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

Question

Is there a safe standard way to convert std::string_view to int?
Since C++11 std::string lets us use stoi to convert to int:
std::string str = "12345";
int i1 = stoi(str); // Works, have i1 = 12345
int i2 = stoi(str.substr(1,2)); // Works, have i2 = 23
try {
int i3 = stoi(std::string("abc"));
}
catch(const std::exception& e) {
std::cout << e.what() << std::endl; // Correctly throws 'invalid stoi argument'
}
But stoi does not support std::string_view. So alternatively, we could use atoi, but one has to be very careful, e.g.:
std::string_view sv = "12345";
int i1 = atoi(sv.data()); // Works, have i1 = 12345
int i2 = atoi(sv.substr(1,2).data()); // Works, but wrong, have i2 = 2345, not 23
So atoi does not work either, since it is based off the null-terminator '\0' (and e.g. sv.substr cannot simply insert/add one).
Now, since C++17 there is also from_chars, but it does not seem to throw when providing poor inputs:
try {
int i3;
std::string_view sv = "abc";
std::from_chars(sv.data(), sv.data() + sv.size(), i3);
}
catch (const std::exception& e) {
std::cout << e.what() << std::endl; // Does not get called
}

Answer 1

The std::from_chars function does not throw, it only returns a value of type from_chars_result which is 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.";
}
}

Answer 2

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 3

Building on #Ron and #Holt's excellent answers, here's a small wrapper around std::from_chars() that returns an optional (std::nullopt when the input fails to parse).
#include <charconv>
#include <optional>
#include <string_view>
std::optional<int> to_int(const std::string_view & input)
{
int out;
const std::from_chars_result result = std::from_chars(input.data(), input.data() + input.size(), out);
if(result.ec == std::errc::invalid_argument || result.ec == std::errc::result_out_of_range)
{
return std::nullopt;
}
return out;
}