeremy-desktop:~$ ./test
Segmentation fault
So apparently it is not safe at all.
Keep in mind that when you allocate a block of memory using new (or malloc for that matter), the actual block of memory allocated will be larger than what you asked for.
The memory block will also contain some bookkeeping information so that when you free the block, it can easily be put back into the free pool and possibly be coalesced with adjacent free blocks.
When you try to free any memory that you didn't receive from new, that bookkeeping information wont be there but the system will act like it is and the results are going to be unpredictable (usually bad).
Yes, it is undefined behavior: passing to delete anything that did not come from new is UB:
C++ standard, section 3.7.3.2.3:
The value of the first argument supplied to one of thea deallocation functions provided in the standard library may be a null pointer value; if so, and if the deallocation function is one supplied in the standard library, the call to the deallocation function has no effect. Otherwise, the value supplied to operator delete(void*) in the standard library shall be one of the values returned by a previous invocation of either operator new(std::size_t) or operator new(std::size_t, const std::nothrow_t&) in the standard library.
The consequences of undefined behavior are, well, undefined. "Nothing happens" is as valid a consequence as anything else. However, it's usually "nothing happens right away": deallocating an invalid memory block may have severe consequences in subsequent calls to the allocator.
After playing a bit with g++ 4.4 in windows, I got very interesting results:
calling delete on a stack variable doesn't seem to do anything. No errors throw, but I can access the variable without problems after deletion.
Having a class with a method with delete this successfully deletes the object if it is allocated in the heap, but not if it is allocated in the stack (if it is in the stack, nothing happens).
Nobody can know what happens. This invokes undefined behavior, so literally anything can happen. Don't do this.
No,
Memory allocated using new should be deleted using delete operator
and that allocated using malloc should be deleted using free.
And no need to deallocate the variable which are allocated on stack.
An angel loses its wings... You can only call delete on a pointer allocated with new, otherwise you get undefined behavior.
here the memory is allocated using stack so no need to delete it exernally but if you have allcoted dynamically
like
int *a=new int()
then you have to do delete a and not delete &a(a itself is a pointer), because the memory is allocated from free store.
You already answered the question yourself. delete must only be used for pointers optained through new. Doing anything else is plain and simple undefined behaviour.
Therefore there is really no saying what happens, anything from the code working fine through crashing to erasing your harddrive is a valid outcome of doing this. So please never do this.
It's UB because you must not call delete on an item that has not been dynamically allocated with new. It's that simple.
Motivation: I have two objects, A and B. I know that A has to be instantiated before B, maybe because B needs information calculated by A. Yet, I want to destruct A before B. Maybe I am writing an integration test, and I want server A to shut-down first. How do I accomplish that?
A a{};
B b{a.port()};
// delete A, how?
Solution: Don't allocate A on the stack. Instead, use std::make_unique and keep a stack-allocated smart pointer to a heap-allocated instance of A. That way is the least messy option, IMO.
auto a = std::make_unique<A>();
B b{a->port()};
// ...
a.reset()
Alternatively, I considered moving the destruction logic out of A's destructor and calling that method explicitly myself. The destructor would then call it only if it has not been called previously.
I've been reading somewere that when you use placement new then you have to call the destructor manually.
Consider the folowing code:
// Allocate memory ourself
char* pMemory = new char[ sizeof(MyClass)];
// Construct the object ourself
MyClass* pMyClass = new( pMemory ) MyClass();
// The destruction of object is our duty.
pMyClass->~MyClass();
As far as I know operator delete normally calls the destructor and then deallocates the memory, right? So why don't we use delete instead?
delete pMyClass; //what's wrong with that?
in the first case we are forced to set pMyClass to nullptr after we call destructor like this:
pMyClass->~MyClass();
pMyClass = nullptr; // is that correct?
BUT the destructor did NOT deallocate memory, right?
So would that be a memory leak?
I'm confused, can you explain that?
Using the new expression does two things, it calls the function operator new which allocates memory, and then it uses placement new, to create the object in that memory. The delete expression calls the object's destructor, and then calls operator delete. Yeah, the names are confusing.
//normal version calls these two functions
MyClass* pMemory = new MyClass; void* pMemory = operator new(sizeof(MyClass));
MyClass* pMyClass = new( pMemory ) MyClass();
//normal version calls these two functions
delete pMemory; pMyClass->~MyClass();
operator delete(pMemory);
Since in your case, you used placement new manually, you also need to call the destructor manually. Since you allocated the memory manually, you need to release it manually.
