Formatted Output

Introduction

C++20 introduced the formatting library std::format. That library makes it easy to create strings with interpolated values and has facilities for adding your types as values in those strings.

The <utilities/format.h> header augments that facility.

Many classes already implement a std::string to_string() const method that returns a string representation of an instance — we can use that to push values into std::format.

We have a concept that captures all types with an appropriate to_string() method.

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

We also supply a std::formatter that automatically connects any has_string_method class to the standard formatting library.

Example

#include <utilities/format.h>
#include <iostream>

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

int main()
{
    Whatever w;
    std::cout << std::format("Struct with a to_string() method: '{}'\n", w);
    return 0;
}

Output

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

Ranges workaround

C++23 will have facilities to allow std::format to work with ranges, which will make it easier to create formatted strings with interpolated values from arrays, vectors, lists, etc. If your compiler does not yet support this type of interpolation, <utilities/format.h> supplies a workaround.

Example

#include <utilities/format.h>
#include <iostream>
#include <vector>
int main()
{
1#ifdef __cpp_lib_format_ranges
    std::cout << "The compiler natively supports formatting `ranges`!\n";
#else
    std::cout << "I will format `ranges` using the `<utilities>` library!\n";
#endif

    std::vector v = {1.123123, 2.1235, 3.555555};
    std::cout << std::format("Unformatted vector: {}\n", v);
    std::cout << std::format("Formatted vector:   {::3.2f}\n", v);
}

1

__cpp_lib_format_ranges is a standard preprocessor flag indicating whether your compiler can format ranges.

Output

I will format `ranges` using the `<utilities>` library!
Unformatted vector: [1.123123, 2.1235, 3.555555]
Formatted vector:   [1.12, 2.12, 3.56]

See Also

print.h
std::format
std::formatter