How is the usage of define in C++?

In C++, #define is a preprocessing directive used to define macros. Macros are a simple text replacement mechanism that can be used to define constants, functions, and conditional compilation, among other things.

The syntax format of #define is:

#define 宏名 值

The macro name is an identifier that can be any legal identifier name, while the value can be a constant, an expression, or a function.

Here are some common usages of #define:

1. Define a constant:
2. Declare a constant value for the mathematical constant PI as 3.14159 and set a maximum size constant as 100.
3. Define a function macro:
4. Define the function SQUARE(x) as the square of x, and MAX(a, b) as the maximum value between a and b.
5. Conditional compilation:
6. #ifdef DEBUG
   // Code to execute in debug mode
   #endif

   #ifndef DEBUG
   // Code to execute in non-debug mode
   #endif

7. Prohibition of redefining macros:
8. #ifndef MACRO_NAME
   #define MACRO_NAME
   // Definition of the macro
   #endif

The scope of a macro definition is from where it is defined until the end of the source file or until another #undef directive is encountered. Macro value replacement occurs in the pre-processing stage before compilation, simply replacing text without any type checking. Therefore, when using macro definitions, it is important to pay attention to their syntax and replacement rules in order to avoid potential errors.