# std::format

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< cpp‎ | utility‎ | format

Defined in header `<format>` |  |
template< class... Args > std::string format( std::format_string <Args...> fmt, Args&&... args ); | (1) | (since C++20)
template< class... Args > std::wstring format( std::wformat_string <Args...> fmt, Args&&... args ); | (2) | (since C++20)
template< class... Args > std::string format( const std::locale & loc, std::format_string <Args...> fmt, Args&&... args ); | (3) | (since C++20)
template< class... Args > std::wstring format( const std::locale & loc, std::wformat_string <Args...> fmt, Args&&... args ); | (4) | (since C++20)

Format args according to the format string fmt, and return the result as a string. If present, loc is used for locale-specific formatting.

1) Equivalent to return std::vformat (fmt.get(), std::make_format_args (args...));.
2) Equivalent to return std::vformat (fmt.get(), std::make_wformat_args (args...));.
3) Equivalent to return std::vformat (loc, fmt.get(), std::make_format_args (args...));.
4) Equivalent to return std::vformat (loc, fmt.get(), std::make_wformat_args (args...));.

Since P2216R3, `std::format` does a compile-time check on the format string (via the helper type std::format_string or std::wformat_string). If it is found to be invalid for the types of the arguments to be formatted, a compilation error will be emitted. If the format string cannot be a compile-time constant, or the compile-time check needs to be avoided, use std::vformat or `std::runtime_format` on fmt(since C++26) instead.

The following requirements apply to each type `T` in `Args`, where `CharT` is char for overloads (1,3), wchar_t for overloads (2,4):

- std::formatter <T, CharT> must satisfy BasicFormatter
- std::formatter <T, CharT>::parse() must be constexpr since P2216R3 (std::vformat does not have this requirement)

### 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

A string object holding the formatted result.

### Exceptions

Throws std::bad_alloc on allocation failure. Also propagates exception thrown by any formatter.

### Notes

It is not an error to provide more arguments than the format string requires:

    std::format("{} {}!", "Hello", "world", "something"); // OK, produces "Hello world!"

As of P2216R3, it is an error if the format string is not a constant expression. std::vformat can be used in this case.

    std::string f(std::string_view runtime_format_string)
    {
        // return std::format(runtime_format_string, "foo", "bar"); // error
        return std::vformat(runtime_format_string, std::make_format_args("foo", "bar")); // OK
    }

std::runtime_format can be used directly on `std::format` instead of std::vformat which requires std::basic_format_args as an argument.

    std::string f(std::string_view runtime_format_string)
    {
        return std::format(std::runtime_format(runtime_format_string), "foo", "bar");
    }

(since C++26)

### Example

Run this code

    #include <format>
    #include <iostream>
    #include <string>
    #include <string_view>

    template<typename... Args>
    std::string dyna_print(std::string_view rt_fmt_str, Args&&... args)
    {
        return std::vformat(rt_fmt_str, std::make_format_args(args...));
    }

    int main()
    {
        std::cout << std::format("Hello {}!\n", "world");

        std::string fmt;
        for (int i{}; i != 3; ++i)
        {
            fmt += "{} "; // constructs the formatting string
            std::cout << fmt << " : ";
            std::cout << dyna_print(fmt, "alpha", 'Z', 3.14, "unused");
            std::cout << '\n';
        }
    }

Output:

    Hello world!
    {}  : alpha
    {} {}  : alpha Z
    {} {} {}  : alpha Z 3.14

### 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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