std::basic_format_string, std::format_string, std::wformat_string
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Formatting library (C++20)
Formatting library
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
(C++20)
basic_format_stringformat_stringwformat_string
(C++20)(C++20)(C++20)
(C++26)
(C++23)
Formatter
(C++20)
(C++23)
(C++23)
(C++23)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
(C++23)
(C++23)
Formatting arguments
(C++20)
(C++20)
(C++20)(C++20)(C++20)
(C++20) (deprecated in C++26)
(C++20)(C++20)
(C++20)
Defined in header
<format>
template< class CharT, class... Args >
struct basic_format_string;
(1)
(since C++20)
struct basic_format_string;
template< class... Args >
(2)
(since C++20)
using format_string =
template< class... Args >
(3)
(since C++20)
using wformat_string =
Class template std::basic_format_string wraps a std::basic_string_view that will be used by formatting functions.
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).
Contents
[edit] Member functions
(constructor)
basic_format_string, raising compile error if the argument is not a format string (public member function)
get
(public member function)
std::basic_format_string::basic_format_string
template< class T >
consteval basic_format_string( const T& s );
(1)
consteval basic_format_string( const T& s );
basic_format_string( /* runtime-format-string */<CharT> s ) noexcept;
(2)
(since C++26)
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
s
-
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 }.
- 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.
- 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.
(since C++26)
- For other formattable types, the format specification is determined by user-defined
formatterspecializations.
std::basic_format_string::get
constexpr std::basic_string_view <CharT> get() const noexcept;
Returns the stored string view.
[edit] 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.
[edit] Example
This section is incomplete
Reason: no example
Reason: no example
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| P2508R1 | C++20 | there's no user-visible name for this facility | the name basic_format_string is exposed
|