However, placement new is designed to work with internal buffers as well (and other scenarios), where the buffers were not allocated with operator new, which is why you shouldn't call operator delete on them.
#include <type_traits>
struct buffer_struct {
std::aligned_storage_t<sizeof(MyClass), alignof(MyClass)> buffer;
};
int main() {
buffer_struct a;
MyClass* pMyClass = new (&a.buffer) MyClass(); //created inside buffer_struct a
//stuff
pMyClass->~MyClass(); //can't use delete, because there's no `new`.
return 0;
}
The purpose of the buffer_struct class is to create and destroy the storage in whatever way, while main takes care of the construction/destruction of MyClass, note how the two are (almost*) completely separate from each other.
*we have to be sure the storage has to be big enough
One reason this is wrong:
delete pMyClass;
is that you must delete pMemory with delete[] since it is an array:
delete[] pMemory;
You can't do both of the above.
Similarly, you might ask why you can't use malloc() to allocate memory, placement new to construct an object, and then delete to delete and free the memory. The reason is that you must match malloc() and free(), not malloc() and delete.
In the real world, placement new and explicit destructor calls are almost never used. They might be used internally by the Standard Library implementation (or for other systems-level programming as noted in the comments), but normal programmers don't use them. I have never used such tricks for production code in many years of doing C++.
You need to distinguish between the delete operator and operator delete. In particular, if you're using placement new, you explicitly invoke the destructor and then call operator delete (and not the delete operator) to release the memory, i.e.
X *x = static_cast<X*>(::operator new(sizeof(X)));
new(x) X;
x->~X();
::operator delete(x);
Note that this uses operator delete, which is lower-level than the delete operator and doesn't worry about destructors (it's essentially a bit like free). Compare this to the delete operator, which internally does the equivalent of invoking the destructor and calling operator delete.
It's worth noting that you don't have to use ::operator new and ::operator delete to allocate and deallocate your buffer - as far as placement new is concerned, it doesn't matter how the buffer comes into being / gets destroyed. The main point is to separate the concerns of memory allocation and object lifetime.
Incidentally, a possible application of this would be in something like a game, where you might want to allocate a large block of memory up-front in order to carefully manage your memory usage. You'd then construct objects in the memory you've already acquired.
Another possible use would be in an optimized small, fixed-size object allocator.
It's probably easier to understand if you imagine constructing several MyClass objects within one block of memory.
In that case, it would go something like:
Allocate a giant block of memory using new char[10*sizeof(MyClass)] or malloc(10*sizeof(MyClass))
Use placement new to construct ten MyClass objects within that memory.
Do something.
Call the destructor of each of your objects
Deallocate the big block of memory using delete[] or free().
This is the sort of thing you might do if you're writing a compiler, or an OS, etc.
In this case, I hope it's clear why you need separate "destructor" and "delete" steps, because there's no reason you will call delete. However, you should deallocate the memory however you would normally do it (free, delete, do nothing for a giant static array, exit normally if the array is part of another object, etc, etc), and if you don't it'll be leaked.
Also note as Greg said, in this case, you can't use delete, because you allocated the array with new[] so you'd need to use delete[].
Also note that you need to assume you haven't overridden delete for MyClass, else it will do something totally different which is almost certainly incompatible with "new".
So I think you're unlikley to want to call "delete" as you describe, but could it ever work? I think this is basically the same question as "I have two unrelated types that don't have destructors. Can I new a pointer to one type, then delete that memory through a pointer to another type?" In other words, "does my compiler have one big list of all allocated stuff, or can it do different things for different types".
I'm afraid I'm not sure. Reading the spec it says:
5.3.5 ... If the static type of the operand [of the delete operator] is different from its dynamic type, the static type shall be a base
class of the operand's dynamic type and the static type shall have a
virtual destructor or the behaviour is undefined.
I think that means "If you use two unrelated types, it doesn't work (it's ok to delete a class object polymorphically through a virtual destructor)."
So no, don't do that. I suspect it may often work in practice, if the compiler does look solely at the address and not the type (and neither type is a multiple-inheritance class, which would mangle the address), but don't try it.
you'll be using placement new for shared memory IPC: one "initializer" process reserves and maps the shared memory and then the mapped memory is shared by all processes
What is the difference between new/delete and malloc/free?
