std::from_chars, std::from_chars_result

std::from_chars_result
    from_chars( const char* first, const char* last,
                /* integer-type */& value, int base = 10 );

constexpr std::from_chars_result
    from_chars( const char* first, const char* last,
                /* integer-type */& value, int base = 10 );

std::from_chars_result
    from_chars( const char* first, const char* last, float& value,
                std::chars_format fmt = std::chars_format::general );
std::from_chars_result
    from_chars( const char* first, const char* last, double& value,
                std::chars_format fmt = std::chars_format::general );
std::from_chars_result
    from_chars( const char* first, const char* last, long double& value,
                std::chars_format fmt = std::chars_format::general );

std::from_chars_result
    from_chars( const char* first, const char* last,
                /* floating-point-type */& value,
                std::chars_format fmt = std::chars_format::general );

struct from_chars_result {
    const char* ptr;
    std::errc ec;
};

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.

char

• the plus sign is not recognized outside of the exponent (only the minus sign is permitted at the beginning)
• if fmt has std::chars_format::scientific set but not std::chars_format::fixed, the exponent part is required (otherwise it is optional)
• if fmt has std::chars_format::fixed set but not std::chars_format::scientific, the optional exponent is not permitted
• if fmt is std::chars_format::hex, the prefix "0x" or "0X" is not permitted (the string "0x123" parses as the value "0" with unparsed remainder "x123")
• leading whitespace is not ignored.

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.

friend bool operator==( const from_chars_result&,
                        const from_chars_result& ) = default;

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 << "That isn't 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:
    auto to_int = [](std::string_view s) -> std::optional<int>
    {
        if (int value; std::from_chars(s.data(), s.data() + s.size(), value).ec == std::errc{})
            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". That isn't a number.
String: " 42". That isn't 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