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

The compiler generates machine code for that. When the compiler sees

CSomeClass* object = new CSomeClass();

(new statement) it generates code that calls the appropriate operator new() (which allocates memory), calls the right constructor, calls destructors of all fully constructed subobjects in case of exception, calls operator delete() in case an exception occurs during construction. All this is done by extra machine code generated by the C++ compiler for that simply looking statement.

Answer 2

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 3

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.