# std::formatted_size

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< cpp‎ | utility‎ | format

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`.

### 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-id﻿s 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 range types, the format specification is interpreted as range format specification.
- For std::pair and std::tuple, the format specification is interpreted as tuple format specification.
- For std::thread::id and `std::stacktrace_entry`, see thread id format specification and stacktrace entry format specification.
- For `std::basic_stacktrace`, no format specifier is allowed.

(since C++23)

- For `std::filesystem::path`, see path format specification.

(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

Propagates any exception thrown by formatter.

### 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."

### 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

### See also

format_to (C++20) | writes out formatted representation of its arguments through an output iterator (function template)
format_to_n (C++20) | writes out formatted representation of its arguments through an output iterator, not exceeding specified size (function template)

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