3.11. Parameters and Variables are Local

Parameters and variables only exist inside their own functions. Within the confines of main, there is no such thing as phil. If you try to use it, the compiler will complain. Similarly, inside print_twice there is no such thing as argument. The value of argument in main is copied into the new variable phil when print_twice is called.

The following code will show the output of the print_twice function. Notice that it is the argument 'b' that is outputted, not the variable 'phil'.

Understanding Parameters
 1#include <iostream>
 2
 3void print_twice (char phil) {
 4    std::cout << phil << phil << '\n';
 5}
 6
 7int main () {
 8    char argument = 'b';
 9    print_twice (argument);
10    return 0;
11}

Try This!

Make the following changes to the previous program:

  • Change the value of 'phil' before printing.

  • Print the value of 'argument' after print_twice is called.

Are the results what you expect?

Variables like this are said to be local. In order to keep track of parameters and local variables, it is useful to draw a stack diagram. Like state diagrams, stack diagrams show the value of each variable, but the variables are contained in larger boxes that indicate which function they belong to.

For example, the stack diagram for print_twice looks like this:

stack diagram showing the print_twice call and its parameter

Fig. 3.1 Stack diagram for print_twice

Keeping similar names separated from eachother in C++ is called scope -- and is one of the most important and powerful concepts in C++ (or any programming language).

Whenever a function is called, it creates a new instance of that function (a new scope) and places it on top of the function call stack. Each instance of a function contains the parameters and local variables for that function. In Fig. 3.1, an instance of a function is represented by a box with the name of the function in the first section and the variables and parameters inside. A instance of a function on the stack are stored in activation records.

In the example, main has one local variable, argument, and no parameters. print_twice has no local variables and one parameter, named phil.

Q1

How many local variables and parameters does main have?

void print_hello_name (string name) {
  cout << "Hello " << name << '!';
}

int main () {
  string name1 = "Phil";
  print_hello_name(name1);
  string name2 = "Joe";
  print_hello_name(name2);
  return 0;
}

Q2

How many local variables and parameters does print_hello_name have?

void print_hello_name (string name) {
  cout << "Hello " << name << '!';
}

int main () {
  string name1 = "Phil";
  print_hello_name(name1);
  string name2 = "Joe";
  print_hello_name(name2);
  return 0;
}

Q3

Whenever we make a function call, we create a(n) of that fucntion, which contiains the parameters and local variables for that function.

Q4

How many calls to hi are made during the exectuion of the entire program?

void hi() {
  cout << "hiii !"<<'\n';
}

void print_greeting(){
  hi();
  cout<<"how are you doing today. "<<'\n';
  hi();
}

int main () {
  hi();
  print_greeting();
  hi();
  return 0;
}

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