Converting a String to a Number Type in C++

std::stoi, std::stol, and std::stoll take a string and return an integer type. Their prototypes are like:

int       stoi ( const std::string& str,
                 std::size_t* pos = nullptr, int base = 10 );
int       stoi ( const std::wstring& str,
                 std::size_t* pos = nullptr, int base = 10 );

where

• str is the string to convert,
• pos is the address of an integer to store the number of characters processed, and
• base is the number base, commonly 10, 2, 8...

These (global) functions are defined in header <string.

The exceptions (<stdexcept>) that may be thrown are:

• std::invalid_argument if no conversion could be performed, and
• std::out_of_range if the converted value would fall out of the range of the result type or if the underlying function (std::strtol or std::strtoll) sets errno to ERANGE.

This is an example showing how to use them:

#include <iomanip>
#include <iostream>
#include <stdexcept>
#include <string>
#include <utility>

int main()
{
    const auto data =
    {
        "45",
        "+45",
        " -45",
        "3.14159",
        "31337 with words",
        "words and 2",
        "12345678901",
    };

    for (const std::string s : data)
    {
        std::size_t pos{};
        try
        {
            std::cout << "std::stoi(" << std::quoted(s) << "): ";
            const int i{std::stoi(s, &pos)};
            std::cout << i << "; pos: " << pos << '\n';
        }
        catch (std::invalid_argument const& ex)
        {
            std::cout << "std::invalid_argument::what(): " << ex.what() << '\n';
        }
        catch (std::out_of_range const& ex)
        {
            std::cout << "std::out_of_range::what(): " << ex.what() << '\n';
            const long long ll{std::stoll(s, &pos)};
            std::cout << "std::stoll(" << std::quoted(s) << "): " << ll
                      << "; pos: " << pos << '\n';
        }
    }

    std::cout << "\nCalling with different radixes:\n";
    for (const auto& [s, base] : {std::pair<const char*, int>
        {"11",  2}, {"22",  3}, {"33",  4}, {"77",  8},
        {"99", 10}, {"FF", 16}, {"jJ", 20}, {"Zz", 36}})
    {
        const int i{std::stoi(s, nullptr, base)};
        std::cout << "std::stoi(" << std::quoted(s)
                  << ", nullptr, " << base << "): " << i << '\n';
    }
}

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