Formatter.md

Formatter Specialisation

The <utilities/formatter.h> header specialises std::formatter for any type that has a to_string() method.

C++20 introduced std::format to create strings with interpolated values; a core feature for the std::print standard output library.

Moreover, you can extend the standard library for your own types by specialising the std::formatter class.

Many classes already implement a to_string() method that returns a string representation of an instance. It seems like a good stopgap measure to connect all those types to std::format and its friends.

We have a concept that captures types with an accessible to_string(...) method that can be called without any arguments (of course, there can be lots of default arguments for the function):

template<typename T>
concept utilities::has_to_string = requires(const T& x) {
    { x.to_string() } -> std::convertible_to<std::string>;
};

We then specialised std::formatter to work with any class that has a to_string method.

Example

#include <utilities/formatter.h>
#include <print>

struct Whatever {
    std::string to_string() const { return "Whatever!"; }
};

int main()
{
    Whatever w;
    std::println("Struct with a `to_string()` method: '{}'", w);
    return 0;
}

Running that outputs:

Struct with a to_string() method: 'Whatever!'

Notes

1. The std::formatter specialisation only works with default formatting. So std::println("{}", w) is fine, but something like std::println("{:x}", w) will cause a program fault.
   Your to_string method may handle various output options, but we only ever call it with no passed arguments.

2. You can safely have a to_string(...) method and provide your own specialisation of std::formatter.
   The compiler will see your specialisation as a better fit than the blanket one provided in <utilities/formatter.h>.

See Also

std::format
std::formatter