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Showing content from https://en.cppreference.com/w/cpp/language/../algorithm/../utility/format/formatted_size.html below:

std::formatted_size - cppreference.com

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.

[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 }.

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 <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}; // "â" or {0342, 130, 128}
    constexpr auto aprox_equ{"\N{APPROXIMATELY EQUAL TO}"sv}; // "â" or {0342, 137, 133}
    constexpr int Ho{42}; // Hâ
 
    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 Hâ â *42* miles/sec/mpc."
Buffer: "Hubble's Hâ â *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 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]

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