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?

---

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
  }

Solution 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.";
    }
}

Solution 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.

Solution 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;
}

Comments

[Yksisarvinen]

That's because std::from_chars does not throw anything. Instead it returns an error code.

[M.M]

What about when ec is some other failure code beside those two?

[s3cur3]

@M.M, as best I can tell from the CppReference docs, those two are the only error codes that std::from_chars() will return.

[303]

Why not simply return out if ec matches std::errc{} and std::nullopt otherwise? Isn't that the lesser verbose alternative? I also think that std::string_views fall in line with iterators and passing them by value is the way to go.