Let ss be a std::stringstream. How to specify a concept for (ss<<some_type).str() is a std::string?

Question

Let ss be a std::stringstream. How to specify a concept for (ss << some_type).str() is a std::string?

Here is the code I have so far:

#include <iostream>
#include <sstream>

namespace detail {
    template< class T, class U >
    concept SameHelper = std::is_same_v<T, U>;
}

template< class T, class U >
concept same_as = detail::SameHelper<T, U> && detail::SameHelper<U, T>;

template<typename T>
concept ssToStr = requires(T a, std::stringstream ss) {
    { (ss << a).str() } -> same_as<std::string>;
};

void call(ssToStr auto obj)
{
    std::stringstream ss;
    std::cout << (ss << obj).str() << std::endl;

}

int main() 
{
    call("Hi");
    return 0;
}

You can check online that the code does not compile. The first part of the error messages reads:

<source>:25:5: error: no matching function for call to 'call'

    call("Hi");

    ^~~~

Answer 1

You're missing #include <sstream>

Fixing that gives the error message that contains:

<source>:14:17: note: because '(ss << a).str()' would be invalid: no member named 'str' in 'std::basic_ostream<char>'

    { (ss << a).str() } -> same_as<std::string>;

Which tells you that (ss << obj) evaluates to a std::ostream , which does not have a .str() member. All you have to check is that ss<<a compiles, and you already know that ss.str() results in a string, so you don't need that in your constraint.

Unfortunately, I don't know enough about constraints to create the working code for you.

Answer 2

Thanks to Mooing Duck I was able to get the code running. The running code reads:

#include <iostream>
#include <sstream>

template<typename T>
concept ssToStr = requires(T a, std::stringstream ss) {
    { ss << a };
};

void call(ssToStr auto obj)
{
    std::stringstream ss;
    ss << obj;
    std::cout << ss.str() << std::endl;
}

int main() 
{
    call("Hi");
    return 0;
}

You can run the code online