< cpp‎ | utility‎ | format
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

Formatting library
Standard format specification
Formatting functions
Format strings
Formatting concepts
Formatting arguments
(C++20) (deprecated in C++26)
Format error
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)

Returns an object that stores a runtime format string directly usable in user-oriented formatting functions and can be implicitly converted to std::basic_format_string.

1,2) Equivalent to return fmt;.


[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 }.

(since C++23)
  • For other formattable types, the format specification is determined by user-defined formatter specializations.

[edit] Return value

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

Class template runtime-format-string <CharT>

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

Member objects

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

Constructors and assignments

/*runtime-format-string*/( std::basic_string_view<CharT> s ) noexcept;
/*runtime-format-string*/( const /*runtime-format-string*/& ) = delete;
/*runtime-format-string*/& operator=( const /*runtime-format-string*/& ) = delete;
1) Initializes str with s.
2) Copy constructor is explicitly deleted. The type is neither copyable nor movable.
3) The assignment is explicitly deleted.

[edit] Notes

Since the return type of runtime_format is neither copyable nor movable, an attempt of passing runtime_fmt as 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 prvalue where copy elision is guaranteed.

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 Value Std Feature
__cpp_lib_format 202311L (C++26) Runtime format strings

[edit] Example

#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");


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

[edit] See also

stores formatted representation of the arguments in a new string
(function template) [edit]
non-template variant of std::format using type-erased argument representation
(function) [edit]
class template that performs compile-time format string checks at construction time
(class template) [edit]