Need help understanding error message

[Cheddar99]

Hi,
I'm doing an online C++ quiz thingy, and the code in one the questions is given below:

#include <iostream>
using namespace std;

class A{
        public:
        int v;
        A():v(1){}
        A(int i):v(i){}
        void operator&&(int a){v=-v;}
};

int main (void) {
        A i = 2;
        i && 2;
        cout << i << endl;
return 0;
}

It doesn't compile - which is the correct answer in the quiz - but I'm having trouble understanding why it doesn't compile. The error message when attempting to compile is "error: no match for 'operator<<' (operand types are ...". The quiz is largely about overloaded operators, hence the overloaded "&&".

I must be missing something very obvious, but I just don't understand the error message. Any assistance greatly appreciated. Thx!

[kooth]

Hi,

You need to define the "<<" operator for class A.

[Cheddar99]

I don't understand why. I mean it's the && operator which is being overloaded, not the "<<" operator. Surely just because you overload one operator doesn't mean you need to overload every operator. This works:

#include <iostream>
using namespace std;

class A{
        public:
        int v;
        A():v(1){}
        A(int i):v(i){}
        void operator&&(int a){v=-v;}
};

int main (void) {
        A i = 2;
        i && 2;
    //  cout << i << endl;
        cout << 1+2 << endl;  
return 0;
}

... presumably because I'm not trying to output "i", but I don't really see why it's any different.

[kooth]

When using iostreams, you must define any operator that you are using. Your "A" class doesn't define one.

Your example works because integers aren't classes - your class needs to define it.

[Cheddar99]

Still not sure I understand what you mean. For example, the following works, but B class obviously hasn't defined any operator:

#include <iostream>
using namespace std;

class A{
        public:
        int v;
        A():v(1){}
        A(int i):v(i){}
        void operator&&(int a){v=-v;}
};

class B{
        public:
        int x=10;  
};

int main (void) {
        A i=2;
        B b;
        i && 2;
    //  cout << i << endl;
        cout << 10 << endl;
        cout << b.x << endl;
return 0;
}

[keskiverto]

That is not a proper comparison. You are comparing A to int:

A foo;
B bar;
std::cout << foo; // error, no << for A
std::cout << bar.x; // ok, there is << for int 

The B fails just like A:

#include <iostream>

class B{
};

int main (void) {
  B b;
  std::cout << b; // error: no match for 'operator<<'
}

 In function 'int main()':
8:13: error: no match for 'operator<<' (operand types are 'std::ostream {aka std::basic_ostream<char>}' and 'B')
8:13: note: candidates are:

You cannot write std::cout << snafu; unless there is operator<< that accepts typeof(snafu) objects.

That has nothing to do with other operators.

Besides,

std::cout << i.v; // ok
std::cout << b.x; // ok 

[mbozzi]

The standard library knows how to print an int. But it doesn't know how to print As. A is a user-defined type, and there are no instructions that describe how to print it.

Provide those instructions by defining the function
std::ostream &operator<<(std::ostream&, A const&).

For example

#include <iostream>

// There is a record type called A
struct A {};

// It is possible to put objects of type A into C++ streams
// In other words, it is possible to print objects of type A.  
std::ostream& operator <<(std::ostream& s, A const& a)
{
  return s << "hi, from an object of type A";
}

int main() {
  // There is an object of type A called a.
  A a;

  // Print it by calling the operator<< above
  // put the object a to the stream std::cout
  std::cout << a;
  std::cout << '\n';
}

[Cheddar99]

Oh, I think I see what you mean. Alright, I wasn't familiar with doing that as I guess I've not seen that or needed to do it before. (still an early-learner!!). Yeah, the last two comments make more sense to me, but thanks to all. Much appreciated.

[kooth]

Good for you for learning and sticking with it. mbozzi and keskiverto did a much better job of explaining this than I did.

There are lots of other good people on this site who are happy to help. Good luck in your journey into the land of C++!

[Cheddar99]

Thanks kooth!