Malloc and constructors

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();