binary '<<': no operator found which takes a right-hand operand of type 'c1'

Question

I was experimenting bit with a c++ then I stumbled upon this problem:- binary '<<': no operator found which takes a right-hand operand of type 'c1'

What is causing such problem?

using c1 = enum class Color : unsigned int {
    red,
    green,
    blue,

};
int main()
{
    c1 col{ Color::red };
    std::cout << "Value of col is " << col;
}

when I use enum followed just the name Color it prompts an warning:-

The enum type 'Color' is unscoped. Prefer 'enum class' over 'enum' (Enum.3).

_"What is causing such problem?"_ that you missed to define an `std::ostream operator<<(std::ostream& os, const Color& col);`. The error message is pretty clear about that, no? — πάντα ῥεῖ, Nov 12 '21 at 14:29
You need to define an operator (I've started it off for you with "red") thus:- std::ostream& operator<<(std::ostream& o, Color c){ return o << "red"; } — QuentinUK, Nov 12 '21 at 14:30
Difference between `enum` and `enum class` https://stackoverflow.com/a/18335862/8309701 — Chris Uzdavinis, Nov 12 '21 at 14:39
Concerning that last line: that's the compiler giving you style advice. It's up to you to decide whether the compiler knows more about the code you're writing than you do. — Pete Becker, Nov 12 '21 at 14:39
its not a problem but a feature. You can decide how a `Color` should be inserted into the stream — 463035818_is_not_an_ai, Nov 12 '21 at 14:50

score 0 · Answer 1 · answered Nov 13 '21 at 08:58

0

to print the index of the elements of the enum class one must use the static_cast or use the ones suggested in the comment box.

 std::cout << static_cast<unsigned int>(Color::red);

//it should print index 0

answered Nov 13 '21 at 08:58

Nice-Willow

13
7

score 0 · Answer 2 · answered Nov 13 '21 at 11:09

You can just cast it to unsigned int if you want to print the value. And if you want the string printed, then you have to write a function for that, like below:

#include <string> // For std::string
#include <iostream>
using c1 = enum class Color : unsigned int {
    red,
    green,
    blue,

};


inline std::string ColorToString(c1 color)
{
    switch (color)
    {
        case c1::red: return "red";
        case c1::green: return "green";
        case c1::blue: return "blue";
    }
    return "";
}


int main()
{
    c1 col{ Color::red };
    
    // If you are trying to output the name of the enum
    std::cout << "Name of col is " << ColorToString(col) << std::endl;
    
    // If you are trying to output the value of the enum
    std::cout << "Value of col is " << static_cast<unsigned int>(col) << std::endl;

}

Demo

binary '<<': no operator found which takes a right-hand operand of type 'c1'

2 Answers2