std::basic_...ormat_string(3)

🌸 NAME 🌸

okay bestie, welcome to the std::basic_...ormat_string(3) page from the C++ Standard Libary manual — yes, that's "Libary" with one r in the header, we're keeping it, iykyk 💅.

• std::basic_format_string, std::format_string, std::wformat_string — that's the lineup. The description line is literally std::basic_format_string, std::format_string, std::wformat_string again, because the page says it twice and we respect the repetition 🎀.

✨ Synopsis ✨

Defined in header <format>.

template< class CharT, class... Args >                           (1)
(since C++20)
struct basic_format_string;
template< class... Args >

using format_string =                                            (2)
(since C++20)

    basic_format_string<char, std::type_identity_t<Args>...>;
template< class... Args >

using wformat_string =                                           (3)
(since C++20)

    basic_format_string<wchar_t, std::type_identity_t<Args>...>;

Class template std::basic_format_string wraps a std::basic_string_view that will be used by formatting functions. That's the whole bit: it's a wrapper around a string view, and formatting functions get to use it 🦋.

The constructor of std::basic_format_string performs compile-time format string checks unless the constructor argument is returned by std::runtime_format (since C++26). So compile-time checking is the default vibe; std::runtime_format is the escape hatch ✨.

💅 Member functions 💅

• (constructor) — constructs a basic_format_string, raising compile error if the constructor argument is not a format string (public member function).
• get — returns the wrapped string (public member function).

🫶 std::basic_format_string::basic_format_string 🫶

template< class T >
(1)
consteval basic_format_string( const T& s );
basic_format_string( /* runtime-format-string */<CharT> s )
(2) (since C++26)
noexcept;

1. Constructs a basic_format_string object that stores a view of string s. If the argument is not a compile-time constant, or if it cannot be parsed as a format string for the formatting argument types Args, the construction is ill-formed. This overload participates in overload resolution only if const T& models std::convertible_to<std::basic_string_view<CharT>>.
2. Constructs a basic_format_string object that stores a view of string s as returned by std::runtime_format. It does not perform format string checks upon construction.

💖 Parameters 💖

The parameter is an object that represents the format string. The format string consists of:

• ordinary characters (except { and }), which are copied unchanged to the output,
• escape sequences {{ and }}, which are replaced with { and } respectively in the output, and
• replacement fields.

Each replacement field has the following format:

{ arg-id (optional) }               (1)
{ arg-id (optional) : format-spec } (2)

1. replacement field without a format specification
2. replacement field with a format specification

• arg-id — specifies the index of the argument in args whose value is to be used for formatting; if it is omitted, the arguments are used in order. The arg-id s in a format string must all be present or all be omitted. Mixing manual and automatic indexing is an error.
• format-spec — the format specification defined by the std::formatter specialization for the corresponding argument. Cannot start with }.

Then the format spec gets interpreted depending on the type, like this:

• For basic types and standard string types, the format specification is interpreted as standard format specification.
• For chrono types, the format specification is interpreted as chrono format specification.
• For range types, the format specification is interpreted as range format specification.
• For std::pair and std::tuple, the format specification is interpreted as tuple format specification. (since C++23)
• For std::thread::id and std::stacktrace_entry, see thread id format specification and stacktrace entry format specification.
• For std::basic_stacktrace, no format specifier is allowed.
• For std::filesystem::path, see path format specification. (since C++26)
• For other formattable types, the format specification is determined by user-defined formatter specializations.

⭐ std::basic_format_string::get ⭐

constexpr std::basic_string_view<CharT> get() const noexcept;

Returns the stored string view. That's it. That's the function. Slay. 💅

🦋 Notes 🦋

The alias templates format_string and wformat_string use std::type_identity_t to inhibit template argument deduction. Typically, when they appear as a function parameter, their template arguments are deduced from other function arguments.

template<class... Args>
std::string format(std::format_string<Args...> fmt, Args&&... args);

auto s = format("{} {}", 1.0, 2);
// Calls format<double, int>. Args are deduced from 1.0, 2
// Due to the use of type_identity_t in format_string, template argument deduction
// does not consider the type of the format string.

So yeah: Args come from 1.0, 2, not from the format string, because type_identity_t is doing its little anti-deduction dance 🧚.

🎀 Example 🎀

This section is incomplete Reason: no example

🔥 Defect reports 🔥

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• P2508R1 — Applied to: C++20 — Behavior as published: there's no user-visible name for this facility — Correct behavior: the name basic_format_string is exposed.

🍓 Category 🍓

• Todo no example

---

okay that's the whole page, bestie! Big love to http://cppreference.com, version 2024.06.10, and the std::basic_...ormat_string(3) page — mwah 💋✨