How to Use C++ Constants: Working with Constant Values in C++

C++ constants are not very different from any C++ variable. They are defined in a similar way and have the same data types and the same memory limitations. However, there is one major difference - once a constant has been created and value assigned to it then that value may not be changed.

Defining Constants with C++

It's also worth noting that there are two types of constant: literal and symbolic.

Literal and Symbolic Constants

A literal constant is simply a value used directly in a block of code, for example:

circumference = 2 * 3.14 * r;

area = 3.14 * r * r;

Here 2 and 3.14 (pi to 2 decimal places) are literal constants. However, there is an obvious disadvantage to using literal constants. If the constant needs to be changed (for example changing pi to 4 decimal places) then the programmer will have to manually change all instances of the constant themselves. It is therefore often more practical to use symbolic constants:

circumference = 2 * pi_val * r;

area = pi_val * r * r;

Now the value of the constant only needs to be defined once. Defining C++ Constants with the Preprocessor The preprocessor is run before the program is compiled. It will look for any lines starting with a hash (#) and will then modify the code according to what it finds. An example of a constant defined in the preprocessor is:

#define pi_val 3.1416

When the preprocessor runs it will search the code for all instances of pi_val and replace it with the number 3.1416. Defining C++ Constants with the Const Key Word C++ constants can also be defined in the same way as C++ variables. The only difference is that the const key word is required:

const float pi_val = 3.1416;

One important advantage of using the const key word to define constants is that the data type will always be enforced. Defining Enumerated Constants An enumerated type is a series of named integer values created by using the enum key word, for example:

enum DAY {SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY};

In this example SUNDAY will have the value 0 and SATURDAY will be 6. However, the values can be initialized:

enum DAY {MONDAY = 1, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY};

This time MONDAY will be 1 and SUNDAY will be 7. The use of enumerators is clarified by seeing a working example:

#include

enum DAY {MONDAY = 1, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY};

int main () {

int day;

cout << "Enter day number: ";

cin >> day;

if ((day == SATURDAY) || (day == SUNDAY)) {

cout << "Weekend"; }

else if ((day >= MONDAY) && (day <= FRIDAY)) {

cout << "Working Day";

} else {

cout << "Invalid Number";

}

return 0;

}

If 1 - 5 are input as the day number then the program will output "Working Day" or "Weekend" for 6 or 7. Summary Constants are used in a C++ program to store information that will not change while the program is running. They can be one of two general types: