std::runtime_format(3)

🌸 NAME 🌸

std::runtime_format — literally std::runtime_format, bestie. (Page title: std::runtime_format(3).) This is from the C++ Standard Libary manual — yes, that’s the spelling up top, and we’re keeping it 💅.

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✨ Synopsis ✨

Defined in header <format>:

/*runtime-format-string*/<char> runtime_format( std::string_view fmt ) noexcept;   (1) (since C++26)
/*runtime-format-string*/<wchar_t> runtime_format( std::wstring_view fmt ) noexcept;   (2) (since C++26)

Okay so what it gives you back is an object that stores a runtime format string directly usable in user-oriented formatting functions, and it can be implicitly converted to std::basic_format_string. It’s giving “format string but make it runtime.” ✨

🍓 Parameters 🍓

• fmt — a string view. That’s the whole parameter, bestie 🫶

💖 Return value 💖

An object holding the runtime format string of the exposition-only type:

🧚 Class template runtime-format-string <CharT> 🧚

template< class CharT >            (exposition only*)
struct /*runtime-format-string*/;

🎀 Member objects 🎀

The returned object contains an exposition-only non-static data member str of type std::basic_string_view<CharT>. lowkey important 🎀

🦋 Constructors and assignments 🦋

/*runtime-format-string*/( std::basic_string_view<CharT> s ) noexcept;          (1)
/*runtime-format-string*/( const /*runtime-format-string*/& ) = delete;         (2)
/*runtime-format-string*/& operator=( const /*runtime-format-string*/& ) = delete;      (3)

1. Initializes str with s.
2. Copy constructor is explicitly deleted. The type is neither copyable nor movable.
3. The assignment is explicitly deleted.

💀 Notes 💀

Okay so here’s the chaotic part: since the return type of runtime_format is neither copyable nor movable, an attempt of passing runtime_fmt as a glvalue inhibits the construction of std::basic_format_string, which results in program ill-formed 😭. To construct std::basic_format_string with runtime_format, the returned value of runtime_format is passed directly on std::basic_format_string as a prvalue where copy elision is guaranteed. No cap, that’s the rule.

auto runtime_fmt = std::runtime_format("{}");

auto s0 = std::format(runtime_fmt, 1); // error
auto s1 = std::format(std::move(runtime_fmt), 1); // still error
auto s2 = std::format(std::runtime_format("{}"), 1); // ok

🔥 Feature-test macro 🔥

• __cpp_lib_format — value 202311L, std (C++26), feature: Runtime format strings.

🌈 Example 🌈

// Run this code

#include <format>
#include <print>
#include <string>
#include <string_view>

int main()
{
    std::print("Hello {}!\n", "world");

    std::string fmt;
    for (int i{}; i != 3; ++i)
    {
        fmt += "{} "; // constructs the formatting string
        std::print("{} : ", fmt);
        std::println(std::runtime_format(fmt), "alpha", 'Z', 3.14, "unused");
    }
}

🌟 Output 🌟

Hello world!
{}  : alpha
{} {}  : alpha Z
{} {} {}  : alpha Z 3.14

👑 See also 👑

• format — stores formatted representation of the arguments in a new string (C++20) (function template).
• vformat — non-template variant of std::format using type-erased argument representation (C++20) (function).
• basic_format_string, format_string, wformat_string — class template that performs compile-time format string checks at construction time (C++20) (class template).

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💖 sourced from http://cppreference.com · version/date 2024.06.10 · page std::runtime_format(3) · k bye bestie 🫶