Why can't I access a std::vector<std::pair<std::string, std::string>> through vec[i].first()?

Question

I am attempting to print data from a std::vector<std::pair<std::string,std::string>> via a for loop. MSVC says that I can't call make a call through this vector. I tried it with std::vector<std::pair<int, int>> as well and got the same error. I tried iterating with a for loop on a std::vector<int> and it worked fine. I haven't tried on another compiler.

Sample code

    std::vector<std::pair<std::string, std::string>> header_data = get_png_header_data(file_contents);

    for (unsigned int i = 0; i < header_data.size(); i++)
    {
        std::cout << header_data[i].first(); //throws an error on this line "call of an object of a class type without an appropriate operator() or conversion functions to pointer-to-function type
    }

I would appreciate an alternative way to access my vector or another storage type that I could use.

Thanks :)

Answer 1

Your std::pair is basically (in a manner of speaking):

    struct std::pair {
        std::string first;
        std::string second;
    };

That's what std::pairs are. first and second are ordinary class members, not methods/functions. Now you can easily see what's happening: .first() attempts to call first's () operator overload. Obviously, std::strings have no such overloads. That's what your C++ compiler's error message is telling you. If you reread your compiler's error message it now becomes crystal clear.

You obviously meant to write std::cout << header_data[i].first;.