How operator new calls the constructor of class?

Question

I know that, new operator will call the constructor of class.

But how it's happening, what is ground level techniques used for this.

Answer 1

Here is how I imagine it:

T* the_new_operator(args)
{
    void* memory = operator new(sizeof(T));
    T* object;
    try
    {
        object = new(memory) T(args);   // (*)
    }
    catch (...)
    {
        operator delete(memory);
        throw;
    }
    return object;
}

(*) Technically, it's not really calling placement-new, but as long as you don't overload that, the mental model works fine :)

Answer 2

It's not really the new operator that calls the constructor. It is more the compiler that translate the following line:

MyClass * mine = new MyClass();

Into the following:

MyClass * mine = malloc(sizeof(MyClass));  // Allocates memory
mine->MyClass();                           // Calls constructor

With other error handling code that other answers have noted.