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
Related
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.
Does anyone know a C++ delete procedure that is safe for both placement new and for regular new?
Object* regular = new Object();
delete_procedure(regular);
void* buf = ::new(sizeof(Object));
Object* placement = new(buf) Object();
delete_procedure(placement);
This function seems to work, but I can't figure out if it's actually safe for both instances (it's the standard method of deleting placement new).
delete_procedure(Object* obj){ // Not sure if safe for regular new
obj->~Object();
::delete(obj);
}
I don't believe there's any unified way of doing this without introducing some extra flags to mark things. The C++ philosophy is "you need to keep track of how you allocated things and ensure they're all destroyed properly," and it does this for efficiency purposes. Think about delete[] versus delete. There could be a unified delete keyword that cleans up any block of memory, but that would incur a performance overhead because of the cost of determining which deallocation routine needs to be invoked. Or think about virtual destructors: C++ could say that delete always does some kind of type introspection, but that would add overhead to all object types, much of it unnecessarily.
C++'s philosophy is "don't pay for what you don't use," and so if you want to build something unified, you'll need to set up some extra infrastructure to track what sorts of deletion routine you need to do.
No, because of arrays and details. Sure, because these problems probably don't matter in your code base.
All deletes are either type specific deletes, or:
The behavior of the standard library implementation of this function is undefined unless ptr is a null pointer or is a pointer previously obtained from the standard library implementation of operator new(size_t) or operator new(size_t, std::nothrow_t).
from http://en.cppreference.com/w/cpp/memory/new/operator_delete
This means passing memory allocated by new[](size_t) to delete(void*) is UB, and new[](size_t) to delete[](void*) is UB.
Second, users are free to replace new/delete on a global or per-class basis.
Third:
void delete_procedure(Object* obj){
obj->~Object();
::operator delete((void*)obj);
}
you'll want to cast to void pointer there and use operator delete.
Forth, the pointer passed to delete_procedure must be the pointer generated by new T or ::new( ::new(sizeof(T)) ) T; it cannot be a pointer to a derives subobject, as its value as a void* may differ.
If T* pt is dynamic castable, dynamic_cast<void*>( pt ) will reconstruct that void*. So this is less of a barrier than it seems. Remember to do the cast before destroying the object!
void delete_procedure_dynamic(Object* obj){
void* ptr=dynamic_cast<void*>(obj);
obj->~Object();
::operator delete(ptr);
}
and use SFINAE/tag dispatching to dispatch between the dynamic and non-dynamic versions.
Fifth, high alignment types need work to handle. They use different new and delete. I am uncertain how dynamic casting and over aligned derived types would interact, but probably you don't have to care.
But new T calls ::new(size_t)`` then constructs an object there.operator delete(void*)must be fed memory produced by::new(size_t). Destroying an object creates bynew Tis legal via.~T()`. So I see nothing fundamentally broken.
In practice, what I might do is override "normal" new to follow the same pattern as your extra storage new (and allocate the block via new(size_t)) to keep things simple. You are in arcane territory, why not make things uniform.
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.
Is there a difference between:
operator delete(some_pointer);
and
delete some_pointer;
and if so what is the difference and where one should use one and where the other version of this operator?
Thanks.
Ironically, the delete operator and operator delete() are not the same thing.
delete some_pointer; calls the destructor of the object pointed to by some_pointer, and then calls operator delete() to free the memory.
You do not normally call operator delete() directly, because if you do, the object's destructor will not be called, and you are likely to end up with memory leaks.
The only time you have to care about operator delete() is when you want to do your own memory management by overriding operator new() and operator delete().
To top it off, you should also be aware that delete and delete [] are two different things.
operator delete() simply frees the memory. delete some_pointer calls some_pointer's destructor, and then calls operator delete().
delete some_pointer; is the "correct" one to use.
operator delete(some_Pointer); exist mainly as an artifact of the syntax for defining you own delete operator. That is, because you define an plus operator as;
myclass::operator+(myclass b) {....}
you really could write:
myclass c = a.operator+(b);
but no one ever does that. They use:
myclass c = a + b;
Similarly, you could write operator delete(some_Pointer);, but no one ever does.
At least in my experience, it's more common to implement operator new and operator delete than to actually use (i.e., call) them, at least directly.
Usually, you use operator new and operator delete indirectly -- you write a new expression, like A *a = new A;. To implement this, the compiler generates code that invokes operator new to allocate raw memory, then invokes a A::A to convert that raw memory into an A object, much as if you'd written:
void *temp = operator new(sizeof A); // allocate raw memory with operator new
A *a = new(temp) A; // convert raw memory to object with placement new
When you're done with the object, you use delete A;. To implement that, the compiler invokes the dtor for the object, and then frees the memory, roughly like you'd done:
a->~A();
operator delete(a);
There are also operator [] new and operator [] delete, which are used when/if you allocate/delete arrays -- but there isn't necessarily any real difference between the normal version and the array version -- they both just allocate a specified amount of raw memory (though you might guess that the array versions will allocate relatively large amounts of memory, and do some optimization on that basis).
In any case, if you want to optimize how memory is allocated for objects of a particular class you overload these to do it. There are a fair number of existing implementations that you can drop-in and use, especially for situations where you expect to allocate a large number of tiny objects so you need to minimize the overhead associated with each allocation (e.g., HeapLayers, Loki's small block allocator).
One interesting little tidbit: operator new, operator [] new, operator delete and operator [] deleteare alwaysstaticclass members, even if you don't explicitly includestatic` in their declaration/definition.
There are also global versions of all four (::operator new, ::operator [] new, ::operator delete and ::operator [] delete). These mark the "border" between the "internal" C++ memory management, and the outside world. Typically they allocate relatively large chunks of memory from the operating system, and then return smaller pieces to the rest of the program upon request. If you want to (try to) optimize memory management for your entire program, you typically do it by overloading (or, really, replacing) these. Again, the typical reason would be if you expect to allocate a lot of small objects (but not in just a few classes). One example of this is the Boost Pool library.
Direct use of any of the above is generally restricted to situations where you need a block of raw memory, not objects. One example would be implementing your own container classes. For example, std::vector normally uses ::operator new (via an Allocator object) to allocate memory in which to store objects. Since it needs to be able to allocate storage, but only later (or perhaps never) create objects in that storage, it can't just use something like data = new T[size]; -- it has to allocate raw memory, then use placement new to create objects in the memory as you add them to the collection (e.g., when you push_back an object). The same is true with std::deque. If you wanted (for example) to implement your own circular buffer "from the ground up", handling all the memory management directly instead of using something like vector for storage, you'd probably need/want to do the same.
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.