Reference Parameters

Passing parameter by reference,
cpp code

#include <iostream>

using namespace std;

// Swap two values.
void swap(double &a, double &b) 
{
        double tmp = a;
        a = b;
        b = tmp;
}

// Return multiple results.
void stats(double x, double y, double &absdif, double &mean, double &sumsqr)
{
        if(x > y) swap(x, y);

        absdif = y - x;
        mean = (x + y) / 2.0;
        sumsqr = x*x + y*y;
}

int main()
{
        double a = 10.45;
        double b = 20.22;

        cout << a << " " << b << endl;
        swap(a, b);
        cout << a << " " << b << "\n" << endl;

        double c = 4.56;
        double d = -3.45;
        double absdif, mean, ss;
        stats(c, d, absdif, mean, ss);
        cout << c << " " << d << " " << absdif << " " << 
                mean << " " << ss << endl;
        stats(22.0, 41.5, absdif, mean, ss);
        cout << absdif << " " << mean << " " << ss << endl;
}

The usual way of passing parameters in C++, and the only way in C, is by value. Each parameter becomes a copy of its argument, and changes to in the function body do not make changes in the caller. This is similar to Java. C++ provides reference parameters, which are denoted using an & in front of the parameter name. Reference parameters become aliases for their arguments, and changes to the parameters immediately change the arguments. You cannot send an expression argument to a reference parameter; you must send something which could appear on the left side of an assignment.

The swap function is a classic use of reference parameters. When the reference parameters are exchanged, the arguments are exchanged since the parameters are aliases for them. Also notice that the stats function receives x and y by value, and uses swap to exchange its copies, but this does not effect the values of c and d in the main.

Reference parameters are by far the main use of the reference mechanism in C++. It can also be used for return values, and, though it seems to have no practical use, ordinary variables.