重载运算符>>从字符串创建数组

Question

I would like to create an std::array from a std::string .

For this, I would like to overload the operator>> .

I have the following test case:

std::istream& operator>>(std::istream& is, const std::array<double, 3>& a)
{
    char p1, p2;

    is >> p1;
    // if fail warn the user

    for (unsigned int i = 1; i < a.size(); ++i) 
    {
        // something to ignore/ check if numeric
    }

    is >> p2;
    // if fail warn the user
    
    return is;
}

int main()
{
    std::string a = "[1 2 3]";
    std::array<double, 3> arr;
    std::istringstream iss (a);
    iss >> arr;
    
    return 0;
}

I would like for the operator to check if the characters [ and ] are in the correct place and to construct the array with the elements inside.

How can I do checks if the extraction was successfull? How can I check the string between the parenthesis is numeric and if so construct my array from it?

Kind regards

Answer 1

I'd add a helper class that checks for a certain character in the stream and removes it if it's there. If it's not, sets the failbit.

Example:

#include <cctype>
#include <istream>

template <char Ch, bool SkipWhitespace = false>
struct eater {
    friend std::istream& operator>>(std::istream& is, eater) {
        if /*constexpr*/ (SkipWhitespace) { // constexpr since C++17
            is >> std::ws;
        }
        if (is.peek() == Ch) // if the expected char is there, remove it
            is.ignore();
        else                 // else set the failbit
            is.setstate(std::ios::failbit);
        return is;
    }
};

And it could then be used like this:

std::istream& operator>>(std::istream& is, std::array<double, 3>& a) {
    // use the `eater` class template with `[` and `]` as template parameters:
    return is >> eater<'['>{} >> a[0] >> a[1] >> a[2] >> eater<']'>{};
}

int main() {
    std::string a = "[1 2 3]";
    std::array<double, 3> arr;
    std::istringstream iss (a);
    // iss.exceptions(std::ios::failbit); // if you want exceptions on failure

    if(iss >> arr) {
        std::cout << "success\n";
    } else {
        std::cout << "fail\n";
    }
}

Demo where , is used as separator in the input.