std::to_chars(3)

✨ std::to_chars — C++ Standard Libary 💅

🌸 NAME 🌸

std::to_chars - std::to_chars

🌟 Synopsis 🌟

          Defined in header <charconv>
          std::to_chars_result
                                                                       (since
       C++17)
              to_chars( char* first, char* last,                   (1)
       (constexpr since C++23)

                        /* integer-type */ value, int base = 10 );
          std::to_chars_result                                     (2) (since
       C++17)
              to_chars( char*, char*, bool, int = 10 ) = delete;
          std::to_chars_result
              to_chars( char* first, char* last, /*                (3) (since
       C++17)
          floating-point-type */ value );
          std::to_chars_result

              to_chars( char* first, char* last, /*                (4) (since
       C++17)
          floating-point-type */ value,

                        std::chars_format fmt );
          std::to_chars_result

              to_chars( char* first, char* last, /*                (5) (since
       C++17)
          floating-point-type */ value,

                        std::chars_format fmt, int precision );

💖 DESCRIPTION 💖

Converts value into a character string by successively filling the range [first, last), where [first, last) is required to be a valid range.

1. Integer formatters: value is converted to a string of digits in the given base (with no redundant leading zeroes). Digits in the range 10..35 (inclusive) are represented as lowercase characters a..z. If value is less than zero, the representation starts with a minus sign. The library provides overloads for all cv-unqualified (since C++23) signed and unsigned integer types and for the type char as the type of the parameter value.
2. Overload for bool is deleted. std::to_chars rejects argument of type bool because the result would be "0"/"1" but not "false"/"true" if it is permitted.
3. value is converted to a string as if by std::printf in the default ("C") locale. The conversion specifier is f or e (resolving in favor of f in case of a tie), chosen according to the requirement for a shortest representation: the string representation consists of the smallest number of characters such that there is at least one digit before the radix point (if present) and parsing the representation using the corresponding std::from_chars function recovers value exactly. If there are several such representations, one with the smallest difference to value is chosen, resolving any remaining ties using rounding according to std::round_to_nearest. The library provides overloads for all cv-unqualified standard (until C++23) floating-point types as the type of the parameter value.
4. Same as (3), but the conversion specified for the as-if printf is f if fmt is std::chars_format::fixed, e if fmt is std::chars_format::scientific, a (but without leading "0x" in the result) if fmt is std::chars_format::hex, and g if fmt is chars_format::general. The library provides overloads for all cv-unqualified standard (until C++23) floating-point types as the type of the parameter value.
5. Same as (4), except the precision is specified by the parameter precision rather than by the shortest representation requirement. The library provides overloads for all cv-unqualified standard (until C++23) floating-point types as the type of the parameter value.

🌈 Parameters 🌈

• first, last — character range to write to
• value — the value to convert to its string representation
• base — integer base to use: a value between 2 and 36 (inclusive).
• fmt — floating-point formatting to use, a bitmask of type std::chars_format
• precision — floating-point precision to use

🎀 Return value 🎀

On success, returns a value of type std::to_chars_result such that ec equals value-initialized std::errc and ptr is the one-past-the-end pointer of the characters written. Note that the string is not NUL-terminated.

On error, returns a value of type std::to_chars_result holding std::errc::value_too_large in ec, a copy of the value last in ptr, and leaves the contents of the range [first, last) in unspecified state.

💀 Exceptions 💀

Throws nothing.

🧚 Notes 🧚

Unlike other formatting functions in C++ and C libraries, std::to_chars is locale-independent, non-allocating, and non-throwing. Only a small subset of formatting policies used by other libraries (such as std::sprintf) is provided. This is intended to allow the fastest possible implementation that is useful in common high-throughput contexts such as text-based interchange (JSON or XML).

The guarantee that std::from_chars can recover every floating-point value formatted by std::to_chars exactly is only provided if both functions are from the same implementation.

It is required to explicitly cast a bool value to another integer type if it is wanted to format the value as "0"/"1".

🌸 Feature-test macro 🌸

• __cpp_lib_to_chars — 201611L (C++17) — Elementary string conversions (std::to_chars, std::from_chars)
• __cpp_lib_to_chars — 202306L (C++26) — Testing for success or failure of <charconv> functions
• __cpp_lib_constexpr_charconv — 202207L (C++23) — Add constexpr modifiers to std::to_chars and std::from_chars overloads (1) for integral types

🌟 Example 🌟

// Run this code

        #include <array>
        #include <charconv>
        #include <iostream>
        #include <string_view>
        #include <system_error>

        void show_to_chars(auto... format_args)
        {
            std::array<char, 10> str;

        #if __cpp_lib_to_chars >= 202306L
            // use C++26 operator bool() for error checking
            if (auto res = std::to_chars(str.data(), str.data() + str.size(),
       format_args...))
                std::cout << std::string_view(str.data(), res.ptr) << '\n';
            else
                std::cout << std::make_error_code(res.ec).message() << '\n';
        #else
            if (auto [ptr, ec]
                    = std::to_chars(str.data(), str.data() + str.size(),
       format_args...);
                ec == std::errc())
                std::cout << std::string_view(str.data(), ptr) << '\n';
            else
                std::cout << std::make_error_code(ec).message() << '\n';
        #endif
        }

        int main()
        {
            show_to_chars(42);
            show_to_chars(+3.14159F);
            show_to_chars(-3.14159, std::chars_format::fixed);
            show_to_chars(-3.14159, std::chars_format::scientific, 3);
            show_to_chars(3.1415926535, std::chars_format::fixed, 10);
        }

Possible output:

42
3.14159
-3.14159
-3.142e+00
Value too large for defined data type

🛠️ Defect reports 🛠️

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

• LWG 2955 (C++17) — behavior as published: this function was in <utility> and used std::error_code; correct behavior: moved to <charconv> and uses std::errc
• LWG 3266 (C++17) — behavior as published: bool argument was accepted and promoted to int; correct behavior: rejected by a deleted overload
• LWG 3373 (C++17) — behavior as published: std::to_chars_result might have additional members; correct behavior: additional members are disallowed

👀 See also 👀

• to_chars_result — the return type of std::to_chars (C++17) (class)
• from_chars — converts a character sequence to an integer or floating-point value (C++17) (function)
• to_string — converts an integral or floating-point value to string (C++11) (function)
• printf, fprintf, sprintf, snprintf — prints formatted output to stdout, a file stream or a buffer (function); snprintf is tagged (C++11)
• operator<< — inserts formatted data (public member function of std::basic_ostream<CharT,Traits>)

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💖 that's the whole page, bestie! source: http://cppreference.com, dated 2024.06.10, page std::to_chars(3) — now go format something iconic 🎀