std:: to_chars_result

From cppreference.com
C++
Utilities library
General utilities
Relational operators (deprecated in C++20)
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

(C++17)
converts an integer or floating-point value to a character sequence
(function)
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