std::to_chars_result

 

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Defined in header <charconv>
struct to_chars_result;		(since C++17)

std::to_chars_result is the return type of std::to_chars. It has no base classes, and only has the following members.

Data members

Member name	Definition
ptr	a pointer of type char* (public member object)
ec	an error code of type std::errc (public member object)

Member and friend functions

operator==(std::to_chars_result)

friend bool operator==( const to_chars_result&,                         const to_chars_result& ) = default;		(since C++20)

Compares the two arguments using default comparisons (which uses operator== to compare ptr and ec respectively).

This function is not visible to ordinary unqualified or qualified lookup, and can only be found by argument-dependent lookup when std::to_chars_result is an associated class of the arguments.

The != operator is synthesized from operator==.

operator bool

constexpr explicit operator bool() const noexcept;		(since C++26)

Checks whether the conversion is successful. Returns ec == std::errc{}.

Notes

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

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 and __cpp_structured_bindings >= 202406L
    // use C++26 structured bindings declaration as condition (P0963)
    // and C++26 to_chars_result::operator bool() for error checking (P2497)
    if (auto [ptr, ec] =
            std::to_chars(str.data(), str.data() + str.size(), format_args...))
        std::cout << std::string_view(str.data(), ptr) << '\n';
    else
        std::cout << std::make_error_code(ec).message() << '\n';
#elif __cpp_lib_to_chars >= 202306L
    // use C++26 to_chars_result::operator bool() for error checking (P2497)
    if (auto result =
            std::to_chars(str.data(), str.data() + str.size(), format_args...))
        std::cout << std::string_view(str.data(), result.ptr) << '\n';
    else
        std::cout << std::make_error_code(result.ec).message() << '\n';
#else
    // fallback to C++17 if-with-initializer and structured bindings
    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 - str.data()) << '\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

See also

to_chars (C++17)

converts an integer or floating-point value to a character sequence (function)