uso de libreria archivos

#include <iostream>
#include <cstdlib>
#include <fstream>
#include <string>
#include <array>

template<typename T>
std::ostream& writeblock(std::ostream& out, const T& block){
    return out.write(
        reinterpret_cast<const char*>(&block),
        sizeof block
    );
}


template<typename T>
std::istream& readblock(std::istream& in, T& block){
    return in.read(
        reinterpret_cast<char*>(&block),
        sizeof block
    );
}

template<typename T>
std::string packstring(T& a, const std::string& s)
{
    if (s.length() >= a.size()) // no entra
        a.at(a.size()-1) = '\0'; // pone un 0 en el ultimo
    else
        a.at(s.length()) = '\0'; // pone un 0 despues del string
    s.copy(a.data(), a.size()-1); // donde copiar, cuanto copiar
    return s;
}

template<typename T>
std::string unpackstring(const T& a){
    return std::string(a.data(), a.size());
}

template<typename stream_type>
void check_open (const stream_type& file)
{
    if (not file.is_open())
    {
        std::cout << "Archivo no abierto." << std::endl;
        std::exit(1);
    }
}

template<typename stream_type>
void close_file (stream_type& file)
{
    file.close();
}

struct record {
    char numero;
    char letra;
};

using namespace std;

int main()
{
    array<char,10> a;
    record rec = {'A', 'B'};

    string s = "Hello world!";
    packstring(a, s);
    s = unpackstring(a);

    cout << s << endl;

    ofstream out("filename.bin", ios::binary);
    writeblock(out, rec);
    out.close();

    ifstream in("filename.bin", ios::binary);
    readblock(in, rec);
    close_file(in);

    cout << rec.letra << ' ' << rec.numero << endl;
    //cout << rec[0] << rec[1] << endl;

    return 0;
}