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std::formatted_size

From cppreference.com
< cpp‎ | utility‎ | format
 
 
Utilities library
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Formatting library
Formatting functions
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formatted_size
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Formatter
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Formatting arguments
Format error
 
Defined in header <format>
template< class... Args >
std::size_t formatted_size( std::format_string<Args...> fmt, Args&&... args );
(1) (since C++20)
template< class... Args >
std::size_t formatted_size( std::wformat_string<Args...> fmt, Args&&... args );
(2) (since C++20)
template< class... Args >

std::size_t formatted_size( const std::locale& loc,

                            std::format_string<Args...> fmt, Args&&... args );
(3) (since C++20)
template< class... Args >

std::size_t formatted_size( const std::locale& loc,

                            std::wformat_string<Args...> fmt, Args&&... args );
(4) (since C++20)

Determine the total number of characters in the formatted string by formatting args according to the format string fmt. If present, loc is used for locale-specific formatting.

The behavior is undefined if std::formatter<std::remove_cvref_t<Ti>, CharT> does not meet the BasicFormatter requirements for any Ti in Args.

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-ids 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.
  • 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 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

The total number of characters in the formatted string.

[edit] Exceptions

Propagates any exception thrown by formatter.

[edit] Example

#include <format>
#include <iostream>
#include <vector>
#include <string_view>
 
int main()
{
    using namespace std::literals::string_view_literals;
 
    constexpr auto fmt_str { "Hubble's H{0} {1} {2:*^4} miles/sec/mpc."sv };
    constexpr auto sub_zero { "₀"sv };  // { "\u2080"sv } => { 0xe2, 0x82, 0x80 };
    constexpr auto aprox_equ { "≅"sv }; // { "\u2245"sv } => { 0xe2, 0x89, 0x85 };
    constexpr int Ho { 42 }; // H₀
 
 
    const auto min_buffer_size = std::formatted_size(fmt_str, sub_zero, aprox_equ, Ho);
 
    std::cout << "Min buffer size = " << min_buffer_size << '\n';
 
    // Use std::vector as dynamic buffer. Note: buffer does not include the trailing '\0'.
    std::vector<char> buffer(min_buffer_size); 
 
    std::format_to_n(buffer.data(), buffer.size(), fmt_str, sub_zero, aprox_equ, Ho);
 
    std::cout << "Buffer: \"" << std::string_view{buffer.data(), min_buffer_size} << "\"\n";
 
    // Or we can print the buffer directly by adding the trailing '\0'.
    buffer.push_back('\0');
    std::cout << "Buffer: \"" << buffer.data() << "\"\n";
}

Output:

Min buffer size = 37
Buffer: "Hubble's H₀ ≅ *42* miles/sec/mpc."
Buffer: "Hubble's H₀ ≅ *42* miles/sec/mpc."

[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

(C++20)
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]