What is the purpose of defining in C++?

In C++, the #define directive is used to create macro definitions. Macro definitions are a type of preprocessor directive that performs text replacement during the compilation phase. The #define directive allows you to define an identifier and assign it a value or code snippet.

The functions of macro definitions include:

1. Defining constants: You can use macro definitions to define constants, allowing you to reuse the same value multiple times in your code, improving readability and maintainability. For example:

#define PI 3.14159
const double pi = PI;

1. Creating function-style macros: You can use macro definitions to create function-style macros, which insert code snippets into the program during the compilation phase to achieve code reuse and simplification. For example:

#define MAX(a, b) ((a) > (b) ? (a) : (b))
int maxNum = MAX(10, 20);

1. Conditional compilation: You can use macro definitions to selectively compile code based on different conditions. For example:

#define DEBUG
#ifdef DEBUG
    // 调试代码
#else
    // 发布代码
#endif

It is important to note that macros are only for text replacement and do not involve type checking or syntax analysis, which may lead to hidden issues. Therefore, it is advisable to carefully consider the scenarios and potential problems when using macros.