在文件结构中使用read（）和write（）使用字符串代替char数组

Question

Is there a way a string could be used instead of a char[] array in the below struct , and still be saved to a file using read and write functions without getting a runtime error?

#include <iostream>
#include <cstdlib>
#include <fstream>

using namespace std;

struct details
{
    char name[30];
    int add_year;
};

const char * file = "students.dat";

int main()
{
    details x;
    ifstream ifile;
    ifile.open(file,ios_base::in|ios_base::binary);
    if(ifile.is_open())
    {
        cout<<"Contents of "<<file<<" is"<<endl;
        while(ifile.read((char *)&x,sizeof x))
        {
            cout<<x.name<<" - "<<x.add_year<<endl;
        }
        ifile.close();
    }
    ofstream ofile(file,ios_base::out|ios_base::app|ios_base::binary);
    if(!ofile.is_open())
    {
        cerr<<"Can't open "<<file<<endl;
        exit(EXIT_FAILURE);
    }
    cout<<"Name"<<endl;
    cin.get(x.name,30);
    cout<<"Year added"<<endl;
    cin>>x.add_year;
    ofile.write((char *) &x, sizeof x);
    ofile.close();
    cin.get();
    return 0;
}

Answer 1

A std::string (or any other dynamic sized container) contains a pointer to its character data that is stored elsewhere in memory. If you were to read() / write() the std::string itself as-is, you would only be reading/writing the pointer value, not the actual character data. This is not the case with a fixed array.

In order to use std::string in your struct , you would have to (de)serialize the content of the struct back and forth so you can take dynamic data into account.

For example:

#include <iostream>
#include <fstream>
#include <string>
#include <cstdint> 

using namespace std;

struct details
{
    string name;
    int32_t add_year;
};

istream& operator>>(istream &in, details &out)
{
    int32_t len;
    if (in.read((char*) &len, sizeof(len)))
    {
        out.name.resize(len);
        if (len > 0) in.read(&(out.name[0]), len);
        in.read((char*) &(out.add_year), sizeof(out.add_year));
    }
    return in;
}

ostream& operator<<(ostream &out, const details &in)
{
    int32_t len = in.name.size();
    out.write((char*) &len, sizeof(len));
    out.write(in.name.c_str(), len);
    out.write((char*) &(in.add_year), sizeof(in.add_year));
    return out;
}

const char * file = "students.dat";

int main()
{
    details x;

    ifstream ifile;
    ifile.open(file, ios_base::binary);
    if (ifile.is_open())
    {
        cout << "Contents of " << file << " is" << endl;
        while (ifile >> x)
        {
            cout << x.name << " - " << x.add_year << endl;
        }
        ifile.close();
    }

    cout << "Name:" << endl;
    getline(cin, x.name);
    cout << "Year added:" << endl;
    cin >> x.add_year;

    ofstream ofile(file, ios_base::app|ios_base::binary);
    if (!ofile.is_open())
    {
        cerr << "Can't open " << file << endl;
        exit(EXIT_FAILURE);
    }
    ofile << x;
    ofile.close();

    cin.get();
    return 0;
}