Related (duplicate?): In what cases do I use malloc vs new?
new / delete
Allocate / release memory
Memory allocated from 'Free Store'.
Returns a fully typed pointer.
new (standard version) never returns a NULL (will throw on failure).
Are called with Type-ID (compiler calculates the size).
Has a version explicitly to handle arrays.
Reallocating (to get more space) not handled intuitively (because of copy constructor).
Whether they call malloc / free is implementation defined.
Can add a new memory allocator to deal with low memory (std::set_new_handler).
operator new / operator delete can be overridden legally.
Constructor / destructor used to initialize / destroy the object.
malloc / free
Allocate / release memory
Memory allocated from 'Heap'.
Returns a void*.
Returns NULL on failure.
Must specify the size required in bytes.
Allocating array requires manual calculation of space.
Reallocating larger chunk of memory simple (no copy constructor to worry about).
They will NOT call new / delete.
No way to splice user code into the allocation sequence to help with low memory.
malloc / free can NOT be overridden legally.
Table comparison of the features:
Feature
new / delete
malloc / free
Memory allocated from
'Free Store'
'Heap'
Returns
Fully typed pointer
void*
On failure
Throws (never returns NULL)
Returns NULL
Required size
Calculated by compiler
Must be specified in bytes
Handling arrays
Has an explicit version
Requires manual calculations
Reallocating
Not handled intuitively
Simple (no copy constructor)
Call of reverse
Implementation defined
No
Low memory cases
Can add a new memory allocator
Not handled by user code
Overridable
Yes
No
Use of constructor / destructor
Yes
No
Technically, memory allocated by new comes from the 'Free Store' while memory allocated by malloc comes from the 'Heap'. Whether these two areas are the same is an implementation detail, which is another reason that malloc and new cannot be mixed.
The most relevant difference is that the new operator allocates memory then calls the constructor, and delete calls the destructor then deallocates the memory.
new calls the ctor of the object, delete call the dtor.
malloc & free just allocate and release raw memory.
new/delete is C++, malloc/free comes from good old C.
In C++, new calls an objects constructor and delete calls the destructor.
malloc and free, coming from the dark ages before OO, only allocate and free the memory, without executing any code of the object.
In C++ new/delete call the Constructor/Destructor accordingly.
malloc/free simply allocate memory from the heap. new/delete allocate memory as well.
The main difference between new and malloc is that new invokes the object's constructor and the corresponding call to delete invokes the object's destructor.
There are other differences:
new is type-safe, malloc returns objects of type void*
new throws an exception on error, malloc returns NULL and sets errno
new is an operator and can be overloaded, malloc is a function and cannot be overloaded
new[], which allocates arrays, is more intuitive and type-safe than malloc
malloc-derived allocations can be resized via realloc, new-derived allocations cannot be resized
malloc can allocate an N-byte chunk of memory, new must be asked to allocate an array of, say, char types
Looking at the differences, a summary is malloc is C-esque, new is C++-esque. Use the one that feels right for your code base.
Although it is legal for new and malloc to be implemented using different memory allocation algorithms, on most systems new is internally implemented using malloc, yielding no system-level difference.
The only similarities are that malloc/new both return a pointer which addresses some memory on the heap, and they both guarantee that once such a block of memory has been returned, it won't be returned again unless you free/delete it first. That is, they both "allocate" memory.
However, new/delete perform arbitrary other work in addition, via constructors, destructors and operator overloading. malloc/free only ever allocate and free memory.
In fact, new is sufficiently customisable that it doesn't necessarily return memory from the heap, or even allocate memory at all. However the default new does.
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.
In a word, if you use C++, try to use new as much as possible.
also,
the global new and delete can be overridden, malloc/free cannot.
further more new and delete can be overridden per type.
new and delete are C++ primitives which declare a new instance of a class or delete it (thus invoking the destructor of the class for the instance).
malloc and free are C functions and they allocate and free memory blocks (in size).
Both use the heap to make the allocation. malloc and free are nonetheless more "low level" as they just reserve a chunk of memory space which will probably be associated with a pointer. No structures are created around that memory (unless you consider a C array to be a structure).
new is an operator, whereas malloc() is a fucntion.
new returns exact data type, while malloc() returns void * (pointer of type void).
malloc(), memory is not initialized and default value is garbage, whereas in case of new, memory is initialized with default value, like with 'zero (0)' in case on int.
delete and free() both can be used for 'NULL' pointers.
new and delete are operators in c++; which can be overloaded too.
malloc and free are function in c;
malloc returns null ptr when fails while new throws exception.
address returned by malloc need to by type casted again as it returns the (void*)malloc(size)
New return the typed pointer.
