Although I am a programmer for a long time, I am still a beginner in C++. Sometimes, I saw the argument of a method of class will pass an class object by const reference. I try to figure it out how it works and why to use it. The following is one example from a forum discussion, which clearly explain my question:
Note the following is partly cited from http://www.daniweb.com/software-development/cpp/threads/159715
Let us start with machine.
It has a method increment, Let us say that add a value to to rackID.
That doesn't need to take ANOTHER machine object.
Then we are going to write a method add that takes another machine object and adds their rackIDs together.
[I could use operator+ BUT that adds a complication so I am not. I use a reference because (a) a copy would be expensive (b) you have not provided a copy constructor and that can lead to problems, and you have a string object, so it will.]
Note in add, we add TWO Machine objects and the SECOND object does not change. I reference the second object without changing it. Since A is also an object then you can access A's private members. This would not be the case for saying
void Machine::Test(const Other& X) .
For the first Machine objects we defined, it has a default pointer: this.
Here is the class definition:
#include<iostream>
#include<string>
using namespace std;
class Machine
{
public:
string owner;
string make;
int rackID;
Machine() : rackID(0) {}
virtual ~Machine() {}
virtual void displayMachineDetails();
void increment(const int =1);
void add(const Machine&);
};
void Machine::displayMachineDetails()
{
cout <<"\nMachine owner: " << owner << "\nMake: " << make
<< "\nRack ID: " << rackID << endl;
}
void Machine::increment(const int Index)
{
rackID+=Index;
}
void Machine::add(const Machine& A)
{
rackID+=A.rackID;
return;
}
int main()
{
Machine a = Machine("Me", "IBM", 1);
a.increment(111);
}