What arguments does new take?

Question

Does new in C++ call a constructor behind the scenes? Or is it the other way around?

I have seen code like new MyClass(*this) which confuses me, since I didn't know that new could take arguments.

Maybe that's because new can call a constructor and, as a result, it can take the arguments declared by a constructor?

Answer 1

MyClass(*this) creates an object of type MyClass by calling its constructor and passing *this as an argument. Putting new behind it means the object is allocated on the heap instead of the stack. It's not new which is taking the argument, it's MyClass .

Answer 2

There is a difference between new and operator new . The new operator uses a hidden operator new function to allocate memory and then it also "value-initializes" the object by calling its constructor with the parameters after the class name.

In your case you call new which allocates memory using ::operator new() and then it initializes an object of MyClass class in that memory using a constructor with the parameter *this .

#include <iostream>

class A {
    public:
        int m_value; 
        A(int value): m_value(value){};
};

int main (){

    int *a  = new int; 

    auto cb= new A(1); 

    std::cout << *a << std::endl; 
    std::cout << b->m_value << std::endl; 
    printf("%02x ", *b);

}

Program returned: 
0
15
0f

As you can see, the new for the a variable creates only a pointer that is value-initialized to 0. That's why when we dereference it we have 0 (all bit all 0, int is 4 bytes most of the time and the pointer point to a memory content = to 0x0000)

But for the b variable we pass parameters. And if we look at the memory content of the b object we can read 0f which means it contains 15 (the member value)

Answer 3

This is not new taking arguments, it's the constructor taking arguments.