To use the malloc(), we need to include <stdlib.h> or
<alloc.h> in the program which is not required for new.
new and delete can be overloaded but malloc can not.
Using the placement new, we can pass the address where we want to
allocate memory but this is not possible in case of malloc.
This code for use of delete keyword or free function. But when create a
pointer object using 'malloc' or 'new' and deallocate object memory using
delete even that object pointer can be call function in the class. After
that use free instead of delete then also it works after free statement ,
but when use both then only pointer object can't call to function in class..
the code is as follows :
#include<iostream>
using namespace std;
class ABC{
public: ABC(){
cout<<"Hello"<<endl;
}
void disp(){
cout<<"Hi\n";
}
};
int main(){
ABC* b=(ABC*)malloc(sizeof(ABC));
int* q = new int[20];
ABC *a=new ABC();
b->disp();
cout<<b<<endl;
free(b);
delete b;
//a=NULL;
b->disp();
ABC();
cout<<b;
return 0;
}
output :
Hello
Hi
0x2abfef37cc20
1.new syntex is simpler than malloc()
2.new/delete is a operator where malloc()/free()
is a function.
3.new/delete execute faster than malloc()/free() because new assemly code directly pasted by the compiler.
4.we can change new/delete meaning in program with the help of operator overlading.
What is wrong with using delete instead of delete[]?
Is there something special happening under the covers for allocating and freeing arrays?
Why would it be different from malloc and free?
Objects created with new[] must use delete[]. Using delete is undefined on arrays.
With malloc and free you have a more simple situation. There is only 1 function that frees the data you allocate, there is no concept of a destructor being called either. The confusion just comes in because delete[] and delete look similar. Actually they are 2 completely different functions.
Using delete won't call the correct function to delete the memory. It should call delete[](void*) but instead it calls delete(void*). For this reason you can't rely on using delete for memory allocated with new[]
See this C++ FAQ
[16.13] Can I drop the [] when
deleteing array of some built-in type
(char, int, etc)?
No!
Sometimes programmers think that the
[] in the delete[] p only exists so
the compiler will call the appropriate
destructors for all elements in the
array. Because of this reasoning, they
assume that an array of some built-in
type such as char or int can be
deleted without the []. E.g., they
assume the following is valid code:
void userCode(int n) {
char* p = new char[n];
...
delete p; // ← ERROR! Should be delete[] p !
}
But the above code is wrong, and it
can cause a disaster at runtime. In
particular, the code that's called for
delete p is operator delete(void*),
but the code that's called for
delete[] p is operator
delete[](void*). The default behavior
for the latter is to call the former,
but users are allowed to replace the
latter with a different behavior (in
which case they would normally also
replace the corresponding new code in
operator new[](size_t)). If they
replaced the delete[] code so it
wasn't compatible with the delete
code, and you called the wrong one
(i.e., if you said delete p rather
than delete[] p), you could end up
with a disaster at runtime.
Why does delete[] exist in the first place?
Whether you do x or y:
char * x = new char[100];
char * y = new char;
Both are stored in char * typed variables.
I think the reason for the decision of delete, and delete[] goes along with a long list of decisions that are in favor of efficiency in C++. It is so that there is no enforced price to do a lookup of how much needs to be deleted for a normal delete operation.
Having 2 new and new[] seems only logical to have delete and delete[] anyway for symmetry.
The difference is that delete will only delete the entire memory range, but will only call the destructor for 1 object. delete[] will both delete the memory and call the destructor for every single object. If you do not use delete[] for arrays, it's only a matter of time before you introduce a resource leak into your application.
EDIT Update
According to the standard, passing an object allocated with new[] to delete is undefined. The likely behavior is that it will act as I described.
Stroustrup talks about the reasons for separate new/new[] and delete/delete[]` operators in "The Design and Evolution of C++" in sections 10.3 through 10.5.1:
10.3 Array Allocation - discusses that they wanted a way to allow arrays of objects to be allocated using a separate scheme from allocation single objects (ie., allocating arrays from a separate store). Adding the array versions of new and delete was a solution for this;
10.5.1 Deallocating Arrays - discusses how a problem with deallocating arrays using just a single delete operator is that there needs to be more information than just the pointer in order to determine if the pointer points to the first element of an array or if it just points to a single object. Instead of "complicating the common case of allocating and deallocating individual objects", the delete[] operator is used to handle arrays. This fits in with the general C++ design philiosophy of "don't pay for what you don't use".
