read/write a complex struct into the binary file

Question

I have the following struct

typedef struct a_t
{
    vector <int> a;
    int p;
}a;

typedef struct b_t
{
    a x;
    int y;
}b;

struct a is a struct contain a vector,struct b contain struct a I wanna write/read the struct b into a binary file. The following code doesn't work

int main()
{

    b m;
    m.x.a.push_back(1);
    m.x.a.push_back(2);
    m.x.a.push_back(3);
    m.x.p = 5;
    m.y = 7;
    cout << sizeof(m.y) << endl;
    cout << sizeof(m.x) << endl;
    cout << sizeof(m) << endl;
    ofstream outfile("model",ios::out|ios::binary);
    outfile.write((char*)&m,sizeof(m));
    outfile.close();


    b p;
    ifstream iffile("model", ios::in|ios::binary);
    iffile.read((char*)&p,sizeof(a));
    iffile.close();

    cout << p.y << endl;;
    cout << p.x.p << endl;
    cout << p.x.a[0] << endl;
    cout << p.x.a[1] << endl;
    cout << p.x.a[2] << endl;
    return 0;
}

The error message is " * glibc detected double free or corruption (top): 0x0000000000504010 * * Aborted (core dumped)" beside,it does't write the struct into the file.

Answer 1

You cannot write and read vectors because they allocate data on the heap. If you have real limits in your application you may decide it's better (for speed and clarity, potentially sacrificing storage) to use a normal C array:

#define MAXARR_A 100

typedef struct a_t
{
    int a[MAXARR_A];
    int p;
} a;

Otherwise you have to serialise your vector by writing out a size and then dumping the array data. To read it back in you read the size, then resize the vector and read the bytes into it.

Edit : Decided to add some code for you... It might compile! =)

void write_a( a& data, ostream& s )
{
    size_t len = data.a.size();
    s.write( (char*)&len, sizeof(len) );
    s.write( (char*)&data.a[0], len * sizeof(int) );
    s.write( (char*)&data.p, sizeof(int) );
}

void write_b( b& data, ostream& s )
{
    write_a( data.x, s );
    s.write( (char*)&data.y, sizeof(int) );
}

void read_a( a& data, istream& s )
{
    size_t len;
    s.read( (char*)&len, sizeof(len) );
    data.a.resize(len);
    s.read( (char*)&data.a[0], len * sizeof(int) );
    s.read( (char*)&data.p, sizeof(int) );
}

void read_b( b& data, istream& s )
{
    read_a( data.x, s );
    s.read( (char*)&data.y, sizeof(int) );
}

Note that I haven't included any error checking... Also, it would be more C++-like to make these functions as read and write member functions on the structures.