Call class constructor from new operator on GNU - use invalid class

Question

The closest thread to my question is here. I am trying to compile the following code with gcc:

#include <malloc.h>

class A
{
public:
  A(){};  
  ~A(){};
};//class A

int main()
{
  A* obj = (A*) malloc( sizeof(A) );
  if(obj==0) return 1 ;
  obj->A::A(); /*error: invalid use of 'class A' */
  obj->A::~A();
  free(obj);
  return 0;  
};//

From the command line I compile the code with:

$ g++ -o main main.cpp  
main.cpp: In function 'int main()':  
main.cpp:22: error: invalid use of 'class A'

Can you please point me in the right direction?

Answer 1

There is no need for malloc business in C++. Simply do:

A* obj = new A();

new allocates the memory and calls the constructor for you.

And, to destroy it, do:

delete a;

delete calls the deconstructor method and frees the memory.

Answer 2

You want a placement-new instead.

void* ptr = malloc(sizeof(A));
A* obj = new(ptr) A;

Answer 3

You can't call a constructor on an object; a constructor can only be called in the creation of an object so by definition the object can't exist yet.

The way to do this is with placement new. There's no need to cast your malloc return. It should be void * as it doesn't return a pointer to an A; only a pointer to raw memory in which you plan to construct an A.

E.g.

void* mem = malloc( sizeof(A) );

A* obj = new (mem) A();
obj->~A();

free(mem);

Answer 4

Since you are using C++, falling back to malloc is unnecessary. You can write it in terms of operator new

int main() {
  if(A* obj = new (std::nothrow) A()) {
    delete obj;
    return 0;
  }
  return 1;
}

The nothrow version of new returns a null pointer if allocation fails, instead of throwing an exception.