std::format - cppreference.com (original) (raw)
| Defined in header | ||
|---|---|---|
| template< class... Args > std::string format( std::format_string<Args...> fmt, Args&&... args ); | (1) | (since C++20) |
| template< class... Args > std::wstring format( std::wformat_string<Args...> fmt, Args&&... args ); | (2) | (since C++20) |
| template< class... Args > std::string format( const std::locale& loc, std::format_string<Args...> fmt, Args&&... args ); | (3) | (since C++20) |
| template< class... Args > std::wstring format( const std::locale& loc, std::wformat_string<Args...> fmt, Args&&... args ); | (4) | (since C++20) |
Format args according to the format string fmt, and return the result as a string. If present, loc is used for locale-specific formatting.
The format string fmt is checked at compile time unless it is initialized from the result of std::runtime_format(since C++26). If, at compile time, the format string is found to be invalid for the types of the arguments to be formatted, a compilation error will be emitted.
The following requirements apply to each type T in Args, where CharT is char for overloads (1,3), wchar_t for overloads (2,4):
- std::formatter<T, CharT> must satisfy BasicFormatter
- std::formatter<T, CharT>::parse() must be constexpr for the purpose of compile-time format string check.
Contents
[edit] Parameters
| fmt | - | 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++23) For other formattable types, the format specification is determined by user-defined formatter specializations. |
|---|---|---|
| args... | - | arguments to be formatted |
| loc | - | std::locale used for locale-specific formatting |
[edit] Return value
A string object holding the formatted result.
[edit] Exceptions
Throws std::bad_alloc on allocation failure. Also propagates exception thrown by any formatter.
[edit] Notes
It is not an error to provide more arguments than the format string requires:
std::format("{} {}!", "Hello", "world", "something"); // OK, produces "Hello world!"
It is an error if the format string is not a constant expression unless it is initialized from the result of std::runtime_format(since C++26). std::vformat does not have this requirement.
[edit] Example
#include
#include
#include
#include
#include
template<typename... Args>
std::string dyna_print(std::string_view rt_fmt_str, Args&&... args)
{
return std::vformat(rt_fmt_str, std::make_format_args(args...));
}
int main()
{
#ifdef __cpp_lib_format_ranges
const std::set<std::string_view> continents
{
"Africa", "America", "Antarctica",
"Asia", "Australia", "Europe"
};
std::cout << std::format("Hello {}!\n", continents);
#else
std::cout << std::format("Hello {}!\n", "continents");
#endif
std::string fmt;
for (int i{}; i != 3; ++i)
{
fmt += "{} "; // constructs the formatting string
std::cout << fmt << " : ";
std::cout << dyna_print(fmt, "alpha", 'Z', 3.14, "unused");
std::cout << '\n';
}
}
Possible output:
Hello {"Africa", "America", "Antarctica", "Asia", "Australia", "Europe"}! {} : alpha {} {} : alpha Z {} {} {} : alpha Z 3.14
[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 |
|---|---|---|---|
| P2216R3 | C++20 | throws std::format_error for invalid format string | invalid format string results in compile-time error |
| P2418R2 | C++20 | objects that are neither const-usable nor copyable(such as generator-like objects) are not formattable | allow formatting these objects |
| P2508R1 | C++20 | there's no user-visible name for this facility | the name basic_format_string is exposed |
[edit] See also
| | writes out formatted representation of its arguments through an output iterator (function template) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | writes out formatted representation of its arguments through an output iterator, not exceeding specified size (function template) [edit] |