What is the difference between new/delete and malloc/free?

new / delete

• Allocate / release memory
  1. Memory allocated from 'Free Store'.
  2. Returns a fully typed pointer.
  3. new (standard version) never returns a NULL (will throw on failure).
  4. Are called with Type-ID (compiler calculates the size).
  5. Has a version explicitly to handle arrays.
  6. Reallocating (to get more space) not handled intuitively (because of copy constructor).
  7. Whether they call malloc / free is implementation defined.
  8. Can add a new memory allocator to deal with low memory (std::set_new_handler).
  9. operator new / operator delete can be overridden legally.
  10. Constructor / destructor used to initialize / destroy the object.

malloc / free

• Allocate / release memory
  1. Memory allocated from 'Heap'.
  2. Returns a void*.
  3. Returns NULL on failure.
  4. Must specify the size required in bytes.
  5. Allocating array requires manual calculation of space.
  6. Reallocating larger chunk of memory simple (no copy constructor to worry about).
  7. They will NOT call new / delete.
  8. No way to splice user code into the allocation sequence to help with low memory.
  9. malloc / free can NOT be overridden legally.

Table comparison of the features:

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 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.

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).

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