Why put const (...)& in C++

Question

I was wondering - why is putting const (...)& in C++ a good idea? I found this code in some class tutorials. I understand that const is making something constant and it can't be changed, while & is passing it to the function. Why do both? If I can't change something, why should I be passing it to the function?

Here is an example:

class CVector {
      public:
        int x,y;
        CVector () {};
        CVector (int a,int b) : x(a), y(b) {}
        CVector operator + (const CVector&);
    };

    CVector CVector::operator+ (const CVector& param) { //example 
      CVector temp;
      temp.x = x + param.x;
      temp.y = y + param.y;
      return temp;
    }

Answer 1

Even though the const is keeping you from changing the value of your parameter, it's better to send a reference to it, rather than send it by value, because, in large objects, sending it by value would require copying it first, which is not necessary when calling by reference

Answer 2

It speeds up the code execution, it offers usage simplicity and it also ensures the caller that you can not change the value of the variable by setting const keyword. It maybe have some other benefits, but I think the performance is the main reason. In this article and related articles, there are many useful things about References in C++.

When you pass a parameter by const type & (pass by reference) like this:

void printStudent(const Student &s) 
{ 
    cout << s.name << "  " << s.address << "  " << s.rollNo; 
} 

int main(){
    Student s1;
    printStudent(s1);
    return 0;
}

Benefits are:

1. Work easily with object(don't think about pointers)
2. It does not copy the Student object(so it is fast)
3. You can not create a new student and assign it to s variable. if you write s=new Student(); the compiler error will happen. So the caller can pass the variable with more confidence.
4. The parameter can not be null.