The text formatting library offers a safe and extensible alternative to the printf family of functions. It is intended to complement the existing C++ I/O streams library.
Defined in header <format> |
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(C++20) | stores formatted representation of the arguments in a new string (function template) |
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(C++20) | writes out formatted representation of its arguments through an output iterator (function template) |
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(C++20) | writes out formatted representation of its arguments through an output iterator, not exceeding specified size (function template) |
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(C++20) | determines the number of characters necessary to store the formatted representation of its arguments (function template) |
Defined in header <format> |
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(C++20) | non-template variant of std::format using type-erased argument representation (function) |
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(C++20) | non-template variant of std::format_to using type-erased argument representation (function template) |
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(C++20)(C++20) | creates a type-erased object referencing all formatting arguments, convertible to format_args (function template) |
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(C++20) | argument visitation interface for user-defined formatters (function template) |
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(C++20) | class template that defines formatting rules for a given type (class template) |
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(C++20) | class template that provides access to a formatting argument for user-defined formatters (class template) |
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(C++20)(C++20)(C++20) | class that provides access to all formatting arguments (class template) |
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(C++20)(C++20)(C++20) | class template that performs compile-time format string checks at construction time (class template) |
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(C++20)(C++20)(C++20) | formatting state, including all formatting arguments and the output iterator (class template) |
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(C++20)(C++20)(C++20) | formatting string parser state (class template) |
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(C++20) | exception type thrown on formatting errors (class) |
| Feature-test macro | Value | Std | Comment |
|---|---|---|---|
__cpp_lib_format | 201907L | (C++20) | Text formatting |
| 202106L |
(C++20) (DR) | Compile-time format string checks; Reducing parameterization of std::vformat_to |
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| 202110L |
(C++20) (DR) | Fixing locale handling in chrono formatters; Supporting non-const-formattable types |
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| 202207L | (C++23) | Exposing std::basic_format_string;Clarify handling of encodings in localized formatting of chrono types |
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__cpp_lib_format_ranges | 202207L | (C++23) | Formatting ranges |
We intentionally treat the addition of std::basic_format_string (P2508) as a defect report because all known implementations make these components available in C++20 mode, although it is not so categorized officially.
#include <cassert>
#include <format>
int main()
{
std::string message = std::format("The answer is {}.", 42);
assert(message == "The answer is 42.");
}The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| P2418R2 | C++20 | objects that are neither const-formattable nor copyable (such as generator-like objects) are not formattable | allow formatting these objects (relaxed formatter requirements) |
| P2508R1 | C++20 | there's no user-visible name for this facility | the name basic_format_string is exposed |
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(C++23) | prints to stdout or a file stream using formatted representation of the arguments (function template) |
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(C++23) | same as std::print except that each print is terminated by additional new line (function template) |
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(C++23) | outputs formatted representation of the arguments (function template) |
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