Namespaces
Variants
Views
Actions

std::formatted_size

From cppreference.com
< cpp‎ | utility‎ | format
 
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
Integer comparison functions
(C++20)(C++20)(C++20)   
(C++20)
Swap and type operations
(C++14)
(C++11)

(C++11)
(C++11)
(C++17)
Common vocabulary types
(C++11)
(C++17)
(C++17)
(C++17)
(C++11)
(C++17)

Elementary string conversions
(C++17)
(C++17)

Stacktrace
 
Formatting library
Formatting functions
(C++20)
(C++20)
formatted_size
(C++20)
(C++20)
(C++20)
Formatter
(C++20)
Formatting arguments
Format error
 
Defined in header <format>
template< class... Args >
std::size_t formatted_size( /*format_string<Args...>*/ fmt, const Args&... args );
(1) (since C++20)
template< class... Args >
std::size_t formatted_size( /*wformat_string<Args...>*/ fmt, const Args&... args );
(2) (since C++20)
template< class... Args >

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

                            /*format_string<Args...>*/ fmt, const Args&... args );
(3) (since C++20)
template< class... Args >

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

                            /*wformat_string<Args...>*/ fmt, const 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<Ti, CharT> does not meet the Formatter requirements for any Ti in Args.

Contents

[edit] Parameters

fmt - parameter of unspecified type, whose initialization is valid only if the argument is convertible to std::string_view (for (1,3)) or std::wstring_view (for (2,4)), and the result of conversion is a constant expression and a valid format string for Args. 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:

  • introductory { character;
  • (optional) arg-id, a non-negative number;
  • (optional) a colon (:) followed by a format specification;
  • final } character.

arg-id specifies the index of the argument in args whose value is to be used for formatting; if arg-id 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.

The format specification is 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

[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]