What are the functionalities of the define keyword in C…

In C++, #define is the directive used for defining preprocessor macros. Its functions include:

1. Constant Definition: An identifier can be defined as a constant using the #define directive for use in a program. For example: #define PI 3.14159.
2. Define a macro function: You can use the #define directive to define a macro function, so that a segment of code can be replaced in the program through macro calls. For example: #define MAX(a, b) ((a) > (b) ? (a) : (b)).
3. Conditional compilation: Conditional compilation macros can be defined using the #define directive, which allows certain portions of code to be compiled based on specified conditions. For instance, by defining DEBUG with #define, the compilation of debug code can be controlled using #ifdef DEBUG and #endif.
4. Macro substitution refers to replacing macro calls in the source code with the content of the macro definition during the preprocessing stage. For example, if #define N 10, then N can be used in the program to represent 10.

It is important to note that #define is a preprocessor directive that is replaced before compilation, not at runtime. Hence, its purpose is to manipulate the source code during compilation, not modify the program during runtime.