"C-language summary" macro definition, preprocessing

Macro definition Simple macro

Format: #define Identifier substitution list

#define N 100int a[N];//N就是100

Macros with parameters

Format: #define identifier (x1, x2, x3,...... xn) Replace list

#define MAX(x, y) ((x) > (y) ? (x) : (y))

Note: When the program compiles, it will replace the uppercase identifier with the original expression, so the program will increase at compile time.

Advantages or Disadvantages

Advantages
① program may be a little faster
② Macros are more versatile
Disadvantages
① compiled code usually gets bigger
② cannot point to a macro with a pointer
③ macro may calculate its parameters more than once

Use of parentheses in macro definitions

① #define TWO_PI (2 * 3.1415)
② #define RES(x) ((x) * 10)
Cause: Because the macro is a substitution expression at compile time, many uncertainties
For example:

#define RES(x) (x * 10) //假如宏是这样定义的，参数没有括号int1);int110//编译器会替换成这样。与结果完全不对

Cancel macro

Format: #define N

Conditional compilation

#if 常量表达式    编译内容#endif

defined运算法仅用于预处理器，N必须是定义的宏#if defined(N)....#endif

The IFDEF directive tests whether an identifier has been defined as a macro

#ifdeft 标示符......#endif

#ifndef 标示符......#endif

"C-language summary" macro definition, preprocessing