Learning C++: Value and Reference Parameters

One of the most difficult concepts for beginning C++ programmers to understand is the difference between passing function parameters by value and passing function parameters by reference.

One of the most difficult concepts for beginning C++ programmers to understand is the difference between passing function parameters by value and passing function parameters by reference. In this article I’m going to explain the difference and demonstrate when and why you should use each parameter type.

Pass by Value Defined

A function parameter is passed by value when the function receives a copy of the parameter. This means the parameter value is stored in the memory reserved for the function call. This is the default behavior of C++.

To demonstrate what passing by value means, here is a simple example:

#include <iostream>
using namespace std;

void changeParam(int param) {
  param += 5;
  cout << "Parameter: " << param << endl;
}
int main ()
{
  int number = 0;
  changeParam(number);
  cout << "Number: " << number << endl;
  return 0;
}

The output from this program is:

Parameter: 5
Number: 0

The variable number is passed as an argument(parameter) to the function. Inside the function, the parameter’s value is increased by 5 and that value is displayed to the screen. The function ends and control passes back to main. The value of number is then accessed and is shown to still be 0.

The variable’s value didn’t change because when number was passed to the function, a copy of its value was placed into the parameter. This value was the value increased by the function body and when the function ended, all the memory associated with the function, which includes the parameter and its value, was erased.

For the most part, this is exactly the behavior we want from C++ functions. Functions are, indeed, written to perform computations but it becomes harder to understand how a program works when the data passed to a function can be changed by the function.

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