A constant declaration statement is similar to a variable declaration and initialisation statement, except that the reserved word const is added in front of the statement in constant declaration. For example:
| C++ statements | Equivalent Pascal statements |
| const float x = 3.7; | const x = 3.7; |
| const int a = 4, b = 7; | const a = 4; b = 7; |
Note that the value of a constant cannot be modified.
An alias is a name (usually more meaningful, easier to remember or type) used in a program. Before compilation, the alias will be translated into another name or string (usually less meaningful, less easy to remember or type).
We can use #define preprocessor directives to define an alias. For
example, the statement
#define MAX 100
defines an alias called MAX which is identical to 100. During compile time, any occurrence of MAX in
the program will be replaced by 100:
| #include <iostream.h> #define MAX 100 ... int a[MAX]; ... for (i = 0; i < MAX; i++) ... if (n >= MAX * 2) ... |
... is translated to ... | #include <iostream.h> ... int a[100]; ... for (i = 0; i < 100; i++) ... if (n >= 100 * 2) ... |
As you can see, #define preprocessor directives can be used to define constants. But the main differences between using const and using #define is that the const statement tells the program to allocate some memory space to store the constant, while the aliases defined by the #define preprocessor directives are not stored in memory; they are translated to their original name during compilation.
In fact, most C++ programmers prefer using #define preprocessor directives to define constants.
See Program 24 for a program using #define preprocessor directives.