Safely convert std::string_view to int (like stoi or atoi)

Question

Is there a safe standard way to convert std::string_view to int?

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Since C++11 std::string lets us use stoi

Answer 1

Unfortunately, there is no standard way that would throw an exception for you but std::from_chars has a return value code that you may use:

#include <charconv>
#include <stdexcept>

template <class T, class... Args>
void from_chars_throws(const char* first, const char* last, T &t, Args... args) {
    std::from_chars_result res = std::from_chars(first, last, t, args... );

    // These two exceptions reflect the behavior of std::stoi.
    if (res.ec == std::errc::invalid_argument) {
        throw std::invalid_argument{"invalid_argument"};
    }
    else if (res.ec == std::errc::result_out_of_range) {
        throw std::out_of_range{"out_of_range"};
    }
}

Obviously you can create svtoi, svtol from this, but the advantage of "extending" from_chars is that you only need a single templated function.

Answer 2

The std::from_chars function does not throw, it only returns a value of type from_chars_result which is basically a struct with two fields:

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

You should inspect the values of ptr and ec when the function returns:

#include <iostream>
#include <string>
#include <charconv>

int main()
{
    int i3;
    std::string_view sv = "abc";
    auto result = std::from_chars(sv.data(), sv.data() + sv.size(), i3);
    if (result.ec == std::errc::invalid_argument) {
        std::cout << "Could not convert.";
    }
}