My C++ program crashes when trying to output data read from a file

Question

I am trying to make a program that stores a database of employees and stores them in an external file. the function that reads a vector from the .dat file and loads it into the program reads the file but whenever I try to display or modify the loaded vector the program crashes.

//displays vector
void Database:: displayAll() const
{
    for(std::vector<Employee>::const_iterator iter = mEmployees.begin(); iter != mEmployees.end(); ++iter)
    {
        iter -> display();
    }

}

std::vector<Employee> mEmployees;

void Database::readData()
{
    ifstream empIn("employee.dat" , ios::binary);
    empIn.seekg(0,ifstream::end);
    long size2 = empIn.tellg();
    empIn.seekg(0,ifstream::beg);
    mEmployees.resize(size2);
    empIn.read((char*)&mEmployees, size2);
    empIn.close();
    cout << mEmployees.size() << endl; //this tests whether or not it reads.
}

Answer 1

You haven't showed us your declaration of mEmployees , but I assume its a std::vector of some kind. The problem is that you are trying to read the file into the vector object, instead of into its content. That will corrupt the vector , as well as any other memory next to it, causing crashes further on.

What you probably want instead is:

empIn.read((char*)&mEmployees[0], size2);

or in C++11 :

empIn.read((char*)mEmployees.data(), size2);

to actually read the contents of the file into the contents of the vector .

To understand why this is happening, you should know that the contents of a vector are not stored within the vector itself (how could they? the size is dynamic) . Instead, a common implementation of vector has pointers to the begin and end of a storage area, somewhere in the free store, where the contents are actually held. When you try to read a file into the memory layout of the vector you are overwriting those pointers, as well as memory beyond the vector itself, corrupting the vector among other things.

Answer 2

The cast to (char*) tells the compiler to discard the type-checking that it would normally do. It is necessary to perform the cast in this case since ifstream requires you to pass a char* . The problem is, this reinterpret_cast is a rather blunt instrument and fails to warn you that you are converting from a vector<Employee>* when you intended to convert from an Employee* .

Just remember that casts like this are low-level operations whose purpose is to tell the compiler that "you know what you are doing, even though it seems wrong". They should be used with extra care.

A better, and corrected, form for what you are doing would be:

empIn.read( reinterpret_cast<char*>(&mEmployees[0]), mEmployees.size() );