[FIXED] How to read data from binary files properly in C++ till the last data?

Issue

I recently learnt that to read properly from a text file we use

while(file>>var1>>var2){
   //do stuff with data
}

instead of

while(file){
   file>>var1>>var2;
   //do stuff with data
}

because the latter performs one extra read even after last data item is read and on the next read it is able to read the eof, so if we have something like std::vector<CLASSNAME>CLASSVECTOR, we end up having one extra entry in the vector, while if we use the first method it reads only till the last record.

My question is if how do I read till the last record in case of a binary file? So if I have something like:

class class1 {
   int a;
   class2 obj2;
   class3 obj3;
public:
   void write_ binary(std::ofstream file) const;
   void read_file(std::ifstream file);
   //rest of class definition
};

And I write this class like so :

void class1::write_ binary(std::ofstream file) const {
   file.write(reinterpret_cast<const char*>(&a),sizeof(a));
   obj2.write_binary(file); //this writes the data in the same way using 
   reinterpret_cast to ofstream file
   obj3.write_binary(file); //this writes the data in the same way using 
   reinterpret_cast to ofstream file
}

And also if I read the file like so :

void class1::read_file(std::ifstream file) {
   file.read(reinterpret_cast<char*>(&a),sizeof(a));
   obj2.read_binary(file); //this reads data for obj2 in the same way using 
   read() and reinterpret_cast
   obj3.read_binary(file); //this reads data for obj3 in the same way using read() and reinterpret_cast
}

And if I want to store this data in a vector like so:

class1 obj1;
std::vector<class1>records;

while(file)
{
   obj1.read_binary(file);
   records.push_back(obj1);
   //reset obj1 to initial state
}

I end up getting an extra record in vector records. I cannot use while(file>>obj1) since I want to use >> for cin. Please explain how do I read from binary file without reading an extra record.

Solution

It's the same as your text example, the test on the file must be after the read not before.

for (;;)
{
     obj1.read_binary(file);
     if (!file) // did the previous read fail?
         break; // if so quit the loop
     records.push_back(obj1);
}

Answered By - john

Answer Checked By - David Goodson (PHPFixing Volunteer)