Difference between revisions of "cpp/utility/from chars"

Revision as of 10:31, 10 March 2022

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 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 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 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 <charconv>
#include <iomanip>
#include <iostream>
#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";
        }
    }
}

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