C++ - How to write and read a structure that contain an object ? (to write and read binary)

Question

I\'m trying to write a C structure in a file (to write in binary) and read it to recover it. I don\'t know if it is possible. Here is what I have :

head.hh:

Answer 1

The file to read from was being opened as write only.

The actual std::string object can't be written that way. The actual object generally contains a couple of pointers and perhaps a size but not the actual character data. It need to be serialized.

If you're going to be writing C++ you should consider learning to use file streams rather than what you've got here.

#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

typedef struct s_test
{
    char cmd[5];
    std::string str;
}t_test;

void Write(int fd, struct s_test* test)
{
    write(fd, test->cmd, sizeof(test->cmd));
    unsigned int sz = test->str.size();
    write(fd, &sz, sizeof(sz));
    write(fd, test->str.c_str(), sz);
}

void Read(int fd, struct s_test* test)
{
    read(fd, test->cmd, sizeof(test->cmd));
    unsigned int sz;
    read(fd, &sz, sizeof(sz));
    std::vector data(sz);
    read(fd, &data[0], sz);
    test->str.assign(data.begin(), data.end());
}

int main()
{
    t_test test;
    int fd = open("test", O_APPEND | O_CREAT | O_TRUNC | O_WRONLY, 0666);

    test.cmd[0] = 's';
    test.cmd[1] = 'm';
    test.cmd[2] = 's';
    test.cmd[3] = 'g';
    test.cmd[4] = 0;
    test.str = "hello world";
    std::cout << "Before Write: " << test.cmd << " " << test.str << std::endl;

    Write(fd, &test);
    close(fd);

    fd = open("test", O_RDONLY, 0666);
    t_test test2;
    Read(fd, &test2);
    std::cout << "After Read: " << test2.cmd << " " << test2.str << std::endl;
    close(fd);

    return (0);
}