std:: from_chars

Analyzes the character sequence [ first , last ) for a pattern described below. If no characters match the pattern or if the value obtained by parsing the matched characters is not representable in the type of value , value is unmodified, otherwise the characters matching the pattern are interpreted as a text representation of an arithmetic value, which is stored in value .

Parameters

Return value

On success, returns a value of type std::from_chars_result such that ptr points at the first character not matching the pattern, or has the value equal to last if all characters match and ec is value-initialized.

If there is no pattern match, returns a value of type std::from_chars_result such that ptr equals first and ec equals std::errc::invalid_argument . value is unmodified.

If the pattern was matched, but the parsed value is not in the range representable by the type of value , returns value of type std::from_chars_result such that ec equals std::errc::result_out_of_range and ptr points at the first character not matching the pattern. value is unmodified.

Exceptions

Throws nothing.

Notes

Unlike other parsing functions in C++ and C libraries, std::from_chars is locale-independent, non-allocating, and non-throwing. Only a small subset of parsing policies used by other libraries (such as std::sscanf ) 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.

A pattern consisting of a sign with no digits following it is treated as pattern that did not match anything.

Example

#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
}

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.

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also