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std::to_chars

From cppreference.com
< cpp‎ | utility
 
 
Utilities library
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(C++20)(C++20)(C++20)   
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(C++14)
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(C++11)
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(C++23)
Elementary string conversions
to_chars
(C++17)
(C++17)
 
Defined in header <charconv>
std::to_chars_result

    to_chars( char* first, char* last,

              /* integer-type */ value, int base = 10 );
(1) (since C++17)
(constexpr since C++23)
std::to_chars_result
    to_chars( char*, char*, bool, int = 10 ) = delete;
(2) (since C++17)
(3)
std::to_chars_result

    to_chars( char* first, char* last, float value );
std::to_chars_result
    to_chars( char* first, char* last, double value );
std::to_chars_result

    to_chars( char* first, char* last, long double value );
(since C++17)
(until C++23)
std::to_chars_result
    to_chars( char* first, char* last, /* floating-point-type */ value );
(since C++23)
(4)
std::to_chars_result

    to_chars( char* first, char* last, float value,
              std::chars_format fmt );
std::to_chars_result
    to_chars( char* first, char* last, double value,
              std::chars_format fmt );
std::to_chars_result
    to_chars( char* first, char* last, long double value,

              std::chars_format fmt );
(since C++17)
(until C++23)
std::to_chars_result

    to_chars( char* first, char* last, /* floating-point-type */ value,

              std::chars_format fmt );
(since C++23)
(5)
std::to_chars_result

    to_chars( char* first, char* last, float value,
              std::chars_format fmt, int precision );
std::to_chars_result
    to_chars( char* first, char* last, double value,
              std::chars_format fmt, int precision );
std::to_chars_result
    to_chars( char* first, char* last, long double value,

              std::chars_format fmt, int precision );
(since C++17)
(until C++23)
std::to_chars_result

    to_chars( char* first, char* last, /* floating-point-type */ value,

              std::chars_format fmt, int precision );
(since C++23)

Converts value into a character string by successively filling the range [firstlast), where [firstlast) 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 floating-point types as the type of the parameter value. (since C++23)
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 floating-point types as the type of the parameter value. (since C++23)
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 floating-point types as the type of the parameter value. (since C++23)

Contents

[edit] 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

[edit] 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 [firstlast) in unspecified state.

[edit] Exceptions

Throws nothing.

[edit] 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 Value Std Feature
__cpp_lib_to_chars 201611L (C++17) Elementary string conversions (std::to_chars, std::from_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

[edit] Example

#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

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

[edit] See also

the return type of std::to_chars
(class) [edit]
converts a character sequence to an integer or floating-point value
(function) [edit]
(C++11)
converts an integral or floating point value to string
(function) [edit]
prints formatted output to stdout, a file stream or a buffer
(function) [edit]
inserts formatted data
(public member function of std::basic_ostream<CharT,Traits>) [edit]