Why does the address of a "size_type" variable is used as an argument of "stoi()" in C++?

Question

The address of a size_type variable is used as an argument of stoi(). Reference link is given below:

stoi()

I can also do the same operation without using size_type. I have read the documentation I have given, but I didn't get when should I use it.

Then, what is the contribution of using the address of a size_type variable here and when sould we use it?

Answer 1

First, it is not mandatory, it can be NULL. The contribution is for case when your string contains several values. This allows to parse them one by one. After a call to stoi, *idx will contain the start index of the next integer. For example:

int main() {
    std::string str = "23 45 56 5656";
    std::string::size_type off = 0;
    do {
        std::string::size_type sz;
        cout << std::stoi(str.substr(off), &sz) << endl;
        off += sz;
    } while (off < str.length());
}

// will print
// 23
// 45
// 56
// 5656

EDIT: as @Surt correctly commented, some error handling can and should added here. So lets make this example complete. The function stoi can throw either invalid_argument or out_of_range, these exceptions should be handled. How to handle them - IDK, your decision here is an example:

int main() {
    std::string str = "23 45 56 5656 no int";
    std::string::size_type off = 0;
    try {
        do {
            std::string::size_type sz;
            std:cout << std::stoi(str.substr(off), &sz) << std::endl;
            off += sz;
        } while (off < str.length());
    } catch(const std::invalid_argument &e) {
        std::cout << "Oops, string contains something that is not a number"
            << std::endl;
    } catch(const std::out_of_range &e) {
        std::cout << "Oops, some integer is too long" << std::endl;
    }
}

Answer 2

If your string contains more data than just a number, you can use idx to parse the rest of the data.

Another situation where this could be useful: if you want to ensure your string contains nothing but a number - you parse the number, look what appears afterwards, and if there is something, you throw an exception: something like 1234heh is not a valid number.