Whether this decision was a mistake or not is debatable - either way has good arguments, but we have what we have.
The reason for this requirement is historical and because new type and new type [size] return different things that need to be cleaned up differently.
Consider this code
Foo* oneEntry = new Foo;
Foo* tenEntries = new Foo[10];
These both return a Foo* pointer, the difference is the second call will result in the Foo constructor being called 10x, and there being roughly 10x as much memory.
So now you want to free your objects.
For a single object you would call delete - e.g. delete oneEntry. This calls the objects destructor and and deallocates the memory.
But here's the problem - oneEntry and tenEntries are both just Foo pointers. The compiler has no idea whether they point to one, ten, or a thousand elements.
When you use the special syntax of delete []. This tells the compiler "this is an array of objects, figure out the count and then destruct them all".
What really happens is that for new type [size] the compiler secretly stores 'size' somewhere else. When you call delete[] it knows that this secret value exists so it can find out how many objects are in that block of memory and destruct them.
The question you could then ask is "why doesn't the compiler always store the size?"
That's a great question and it dates back to the early days of C++. There was a desire that for built-in types (char, int, float, etc) the following would be valid for C++;
int* ptr = new int;
free(ptr);
int* ptr = (int*)malloc(sizeof(int) * someSize);
delete ptr;
The reasoning behind this was an expectation that people would provide libraries that returned dynamically allocated memory, and users of these libraries would have no way of knowing whether to use free/delete.
This desire for compatibility meant that the size of an array could not be stored as part of the array itself and had to be kept elsewhere. Because of this overhead (and remember, this was back in the early 80's) it was decided to do this book keeping only for arrays and not single-elements. Thus arrays need a special delete syntax that looks up this value.
The reason malloc/free do not have this problem is that they simply deal with blocks of memory and do not have to worry about calling constructors/destructors.
As to the "why" in the title: one of the design goals of C++ was that there wouldn't be any hidden costs. C++ was also developed at a time when every byte of memory still mattered a whole lot more than it does today. Language designers also like orthogonality: if you allocate the memory with new[] (instead of new), you should free it with delete[].
I don't think there's any technical reason that new[] couldn't stick an "I'm an array" flag in the header of the memory block for delete (no more delete[]) to look at later.
new and delete are different from malloc and free in that malloc and free only allocate and free memory; they don't call ctors or dtors.
When you use new[] to allocate an array, you are actually telling C++ the size of the array. When you use malloc, you are instead telling it how much memory is allocated. In the former case, freeing based on the size of the array would not make sense. In this case, it does. But since there is no difference between a pointer for an array vs. for a single object, a separate function is needed.
What is the difference between new/delete and malloc/free?
Related (duplicate?): In what cases do I use malloc vs new?
new / delete
Allocate / release memory
Memory allocated from 'Free Store'.
Returns a fully typed pointer.
new (standard version) never returns a NULL (will throw on failure).
Are called with Type-ID (compiler calculates the size).
Has a version explicitly to handle arrays.
Reallocating (to get more space) not handled intuitively (because of copy constructor).
Whether they call malloc / free is implementation defined.
Can add a new memory allocator to deal with low memory (std::set_new_handler).
operator new / operator delete can be overridden legally.
Constructor / destructor used to initialize / destroy the object.
malloc / free
Allocate / release memory
Memory allocated from 'Heap'.
Returns a void*.
Returns NULL on failure.
Must specify the size required in bytes.
Allocating array requires manual calculation of space.
Reallocating larger chunk of memory simple (no copy constructor to worry about).
They will NOT call new / delete.
No way to splice user code into the allocation sequence to help with low memory.
malloc / free can NOT be overridden legally.
Table comparison of the features:
Feature
new / delete
malloc / free
Memory allocated from
'Free Store'
'Heap'
Returns
Fully typed pointer
void*
On failure
Throws (never returns NULL)
Returns NULL
Required size
Calculated by compiler
Must be specified in bytes
Handling arrays
Has an explicit version
Requires manual calculations
Reallocating
Not handled intuitively
Simple (no copy constructor)
Call of reverse
Implementation defined
No
Low memory cases
Can add a new memory allocator
Not handled by user code
Overridable
Yes
No
Use of constructor / destructor
Yes
No
Technically, memory allocated by new comes from the 'Free Store' while memory allocated by malloc comes from the 'Heap'. Whether these two areas are the same is an implementation detail, which is another reason that malloc and new cannot be mixed.
