I need explanation why this code print output 1?

Question

#include <iostream>

struct A
{
int x;
A(): x(0) {cout << "Constructor";)

friend std::ostream& operator <<(std::ostream& stream, const A& obj)
{
return stream << obj.x;
}
};
int main()
{
A a();
cout << a << endl;
}

Answer 1

Very interesting!

"For some reason, using A a(); is different than using A a;"

Well done Zeke! You have perfectly narrowed down the issue. Now, let's take a closer look...

A a; VS. A a();

`A a;` tells us that you are instantiating a new object of type A.
`A a();` tells us that you are prototyping a function called `a` that has no argument and return an object of type A.

What happens if you run the above code?

In GCC, you end up with a warning in regards to the `return address of a` like so
warning: the address of 'A a()' will always evaluate as 'true' [-Waddress]

In VS, you end up with a linker error (LNK 2019) like so
LNK2019 unresolved external symbol "struct A __cdecl a(void)"

Answer 2

It's interesting that when you remove the parentheses from the instantiation of a, it will run as expected. For some reason, using A a(); is different than using A a;

I know there's a rule about this somewhere in my head, but there are so many rules in so many languages that it starts to get all fuzzy. I think calling the constructor with no arguments using the parentheses is not good practice, probably for this reason. That's usually reserved for uses like:

A b = new A();

Any other C++ people want a stab at this?
https://code.sololearn.com/c1chZi2ruHm9

https://stackoverflow.com/questions/5116541/difference-between-creating-object-with-or-without/5116618

Answer 3

"But then why does it output 1?"

Short answer: Because the address of a function is a non-zero value like 0xBAADF00D, and each non-zero value is considered `true` in terms of boolean.

Answer 4

Thanks for all your help C++ Soldier (Babak), Zeke Williams, Ketan Lalcheta, and the others.. Its solved my problem.

Answer 5

Zarthan Maybe C++ Soldier (Babak) can help you with that...

Answer 6

Ketan Lalcheta Yeah buddy! we usually get told to use language structures in the right place but we also know how flexible C++ is and many times it surprises us. It happens to the best of us! 8D

Answer 7

How I wish I know C++ language😢😢😢😢😢.. I'm so lost reading all these comments...

Answer 8

😎iYkE4LiFe😎 LOL! I promise you get to grip with all of it prety sooooon!

Answer 9

wow... Difference between A a(); and A a;...

this is what I was actually looking for since so many days...

C++ Soldier (Babak) , I tried to create class A as empty nd wrote A a(); in main function. this I did in Visual Studio 2012 and build successfully...

A a() to be considered as function prototyping is another thing I could not get... in main function, we generally don't do function prototyping...

Answer 10

C++ Soldier (Babak) But then why does it output 1?
Shouldn't it have thrown some garbage because of printout out an undefined function?