NAME¶

std::from_chars_result - std::from_chars_result

Synopsis¶

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.

Notes¶

Feature-test macro Value Std Feature
201611L (C++17) Elementary string conversions (std::to_chars,
__cpp_lib_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 converts a character sequence to an integer or floating-point value
(C++17) (function)