The most relevant difference is that the new operator allocates memory then calls the constructor, and delete calls the destructor then deallocates the memory.
new calls the ctor of the object, delete call the dtor.
malloc & free just allocate and release raw memory.
new/delete is C++, malloc/free comes from good old C.
In C++, new calls an objects constructor and delete calls the destructor.
malloc and free, coming from the dark ages before OO, only allocate and free the memory, without executing any code of the object.
In C++ new/delete call the Constructor/Destructor accordingly.
malloc/free simply allocate memory from the heap. new/delete allocate memory as well.
The main difference between new and malloc is that new invokes the object's constructor and the corresponding call to delete invokes the object's destructor.
There are other differences:
new is type-safe, malloc returns objects of type void*
new throws an exception on error, malloc returns NULL and sets errno
new is an operator and can be overloaded, malloc is a function and cannot be overloaded
new[], which allocates arrays, is more intuitive and type-safe than malloc
malloc-derived allocations can be resized via realloc, new-derived allocations cannot be resized
malloc can allocate an N-byte chunk of memory, new must be asked to allocate an array of, say, char types
Looking at the differences, a summary is malloc is C-esque, new is C++-esque. Use the one that feels right for your code base.
Although it is legal for new and malloc to be implemented using different memory allocation algorithms, on most systems new is internally implemented using malloc, yielding no system-level difference.
The only similarities are that malloc/new both return a pointer which addresses some memory on the heap, and they both guarantee that once such a block of memory has been returned, it won't be returned again unless you free/delete it first. That is, they both "allocate" memory.
However, new/delete perform arbitrary other work in addition, via constructors, destructors and operator overloading. malloc/free only ever allocate and free memory.
In fact, new is sufficiently customisable that it doesn't necessarily return memory from the heap, or even allocate memory at all. However the default new does.
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.
In a word, if you use C++, try to use new as much as possible.
also,
the global new and delete can be overridden, malloc/free cannot.
further more new and delete can be overridden per type.
new and delete are C++ primitives which declare a new instance of a class or delete it (thus invoking the destructor of the class for the instance).
malloc and free are C functions and they allocate and free memory blocks (in size).
Both use the heap to make the allocation. malloc and free are nonetheless more "low level" as they just reserve a chunk of memory space which will probably be associated with a pointer. No structures are created around that memory (unless you consider a C array to be a structure).
new is an operator, whereas malloc() is a fucntion.
new returns exact data type, while malloc() returns void * (pointer of type void).
malloc(), memory is not initialized and default value is garbage, whereas in case of new, memory is initialized with default value, like with 'zero (0)' in case on int.
delete and free() both can be used for 'NULL' pointers.
new and delete are operators in c++; which can be overloaded too.
malloc and free are function in c;
malloc returns null ptr when fails while new throws exception.
address returned by malloc need to by type casted again as it returns the (void*)malloc(size)
New return the typed pointer.
To use the malloc(), we need to include <stdlib.h> or
<alloc.h> in the program which is not required for new.
new and delete can be overloaded but malloc can not.
Using the placement new, we can pass the address where we want to
allocate memory but this is not possible in case of malloc.
This code for use of delete keyword or free function. But when create a
pointer object using 'malloc' or 'new' and deallocate object memory using
delete even that object pointer can be call function in the class. After
that use free instead of delete then also it works after free statement ,
but when use both then only pointer object can't call to function in class..
the code is as follows :
#include<iostream>
using namespace std;
class ABC{
public: ABC(){
cout<<"Hello"<<endl;
}
void disp(){
cout<<"Hi\n";
}
};
int main(){
ABC* b=(ABC*)malloc(sizeof(ABC));
int* q = new int[20];
ABC *a=new ABC();
b->disp();
cout<<b<<endl;
free(b);
delete b;
//a=NULL;
b->disp();
ABC();
cout<<b;
return 0;
}
output :
Hello
Hi
0x2abfef37cc20
1.new syntex is simpler than malloc()
2.new/delete is a operator where malloc()/free()
is a function.
3.new/delete execute faster than malloc()/free() because new assemly code directly pasted by the compiler.
4.we can change new/delete meaning in program with the help of operator overlading.