Malloc and constructors

Unlike new and delete operators mal开发者_JAVA技巧loc does not call the constructor when an object is created. In that case how must we create an object so that the constructor will also be called.

Er...use new? That's kind of the point. You can also call the constructor explicitly, but there's little reason to do it that way

Using new/delete normally:

A* a = new A();

delete a;

Calling the constructor/destructor explicitly ("placement new"):

A* a = (A*)malloc(sizeof(A));
new (a) A();

a->~A();
free(a);

You can use "placement new" syntax to do that if you really, really need to:

MyClassName* foo = new(pointer) MyClassName();

where pointer is a pointer to an allocated memory location large enough to hold an instance of your object.

Prefer new.

But if for some reason you have raw memory, you can construct it with "placement new":

new (ptr) TYPE(args);

And since you won't be using delete, you'll need to call the destructor directly:

ptr->~TYPE();

You mis-understand what malloc does. malloc does not create objects, it allocates memory. As it does not create objects there is no object for it to call a constructor to create.

If you need to dynamically create an object in C++ you need to use some form of new.

Use placement new. The advice is handy:

ADVICE: Don't use this "placement new" syntax unless you have to. Use it only when you really care that an object is placed at a particular location in memory. For example, when your hardware has a memory-mapped I/O timer device, and you want to place a Clock object at that memory location.

Take a look at placement new operator, which constructs an object on a pre-allocated buffer.

class A
{
    public:
      A() { printf("Constructor of A"); }
};

int main()
{
   A* pA = (A*) malloc(sizeof(A));  // Construtor is not going to be called.

   __asm { MOV EAX, pA}             // Makes pA as "this" pointer in intel based s/m.

   A();                             // Calling constructor explicitly.

   return 0;
}