Parameter Passing

Parameter Passing Mechanisms in C++

There are two ways to pass parameters:

pass by value
pass by reference

Pass by Value

In the case of passing parameters by value, the formal parameters of a function are copies of the actual parameters' values. This means that:

actual parameters can be expressions whose values match the types of formal parameters (or can be implicitly converted to the types of the formal parameters).
the value of the expression (or variable) given as the actual parameter is stored on the stack.
upon exiting the function call, the changes made in the function to the formal parameters have no effect on the actual parameters. The actual parameters remain unchanged!
this is the default way of passing parameters.

Example:

#include < iostream >
using namespace std;

int gcd(int a , int b)
{
    cout << "At the start of the gcd function, a = " << a << " and b = " << b << endl;
    while(b != 0)
    {
        int r = a % b;
        a = b;
        b = r;
    }
    cout << "At the end of the gcd function, a = " << a << " and b = " << b << endl;
    return a;
}

int main()
{
    int x = 24, y = 18;
    cout << gcd(x , y) << endl;
    cout << "After calling the gcd function, x = " << x << " and y = " << y << endl;
    return 0;
}

We observe that although the formal parameters a and b in the gcd function are modified, in main(), after calling the gcd function, the actual parameters x and y have their original values.

Pass by Reference

This is the specific C++ mechanism by which we can modify variables outside the function within the function. In the case of passing parameters by reference, the formal parameters of a function are references to the actual parameters. This means that:

actual parameters can only be variables, or expressions whose results are similar to variables: array elements, structure fields, dereferenced pointers, etc.
the address of the variable given as the actual parameter is stored on the stack.
all changes made in the function call to the formal parameters are actually made to the actual parameters. The actual parameters are modified upon exit from the call!
to specify that a parameter is passed by reference, it will be preceded by the character & in the function's header.

Example:

#include < iostream >
using namespace std;

void doubleValue(int & n)
{
    n = 2 * n;
}

int main()
{
    int x = 24;
    cout << "x = "  << x << endl;
    doubleValue(x);
    cout << "x = "  << x << endl;
    return 0;
}

We observe that, upon exit from the call, the value of the variable x is modified. Moreover, a call like doubleValue(10); represents a syntax error, because the actual parameter must be a variable.

Parameter Passing