Parameter passing mechanisms: how to pass a parameter

Information, information, information
- When a function is invoked (called) with some parameters, some information of the parameter are COPIED into the (formal) parameter variables
- WHAT information is being copied depends on the parameter passing mechanism
- The parameter passing mechanism defines HOW or WHAT INFORMATION from the actual parameter variable is COPIED into the (formal) parameter variables
- Also, since different type of information will be used in a different manner, the parameter passing mechanism also defines HOW the (invoked) function will USE the given (passed) information
Commonly used parameter passing mechanisms
- Pass-by-value: copies the value of the actual parameter into the (formal) parameter variable
- Pass-by-reference: copies the reference (address) of the actual parameter into the (formal) parameter variable

Passing parameters BY VALUE

A parameter is passed by value when:
- the VALUE of the actual parameter is copied into the (formal) parameter variable

Example:

int main(int argc, char *argv[]) { int x, y; void f(int, int); // pass by value x = 10; y = 100; cout << "Before f(x,y), x = " << x << ", y = " << y << "\n"; f(x, y); cout << "After f(x,y), x = " << x << ", y = " << y << "\n"; } void f(int a, int b) { cout << "Before increment a = " << a << ", b = " << b << "\n"; a = a + 10000; b = b + 10000; cout << "After increment a = " << a << ", b = " << b << "\n"; }

Example Program: (Demo above code) --------- click here

What is going on inside the computer

The following figure shows what is going on inside the computer when parameters are passed "by value":

In "call-by-value", the value of the actual parameter variables are copied into the (formal) parameter variables.

Consequently, when the program need to use the value of a parameter, it reads the value stored in the parameter variable to obtain the value.

Also, when the program need to store a new value into a parameter variable, it also uses the memory space reserved for the parameter variable and store the new value in that location. (Thus, the original variable is NOT affected !)

Passing parameters BY REFERENCE

A parameter is passed by reference when:
- the REFERENCE or ADDRESS of the actual parameter is copied into the (formal) parameter variable

Example:

int main(int argc, char *argv[]) { int x, y; void f(int &, int &); // pass by reference x = 10; y = 100; cout << "Before f(x,y), x = " << x << ", y = " << y << "\n"; f(x, y); cout << "After f(x,y), x = " << x << ", y = " << y << "\n"; } void f(int & a, int & b) { cout << "Before increment a = " << a << ", b = " << b << "\n"; a = a + 10000; b = b + 10000; cout << "After increment a = " << a << ", b = " << b << "\n"; }

Example Program: (Demo above code) --------- click here

What is going on inside the computer

The following figure shows what is going on inside the computer when parameters are passed "by reference":

In "call-by-reference", the address of the actual parameter variables are copied into the (formal) parameter variables.

Consequently, when the program need to use the value of a parameter, it uses the address value to locate the variable in memory and obtain the value.

Also, when the program need to store a new value into a parameter variable, it also uses the address value in the parameter variable to locate the variable in memory and store the new value in that location.