C++ Read and write multiple objects of same class

Question

airport air(1,2,3); //an airport constructor
ofstream myfile;
myfile.open("rishab",ios::app||ios::binary);
myfile.write((char*)air,sizeof(airport);
myfile.close();

Such commands are called multiple times in my program to get info of many airports. Basically the binary file is full of airports. I need to read all these airports into an array later on. How do I read the file so that I get the array of airports.

Apologies if this question is too basic. I am in high school learning about pointers and shortest path graphs.

Answer 1

What you are trying to do is serialization. This way of serializing objects is not stable, and highly depends on what airport is. It's better to use explicit serialization.

Here's a description of what serialization is and why is it made this way.

In MessagePack a typical serialization-deserialization scenario would look like this:

struct airport {
 std::string name; //you can name your airports here
 int planeCapacity;
 int acceptPlanesFrom;
 MSGPACK_DEFINE(name,planeCapacity,acceptPlanesFrom);
};

...

// define your airports
std::vector<airport> airports;
airport a={"BLA",1,2};
airport b={"BLB",3,4};
airports.push_back(a);
airports.push_back(b);

// create a platform-independent byte sequence from your data
msgpack::sbuffer sbuf;
msgpack::pack(sbuf, airports) ;
std::string data=sbuf.data();//you can write that into a file

msgpack::unpacked msg;
// get your data safely back
msgpack::unpack(&msg, sbuf.data(), sbuf.size());
msgpack::object obj = msg.get();

std::cout<<obj<<std::endl;

// now convert the bytes back to your objects
std::vector<airport> read_airports;
obj.convert(&read_airports);
std::cout<<read_airports.size()<<std::endl;

with the console output:

[["BLA", 1, 2], ["BLB", 3, 4]]
2

• some more

Answer 2

Well, if you're sure that your file is valid, then you can simply use read() until you reach EOF. Each read() - of sizeof(airport)- will give you a valid airport object.

Note that storing the binary "value" of and object will result in an invalid object when loading it if it contains pointers - or references.

EDIT: myfile.write((char*)&air,sizeof(airport); will write the content of the air object the file. By doing this, you're actually writing the object, not the pointer.

Answer 3

ofstream myfile;

std::vector<airport> vec;
myfile.open("rishab",ios::app||ios::binary);
while(myfile.write(reinterpret_cast<char*>(&air),sizeof(airport)) != 0)
   vec.push_back(air);

myfile.close();

Now use vec for processing

Answer 4

You can program it like this.

struct AirPort
{
    int a;
    int b;
    int c;
};
int main()
{
    std::vector<AirPort> airportList;
    FILE* fp = fopen(filename,"rb");
    if( NULL != fp)
    {
        while(!feof(fp))
        {
            AirPort ap;
            if (fread(&ap,sizeof(ap),1,fp)==1)
            {
                airportList.push_back(ap);
            }
        }
    }
    fclose(fp);
    return 0;
}