Problem reading from file

[dualdark]

Hello everyone, i have this code, but when i tried to read from the file, it crash.

this is my code. When im using nombre as char, it works, but when string it doesnt.

#include<fstream>
#include <iostream>
using namespace std;
struct {
    int matricula;
    string nombre;
    int edad;
}alumno;

 int main() {
    int opc;
    do{
        cout <<endl << "1. Capturar"<<endl;
        cout << "2. Mostrar";
        cin >>opc;

        if(opc==1){
            cout <<"dame matricula :";
            cin >> alumno.matricula;
            cout << "NOmbre:" ;
            fflush(stdin);
            getline(cin,alumno.nombre);
            cout << "dame edad :";
            cin >> alumno.edad;

            FILE *archdisco;
            archdisco = fopen("alumnos.dat","at+");
fwrite(&alumno,sizeof(alumno),1,archdisco);
 fclose(archdisco);
        }
        if(opc==2){

            FILE *archdisco;
            archdisco = fopen("alumnos.dat","at+");
            while(fread(&alumno,sizeof(alumno),1,archdisco)==1){

                cout << " MATRICULA " <<alumno.matricula;
                cout << "Nombre " << alumno.nombre;
                cout << " MESES " <<alumno.edad<<endl;

            }

            fclose(archdisco);

        }
    }
    while(opc!=3);

return 0;
}

thanks in advance.

[kbw]

while(fread(&alumno,sizeof(alumno),1,archdisco)==1)
This does a memory overwrite of alumno. That's ok for built in types (Plain Old Data Types--POD Types), but bad for objects.

You should define stream input/output operators for your type and use those with an fstream.

#include <ostream>
#include <istream>
#include <fstream>
#include <iostream>

struct Alumno {
	int matricula;
	std::string nombre;
	int edad;
};

std::ostream& operator<<(std::ostream& os, const Alumno& n) {
	os << n.matricula << ' ' << n.nombre << ' ' << n.edad;
	return os;
}

std::istream& operator>>(std::istream& is, Alumno& n) {
	is >> n.matricula >> n.nombre >> n.edad;
	return is;
}

int main() {
	int opc;
	Alumno alumno;
	do {
		std::cout << "1. Capturar" << std::endl;
		std::cout << "2. Mostrar" << std::endl;
		std::cin >> opc;

		if (opc == 1) {
			std::cout << "dame matricula :";
			std::cin >> alumno.matricula;
			std::cout << "NOmbre:";
			std::cin >> alumno.nombre;
			std::cout << "dame edad :";
			std::cin >> alumno.edad;

			std::ofstream f("alumnos.dat", std::ios_base::app);
			f << alumno << std::endl;
		}
		if (opc == 2) {
			std::ifstream f("alumnos.dat");
			Alumno n;
			while (f >> n) {
				std::cout
					<< "MATRICULA " << n.matricula
					<< " Nombre " << n.nombre
					<< " MESES " << n.edad << std::endl;
			}
		}
	}
	while (opc != 3);

	return 0;
}

[dualdark]

got it, thank you very much.

But now, if instead of showing data, i want to add the data to an objects array or an objects list, how could i do it?

thanks again.

---------------------------------------------------------------------------------------

nvm, i got it... this is just a test code... now im going to use it in my real program, thank you very much.

#include <ostream>
#include <istream>
#include <fstream>
#include <iostream>
using namespace std;
class Alumno {
	public:
        int matricula;
        string nombre;
        int edad;
        void Mostrar();
};
void Alumno::Mostrar(){
    cout << "MATRICULA " << matricula
    << " Nombre " << nombre
    << " MESES " << edad << endl;
}
ostream& operator<<(ostream& os, const Alumno& n) {
	os << n.matricula << ' ' << n.nombre << ' ' << n.edad;
	return os;
}

istream& operator>>(istream& is, Alumno& n) {
	is >> n.matricula >> n.nombre >> n.edad;
	return is;
}

int main() {
	int opc;
	Alumno alumno;
	Alumno alumno2[3];
	do {
		cout << "1. Capturar" << endl;
		cout << "2. Mostrar" << endl;
		cin >> opc;

		if (opc == 1) {
			cout << "dame matricula :";
			cin >> alumno.matricula;
			cout << "NOmbre:";
			cin >> alumno.nombre;
			cout << "dame edad :";
			cin >> alumno.edad;

			ofstream f("alumnos.dat", ios_base::app);
			f << alumno << endl;
		}
		if (opc == 2) {
            int i=0;
			ifstream f("alumnos.dat");
			Alumno n;
			while (f >> n) {
                cout << "MATRICULA " << n.matricula
					 << " Nombre " << n.nombre
					 << " MESES " << n.edad << endl;
                alumno2[i]=n;
                i++;
			}
			for(int j=0;j<3;j++){
                alumno2[j].Mostrar();
			}
		}
	}
	while (opc != 3);

	return 0;
}

btw, why so many people prefers to write "std::" instead of just add "using namespace std" ??

[ostar2]

Using std:: instead of using namespace std; creates less overhead. In other words it's more efficient.

[kbw]

want to add the data to an objects array

We'd normally use a vector because it can grow, whereas an array has a fixed length.

std::vector<Alumno> alumno2;

Alternatively STL provides a double threaded linked list

std::list<Alumno> alumno3;

http://en.cppreference.com/w/cpp/container

[dualdark]

Thank you very much @kbw, you had helped me a lot.

@ostar2 i didnt know that... ill try to do that wey then =) thanks

[Pured]

@dual, If you're just making simple programs, it's fine to use namespace.

[dualdark]

One last thing, how do i know how many "objects" i have on my file?

Also, the vector is not working

int main() {
	int opc;
	Alumno alumno;
	vector<Alumno> alumno2;
	do {
		cout << "1. Capturar" << endl;
		cout << "2. Mostrar" << endl;
		cin >> opc;

		if (opc == 1) {
			alumno.Capturar();
			ofstream f("alumnos.txt", ios_base::app);
			f << alumno << endl;
		}
		if (opc == 2) {
            int i=0;
			ifstream f("alumnos.txt");
			Alumno n;
			while (f >> n) {
                cout << "MATRICULA " << n.matricula
					 << " Nombre " << n.nombre
					 << " MESES " << n.edad << endl;
                alumno2[i]=n;
                i++;
			}
		}
	}
	while (opc != 3);

[Pured]

add a counter and increment it each loop.

[dualdark]

thanks @Pured , it was so easy, i feel so stupid :-/