define usage of function macro

define definition function macro usage: 1. Define a simple calculation macro, "#define SQUARE(x) ((x) * (x))"; 2. Define a macro with multiple parameters, "#define MAX(a, b) ((a) > (b) ? (a) : (b))"; 3. Define macros with complex expressions, "#define ABS(x) ((x ) < 0 ? -(x) : (x))".

Function macro is a special type of macro defined using `#define`, which is used to use a piece of code as the replacement text of the macro. The basic syntax of a function macro is as follows:

#define 宏名(参数列表) 替换文本

In a function macro, the parameter list is optional and the replacement text is a code fragment. When the preprocessor encounters a call to a function macro, it replaces the parameters of the function macro with the actual parameters and inserts the replacement text into the code.

The following are some common uses of function macros:

1. Define simple calculation macros:

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

When using `SQUARE(5)` in code, preprocessing The compiler will replace it with `((5) * (5))`, which is `25`.

2. Define a macro with multiple parameters:

#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`.

3. Define macros with complex expressions:

#define ABS(x) ((x) < 0 ? -(x) : (x))

When using `ABS(-5)` in your code, the preprocessor will replace it with `((-5 ) < 0 ? -(-5) : (-5))`, that is, `5`.

It should be noted that function macros are just simple text replacement, without type checking and scope restrictions. Therefore, care needs to be taken when using function macros to avoid potential errors and side effects. In addition, since the function macro is replaced during the preprocessing stage, its parameters are not evaluated multiple times. Therefore, expressions with side effects should be avoided in function macros.

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