Detailed explanation of define usage

define Usage: 1. Define constants; 2. Define function macros: 3. Define conditional compilation; 4. Define multi-line macros.

`define` is one of the directives of the C/C preprocessor, used to define macros. Its basic syntax is as follows:

#define 宏名 替换文本

When the preprocessor encounters the `#define` directive, it will replace the macro name with the specified replacement text. When you use a macro name in your code, the preprocessor replaces it with the corresponding replacement text before compilation.

The following are some common uses of `define`:

1. Define constants:

#define PI 3.14159

When using `PI` in the code, the preprocessor will replace it is `3.14159`.

2. Define function macro:

#define SQUARE(x) ((x) * (x))

When using `SQUARE(5)` in the code, the preprocessor will replace it with `((5) * (5))` , that is `25`.

3. Define conditional compilation:

#define DEBUG

Use `#ifdef` or `#ifndef` in the code to determine whether the macro is defined. For example:

#ifdef DEBUG
    // 调试代码
#endif

If the `DEBUG` macro is defined, the preprocessor will compile the code in the `//debug code` section.

4. Define multi-line macro:

#define MAX(a, b) \
    ((a) > (b) ? (a) : (b))

When using `MAX(5, 10)` in the code, the preprocessor will replace it with `((5) > (10 ) ? (5) : (10))`, that is, `10`.

It should be noted that `define` is just a simple text replacement, without type checking and scope restrictions. Therefore, care needs to be taken when using macros to avoid potential errors and side effects.

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