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Difference between revisions of "cpp/utility/format/format to n"

From cppreference.com
< cpp‎ | utility‎ | format
(The standard SI unit should be used for the Hubble's constant.)
m (match code and output)
Line 78: Line 78:
 
     const auto result =
 
     const auto result =
 
         std::format_to_n(buffer, std::size(buffer),  
 
         std::format_to_n(buffer, std::size(buffer),  
−
                         "Hubble's H{0} {1} {2} miles/sec/mpc.",
+
                         "Hubble's H{0} {1} {2} km/sec/mpc.",
−
                         "\u2080", "\u2245", 42);
+
                         "\u2080", "\u2245", 71);
  
 
     std::cout << "Buffer: \"" << std::string_view{buffer, result.size} << "\"\n"
 
     std::cout << "Buffer: \"" << std::string_view{buffer, result.size} << "\"\n"

Revision as of 11:10, 17 December 2020

 
C++
 
Utilities library
General utilities
Relational operators (deprecated in C++20)
 
Formatting library
Standard format specification
Formatting functions
(C++20)
(C++20)
format_to_n
(C++20)
(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 OutputIt, class... Args>

std::format_to_n_result<OutputIt>
    format_to_n(OutputIt out, std::iter_difference_t<OutputIt> n,

                std::string_view fmt, const Args&... args);
 (1) (since C++20)
template<class OutputIt, class... Args>

std::format_to_n_result<OutputIt>
    format_to_n(OutputIt out, std::iter_difference_t<OutputIt> n,

                std::wstring_view fmt, const Args&... args);
 (2) (since C++20)
template<class OutputIt, class... Args>

std::format_to_n_result<OutputIt>
    format_to_n(OutputIt out, std::iter_difference_t<OutputIt> n,

                const std::locale& loc, std::string_view fmt, const Args&... args);
 (3) (since C++20)
template<class OutputIt, class... Args>

std::format_to_n_result<OutputIt>
    format_to_n(OutputIt out, std::iter_difference_t<OutputIt> n,

                const std::locale& loc, std::wstring_view fmt, const Args&... args);
 (4) (since C++20)
template<class OutputIt>

struct format_to_n_result {
    OutputIt out;
    std::iter_difference_t<OutputIt> size;

};
 (5) (since C++20)

Format args according to the format string fmt, and write the result to the output iterator out. At most n characters are written. If present, loc is used for locale-specific formatting.

Let CharT be decltype(fmt)::char_type (char for overloads (1,3), wchar_t for overloads (2,4)).

These overloads participate in overload resolution only if OutputIt satisfies the concept std::output_iterator<const CharT&>.

The behavior is undefined if OutputIt does not model (meet the semantic requirements of) the the concept std::output_iterator<const CharT&>, or if std::formatter<Ti, CharT> does not meet the Formatter requirements for any Ti in Args.

5) std::format_to_n_result has no base classes, or members other than out, size and implicitly declared special member functions.

Contents

Parameters

out - iterator to the output buffer
n - maximum number of characters to be written to the buffer
fmt - string view representing the format string.
[edit]
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

A format_to_n_result such that the out member is an iterator past the end of the output range, and the size member is the total (not truncated) output size.

Exceptions

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

Example

Run this code
#include <format>
#include <string_view>
#include <iostream>
 
int main()
{
    char buffer[64];
 
    const auto result =
        std::format_to_n(buffer, std::size(buffer), 
                         "Hubble's H{0} {1} {2} km/sec/mpc.",
                         "\u2080", "\u2245", 71);
 
    std::cout << "Buffer: \"" << std::string_view{buffer, result.size} << "\"\n"
              << "Buffer size = " << std::size(buffer) << '\n'
              << "Untruncated output size = " << result.size << '\n';
}

Output: 

Buffer: "Hubble's H₀ ≅ 71 km/sec/mpc."
Buffer size = 64
Untruncated output size = 35

See also

(C++20)
 stores formatted representation of the arguments in a new string
(function template) [edit]
(C++20)
 writes out formatted representation of its arguments through an output iterator
(function template) [edit]
(C++20)
 determines the number of characters necessary to store the formatted representation of its arguments
(function template) [edit]
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