Issues with strings and char arrays in C++ - c++

Question

I'm writing a registry generator as a part of a bigger program. I'm very new in C++, but good at other programming languages like PHP.
I'll start by providing the code of the problematic function:
void generacionAleatoria() {
string r_marca, r_nom, r_apellido;
char r_patente[6];
int num_rand;
registroAuto r_auto;
string nombres[8] = {
"Juan", "Pedro", "Roberto", "Miguel", "Guillermo", "Emilio", "Roque", "Gustavo"
} ;
string apellidos[8] = {
"Messi", "Maradona", "Gardel", "Heredia", "Pimpinela", "Nadal", "Mascherano", "Troilo"
};
string marcas[12] = {
"Volvo", "Renault", "Audi", "Ford", "Fiat", "Chevrolet", "Nissan", "Volkswagen", "Mercedes Benz", "Rolls Royce", "Delorean", "Aston Martin"
};
char letras_patentes[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
char numeros_patentes[] = "0123456789";
for (int i = 0; i < cantidad_autos; i++) {
r_marca = marcas[rand() % (sizeof(marcas)/sizeof(marcas[0]) - 1)];
r_nom = nombres[rand() % (sizeof(nombres)/sizeof(nombres[0]) - 1)];
r_apellido = apellidos[rand() % (sizeof(apellidos)/sizeof(apellidos[0]) - 1)];
for(int m = 0; m < 3; ++m) {
r_patente[m] = letras_patentes[rand() % (sizeof(letras_patentes) - 1)];
}
for(int n = 3; n < 6; n++) {
r_patente[n] = numeros_patentes[rand() % (sizeof(numeros_patentes) - 1)];
}
strcpy(r_auto.patente,r_patente);
strcpy(r_auto.marca,r_marca.c_str());
strcpy(r_auto.apellido,r_apellido.c_str());
strcpy(r_auto.nom,r_nom.c_str());
fwrite(&r_auto,sizeof(registroAuto),1,archivo);
if (ver_variables_testeo) {
//cout << (i+1) << ") " << r_auto.patente<<endl;
cout << (i+1) << ") " << r_auto.marca << " - " << r_auto.patente << " - " << r_auto.nom << " " << r_auto.apellido << endl; //Para testear
}
}
}
This creates 100 structs of the following type:
struct registroAuto {
char marca[15];
char patente[6];
char nom[25];
char apellido[25];
};
In case you're wondering, this is meant to be a registry of Uber drivers and their cars: brand, license plate, name and surname. Well, it's not really a registry, it's college homework.
The problem is that when I print out the contents of my new struct, the license plate and the name will be together, as in:
100) Fiat - KWQ293Maria - Maria Gardel
You can see by the position of the hyphens, that the license plate is now "KWQ293Maria", even though it is an array of 6 chars!
A reminder of the cout command:
cout << (i+1) << ") " << r_auto.marca << " - " << r_auto.patente << " - " << r_auto.nom << " " << r_auto.apellido << endl;
I did some tests, but I don't know what to do with the results.
1: commenting out the strcopy of the name fixes the issue
strcpy(r_auto.patente,r_patente);
strcpy(r_auto.marca,r_marca.c_str());
strcpy(r_auto.apellido,r_apellido.c_str());
//strcpy(r_auto.nom,r_nom.c_str());
As you can see, this is the last of the 4 statements in my original code, so I don't know why it would affect r_auto.patente.
Can you please help me? I'm guessing there's a key concept of char array handling that I missed out on in class :-(

Answer 1

When using character arrays as strings they need to be terminated by a null character '\0'. So when you construct your number-plate you need to make the array 7 characters long.
struct registroAuto {
char marca[15];
char patente[7]; // 6 for numbers, 1 for terminator '\0'
char nom[25];
char apellido[25];
};
Same with your working variable:
char r_patente[7];
And you need to manually add the null-terminator when you create the number:
for(int m = 0; m < 3; ++m) {
r_patente[m] = letras_patentes[rand() % (sizeof(letras_patentes) - 1)];
}
for(int n = 3; n < 6; n++) {
r_patente[n] = numeros_patentes[rand() % (sizeof(numeros_patentes) - 1)];
}
r_patente[6] = '\0'; // add the null terminator