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 );

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 Value Std Feature
201611L (C++17) Elementary string conversions
__cpp_lib_to_chars (std::to_chars, std::from_chars)
202306L (C++26) Testing for success or failure of
<charconv> functions
Add constexpr modifiers to
__cpp_lib_constexpr_charconv 202207L (C++23) 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.

DR Applied to Behavior as published Correct behavior
LWG 2955 C++17 this function was in <utility> and used moved to <charconv> and
std::error_code uses std::errc
LWG 3266 C++17 bool argument was accepted and promoted rejected by a deleted
to int overload
LWG 3373 C++17 std::to_chars_result might have additional members are
additional members 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 prints formatted output to stdout, a file stream or a buffer
sprintf (function)
snprintf
(C++11)
operator<< inserts formatted data
(public member function of std::basic_ostream<CharT,Traits>)