如果需要局部变量，则使用花括号并在里面放入break语句。将左花括号放在case语句的同一行switch (a) {
/* OK */
case 0: {
int32_t a, b;
char c;
a = 5;
/* ... */
break;
}
/* Wrong */
case 1:
{
int32_t a;
break;
}
/* Wrong, break shall be inside */
case 2: {
int32_t a;
}
break;
}
宏和预处理指令总是使用宏而不是文字常量，特别是对于数字所有的宏必须是全大写的，并带有下划线_字符(可选)，除非它们被明确标记为function，将来可能会被常规函数语法替换/* OK */
#define MY_MACRO(x) ((x) * (x))
/* Wrong */
#define square(x) ((x) * (x))
总是用圆括号保护输入参数/* OK */
#define MIN(x, y) ((x) < (y) ? (x) : (y))
/* Wrong */
#define MIN(x, y) x < y ? x : y
总是用括号保护最终的宏计算/* Wrong */
#define MIN(x, y) (x) < (y) ? (x) : (y)
#define SUM(x, y) (x) + (y)
/* Imagine result of this equation using wrong SUM implementation */
int32_t x = 5 * SUM(3, 4); /* Expected result is 5 * 7 = 35 */
int32_t x = 5 * (3) + (4); /* It is evaluated to this, final result = 19 which is not what we expect */
/* Correct implementation */
#define MIN(x, y) ((x) < (y) ? (x) : (y))
#define SUM(x, y) ((x) + (y))
当宏使用多个语句时，使用do-while(0)语句保护它typedef struct {
int32_t px, py;
} point_t;
point_t p; /* Define new point */
/* Wrong implementation */
/* Define macro to set point */
#define SET_POINT(p, x, y) (p)->px = (x); (p)->py = (y) /* 2 statements. Last one should not implement semicolon */
SET_POINT(&p, 3, 4); /* Set point to position 3, 4. This evaluates to... */
(&p)->px = (3); (&p)->py = (4); /* ... to this. In this example this is not a problem. */
/* Consider this ugly code, however it is valid by C standard (not recommended) */
if (a) /* If a is true */
if (b) /* If b is true */
SET_POINT(&p, 3, 4);/* Set point to x = 3, y = 4 */
else
SET_POINT(&p, 5, 6);/* Set point to x = 5, y = 6 */
/* Evaluates to code below. Do you see the problem? */
if (a)
if (b)
(&p)->px = (3); (&p)->py = (4);
else
(&p)->px = (5); (&p)->py = (6);
/* Or if we rewrite it a little */
if (a)
if (b)
(&p)->px = (3);
(&p)->py = (4);
else
(&p)->px = (5);
(&p)->py = (6);
/*
* Ask yourself a question: To which `if` statement `else` keyword belongs?
*
* Based on first part of code, answer is straight-forward. To inner `if` statement when we check `b` condition
* Actual answer: Compilation error as `else` belongs nowhere
*/
/* Better and correct implementation of macro */
#define SET_POINT(p, x, y) do { (p)->px = (x); (p)->py = (y); } while (0) /* 2 statements. No semicolon after while loop */
/* Or even better */
#define SET_POINT(p, x, y) do { \ /* Backslash indicates statement continues in new line */
(p)->px = (x); \
(p)->py = (y); \
} while (0) /* 2 statements. No semicolon after while loop */
/* Now original code evaluates to */
if (a)
if (b)
do { (&p)->px = (3); (&p)->py = (4); } while (0);
else
do { (&p)->px = (5); (&p)->py = (6); } while (0);
/* Every part of `if` or `else` contains only `1` inner statement (do-while), hence this is valid evaluation */
/* To make code perfect, use brackets for every if-ifelse-else statements */
if (a) { /* If a is true */
if (b) { /* If b is true */
SET_POINT(&p, 3, 4);/* Set point to x = 3, y = 4 */
} else {
SET_POINT(&p, 5, 6);/* Set point to x = 5, y = 6 */
}
} |
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