Namespaces
Variants
Views
Actions

Difference between revisions of "cpp/utility/format/formatted size"

From cppreference.com
< cpp‎ | utility‎ | format
(P2216R3 std::format improvements)
m (~)
Line 10: Line 10:
 
{{dcl | num=2 | since=c++20 |1=
 
{{dcl | num=2 | since=c++20 |1=
 
template< class... Args >
 
template< class... Args >
−
std::size_t formatted_size( /*format_string<Args...>*/ fmt, const Args&... args );
+
std::size_t formatted_size( /*wformat_string<Args...>*/ fmt, const Args&... args );
 
}}
 
}}
 
{{dcl | num=3 | since=c++20 |1=
 
{{dcl | num=3 | since=c++20 |1=
Line 20: Line 20:
 
template< class... Args >
 
template< class... Args >
 
std::size_t formatted_size( const std::locale& loc,
 
std::size_t formatted_size( const std::locale& loc,
−
                             /*format_string<Args...>*/ fmt, const Args&... args );
+
                             /*wformat_string<Args...>*/ fmt, const Args&... args );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}

Revision as of 03:20, 18 June 2021

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Formatting library
Standard format specification
Formatting functions
(C++20)
(C++20)
(C++20)
formatted_size
(C++20)
(C++20)
(C++20)
Format strings
(C++20)(C++20)(C++20)
(C++26)
Formatting concepts
(C++23)
Formatter
(C++20)
(C++23)
(C++23)
(C++23)
(C++23)
(C++20)(C++20)(C++20)
(C++20)(C++20)(C++20)
(C++23)
(C++23)
Formatting arguments
(C++20)
(C++20)
(C++20)(C++20)(C++20)
(C++20) (deprecated in C++26)
(C++20)(C++20)
Format error
(C++20)
 
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

Parameters

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

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)
(since C++26)
  • 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

Return value

The total number of characters in the formatted string.

Exceptions

Throws std::format_error if fmt is not a valid format string for the provided arguments. Also propagates any exception thrown by formatter.

Example

Run this code
#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."

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

See also

(C++20)
 writes out formatted representation of its arguments through an output iterator
(function template) [edit]
(C++20)
 writes out formatted representation of its arguments through an output iterator, not exceeding specified size
(function template) [edit]
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/utility/format/formatted_size&oldid=130206"