Question

#include <iostream>
using namespace std;

class A {
public:
    A() {
        cout << "Default Ctor" << endl;
    }
};

int main() {
    A(); // <------- Problem

    return 0;
}

It shows Default Ctor on console. My questions

• Is is valid?
• If so, how did it instantiate since I didn’t use new or any object?

Answer 1

You are creating a new object with A().

Is is valid?

Yes it is.

If so, how did it instantiate since I didn’t use new or any object?

new simply creates the object in dynamic memory. You’re creating the object in automatic memory. Also, just because you didn’t give the object a name, doesn’t mean it isn’t created.

Think of this:

int foo() { return 1; }

int main()
{
   foo();
}

Leaving optimizations aside Did foo() actually return 1? Yes it did. Just that you’re not using it.

EDIT:

Let’s break it down a bit:

A();  //creates a temporary unnamed object in automatic storage

A a;   //creates an object a of type A in automatic storage

A a(); //creates no object, but declare a function a returning A and taking no parameters

A a(A());   //these two are equivalent
A a = A();  //creates a temporary unnamed object and creates an object a in automatic storage
            //using the compiler-generated copy constructor

A a;
a = A();    //creates an object a in automatic storage
            //creates an unnamed temporary object and uses the compiler-generated assignment operator
            //to assign it to a

A a = &A(); //invalid, &A() is the address of a temporary a, which is illegal