Understanding the convertible_to concept in c++20

Question

I'm still new to the C++20 concepts and I want to know why this does not work. I want to create a function template that concatenates numbers as strings. So I wanted to try concepts. I used std::convertible_to to check whether the entered datatype (which is int in this case) can be converted to std::string . But I'm facing an error that I don't understand.

//building the concept
template <typename T>
concept ConvertibleToStdString = std::convertible_to<T,std::string>;

//using the concept
template <ConvertibleToStdString T>
std::string concatenate(T a, T b){
    return std::to_string(a) + std::to_string(b);
}

int main(){

    int x{623};
    int y{73};

    auto result = concatenate(x,y);
    std::cout << result << std::endl;
    
    return 0;
}

Error:

main.cpp:21:34: error: use of function 'std::string concatenate(T, T) [with T = int; std::string = std::basic_string<char>]' with unsatisfied constraints
   21 |     auto result = concatenate(x,y);

What am I doing wrong?

Answer 1

You appear to want a concept for types that can be passed to std::to_string() .

This code will achieve that.

template <typename T>
concept ConvertibleToStdString = requires(T a){ std::to_string(a); };

What am I doing wrong?

You are misunderstanding the meaning ofstd::convertible_to<T,std::string> .

That concept validates (among other things) that T can implicitly convert to a std::string , as in:

std::string s;
s = 623;   // This will NOT compile.  int is not convertible_to std::string