std::from_chars_result

 

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

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

Data members

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

Member and friend functions

operator==(std::from_chars_result)

friend bool operator==( const from_chars_result&,                         const from_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::from_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 <cassert>
#include <charconv>
#include <iomanip>
#include <iostream>
#include <optional>
#include <string_view>
#include <system_error>
 
int main()
{
    for (std::string_view const str : {"1234", "15 foo", "bar", " 42", "5000000000"})
    {
        std::cout << "String: " << std::quoted(str) << ". ";
        int result{};
        auto [ptr, ec] = std::from_chars(str.data(), str.data() + str.size(), result);
 
        if (ec == std::errc())
            std::cout << "Result: " << result << ", ptr -> " << std::quoted(ptr) << '\n';
        else if (ec == std::errc::invalid_argument)
            std::cout << "This is not a number.\n";
        else if (ec == std::errc::result_out_of_range)
            std::cout << "This number is larger than an int.\n";
    }
 
    // C++23's constexpr from_char demo / C++26's operator bool() demo:
    auto to_int = [](std::string_view s) -> std::optional<int>
    {
        int value{};
#if __cpp_lib_to_chars >= 202306L
        if (std::from_chars(s.data(), s.data() + s.size(), value))
#else
        if (std::from_chars(s.data(), s.data() + s.size(), value).ec == std::errc{})
#endif
            return value;
        else
            return std::nullopt;
    };
 
    assert(to_int("42") == 42);
    assert(to_int("foo") == std::nullopt);
#if __cpp_lib_constexpr_charconv and __cpp_lib_optional >= 202106
    static_assert(to_int("42") == 42);
    static_assert(to_int("foo") == std::nullopt);
#endif
}

Output:

String: "1234". Result: 1234, ptr -> ""
String: "15 foo". Result: 15, ptr -> " foo"
String: "bar". This is not a number.
String: " 42". This is not a number.
String: "5000000000". This number is larger than an int.

See also

from_chars (C++17)

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