c++iostream

Writing tuple data into a file using a loop


I want to write/read a tuple into a BIN file. I know how to write/read each element of the tuple, but the tuple has 13 elements and I would rather make this all at once using a loop. But I have been unable to do so.

This is the code that I have tried:

#include <fstream>

template <typename T>
std::ostream& writeBin(std::ostream& f, T data) {
    return f.write(reinterpret_cast<char*>(&data), sizeof(data));
}
std::ostream& writeBin(std::ostream& f, const std::string& str) {
    std::string::size_type sz = str.size();
    if (f) { f.write(reinterpret_cast<char*>(&sz), sizeof(sz)); }
    if (f) { f.write(str.data(), str.size()); }
    return f;
}

template <typename T>
std::istream& readBin(std::istream& f, T& data) {
    if (f) { f.read(reinterpret_cast<char*>(&data), sizeof(data)); }
    return f;
}
std::istream& readBin(std::istream& f, std::string& str) {
    std::string::size_type sz = 0;
    if (f) { f.read(reinterpret_cast<char*>(&sz), sizeof(sz)); }
    if (f) { str.resize(sz); f.read(str.data(), sz); }
    return f;
}

class Data {
public:
    Data() {};
    ~Data() {};
    std::tuple<std::string, std::string, int, int, int, int, std::string, int, int, int, int, int, int>tup{
        " "," ",0,0,0,0," ",0,0,0,0,0,0
    };
    bool write(std::ofstream& f);
    bool read(std::ifstream& f);
};

bool Data::write(std::ofstream& f) {
    std::apply([](auto&&... tArg) {
        ((if (!writeBin(f, tArg)) { return false; }), ...);//doesnt work
        }, tup);
    return true;
}

bool Data::read(std::ifstream& f) {
    std::apply([](auto&&... tArg) {
        ((if (!readBin(f, tArg)) { return false; }), ...);//doesnt work
        }, tup);
    return true;
}

Solution

  • Do you mean iterating over all the elements of tuple? That's what std::index_sequence is for:

    C++20:

    bool Data::write(std::ostream& f) {
        using Tuple = decltype(tup);
        return [&os=f]<std::size_t...I> (const auto& tup, std::index_sequence<I...>){
            return (writeBin(os, std::get<I>(tup)) && ...);
        }(tup, std::make_index_sequence<std::tuple_size_v<Tuple>>());
    }
    

    Pre-C++20 you need to provide an extra helper layer, but the principle is generally the same:

    template<typename Tuple, std::size_t...I>
    static bool write_impl(
        std::ostream& f,
        const Tuple& t,
        std::index_sequence<I...>)
    {
        return (writeBin(f, std::get<I>(t)) && ...);
    }
    
    bool Data::write(std::ostream& f) {
        return write_impl(f, tup, std::make_index_sequence<std::tuple_size_v<decltype(tup)>>());
    }