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

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< cpp‎ | utility‎ | format
m (fmt.)
 
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{{dcl begin}}
 
{{dcl begin}}
−
{{dcl header | format }}
+
{{dcl header|format}}
−
{{dcl | num=1 | since=c++20 |1=
+
{{dcl|num=1|since=c++20|1=
−
template<class... Args>
+
template< class... Args >
−
std::size_t formatted_size(std::string_view fmt, const Args&... args);
+
std::size_t formatted_size( std::format_string<Args...> fmt, Args&&... args );
 
}}
 
}}
−
{{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(std::wstring_view fmt, const Args&... args);
+
std::size_t formatted_size( std::wformat_string<Args...> fmt, Args&&... args );
 
}}
 
}}
−
{{dcl | num=3 | since=c++20 |1=
+
{{dcl|num=3|since=c++20|1=
−
template<class... Args>
+
template< class... Args >
−
std::size_t formatted_size(const std::locale& loc, std::string_view fmt, const Args&... args);
+
std::size_t formatted_size( const std::locale& loc,
 +
                            std::format_string<Args...> fmt, Args&&... args );
 
}}
 
}}
−
{{dcl | num=4 | since=c++20 |1=
+
{{dcl|num=4|since=c++20|1=
−
template<class... Args>
+
template< class... Args >
−
std::size_t formatted_size(const std::locale& loc, std::wstring_view fmt, const Args&... args);
+
std::size_t formatted_size( const std::locale& loc,
 +
                            std::wformat_string<Args...> fmt, Args&&... args );
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
  
−
Determine the total number of characters in the formatted string by formatting {{tt|args}} according to the format string {{tt|fmt}}. If present, {{tt|loc}} is used for locale-specific formatting.
+
Determine the total number of characters in the formatted string by formatting {{c|args}} according to the format string {{c|fmt}}. If present, {{c|loc}} is used for locale-specific formatting.
  
−
The behavior is undefined if {{c|std::formatter<Ti, CharT>}} does not meet the {{named req|Formatter}} requirements for each {{tt|Ti}} in {{tt|Args}}.
+
The behavior is undefined if {{c|std::formatter<std::remove_cvref_t<Ti>, CharT>}} does not meet the {{named req|BasicFormatter}} requirements for any {{tt|Ti}} in {{tt|Args}}.
  
 
===Parameters===
 
===Parameters===
 
{{par begin}}
 
{{par begin}}
−
{{par | fmt | string view representing the format string.
+
{{par|fmt|{{cpp/utility/format/format string|const}}}}
−
{{cpp/utility/format/format string}}
+
{{par|args...|arguments to be formatted}}
−
}}
+
{{par|loc|{{lc|std::locale}} used for locale-specific formatting}}
−
{{par | args... | arguments to be formatted}}
+
−
{{par | loc | {{lc|std::locale}} used for locale-specific formatting}}
+
 
{{par end}}
 
{{par end}}
  
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===Exceptions===
 
===Exceptions===
−
Throws {{lc|std::format_error}} if {{tt|fmt}} is not a valid format string for the provided arguments. Also propagates any exception thrown by formatter.
+
Propagates any exception thrown by formatter.
  
 
===Example===
 
===Example===
 
{{example
 
{{example
−
|code=
+
|code=
−
|output=
+
#include <format>
 +
#include <iomanip>
 +
#include <iostream>
 +
#include <string_view>
 +
#include <vector>
 +
 
 +
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{"\N{SUBSCRIPT ZERO}"sv}; // "₀" or {0342, 130, 128}
 +
    constexpr auto aprox_equ{"\N{APPROXIMATELY EQUAL TO}"sv}; // "≅" or {0342, 137, 133}
 +
    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. The 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::quoted(std::string_view{buffer.data(), min_buffer_size})
 +
              << '\n';
 +
 
 +
    // Print the buffer directly after adding the trailing '\0'.
 +
    buffer.push_back('\0');
 +
    std::cout << "Buffer: " << std::quoted(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===
 +
{{dr list begin}}
 +
{{dr list item|paper=P2216R3|std=C++20|before=throws {{lc|std::format_error}} for invalid format string|after=invalid format string results in compile-time error}}
 +
{{dr list item|paper=P2418R2|std=C++20|before=objects that are neither const-usable nor copyable<br>(such as generator-like objects) are not formattable|after=allow formatting these objects}}
 +
{{dr_p2508}}
 +
{{dr list end}}
  
 
===See also===
 
===See also===
 +
{{dsc begin}}
 +
{{dsc inc|cpp/utility/format/dsc format_to}}
 +
{{dsc inc|cpp/utility/format/dsc format_to_n}}
 +
{{dsc end}}
  
−
{{langlinks|ja|zh}}
+
{{langlinks|es|ja|ru|zh}}

Latest revision as of 13:08, 9 July 2024

 
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( 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-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

[edit] Return value

The total number of characters in the formatted string.

[edit] Exceptions

Propagates any exception thrown by formatter.

[edit] Example

Run this code
#include <format>
#include <iomanip>
#include <iostream>
#include <string_view>
#include <vector>
 
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{"\N{SUBSCRIPT ZERO}"sv}; // "₀" or {0342, 130, 128}
    constexpr auto aprox_equ{"\N{APPROXIMATELY EQUAL TO}"sv}; // "≅" or {0342, 137, 133}
    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. The 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::quoted(std::string_view{buffer.data(), min_buffer_size})
              << '\n';
 
    // Print the buffer directly after adding the trailing '\0'.
    buffer.push_back('\0');
    std::cout << "Buffer: " << std::quoted(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]
(C++20)
 writes out formatted representation of its arguments through an output iterator, not exceeding specified size
(function template) [edit]
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