定義一個帶參數的宏
01.#define MACRO_P(A) \
02.{ \
03. some.a = A; \
04. some.b = A+1; \
05. some.c[A] = 0; \
06.}
07./* 用法，參數約定為隻能是整型常量，範圍在0..50 */
08.MACRO_P(0);
09.MACRO_P(33);
複制代碼其中參數A使用時必為常量，且在0..50範圍内。那麼如何在編譯時判斷越界并報錯停止編譯，而不是在運作時？
有可能實作嗎？如果換成C++呢？
公開答案，其實就是用static_assert方法，這個方法是D語言最早官方支援，當然C和C++也都實作了，能保證程式的正确性并且沒有運作時開銷：
1. C_1x标準提供了_Static_assert()。gcc 4.6以後開始支援。
檔案s.c
01./* C_1x standard _Static_assert version */
02.#include <stdio.h>
03.#define R 50
04.#define M(A) \
05.{ \
06._Static_assert( A>=0 && A<=R, "in M(A) A out of range" ); \
07. s.a = A; \
08. s.b = A+1; \
09. s.c[A] = 0; \
10.}
11.typedef struct {
12. int a;
13. int b;
14. int c[R+1];
15.} s_t;
16.s_t s;
17.int main(int argc, char ** argv)
18.{
19. int i=0;
20. M(51);
21. M(i);
22. return 0;
23.}
24.
25./* compile time error message:
26.$ gcc -o s s.c
27.s.c: In function ‘main’:
28.s.c:20:2: error: static assertion failed: "in M(A) A out of range"
29.s.c:21:2: error: expression in static assertion is not constant
30.$
31.*/
複制代碼2.一個不完美的數組下标不能為負的方案，ANSI C實作。
檔案:a.c
01./* ANSI C version */
03.#define STATIC_ASSERT(COND,MSG) typedef char static_assertion_##MSG[(COND)?1:-1]
04.#define R 50
05.#define M(A) \
06.{ \
07.STATIC_ASSERT( A>=0 && A<=R, in_M_A_out_of_range ); \
08. s.a = A; \
09. s.b = A+1; \
10. s.c[A] = 0; \
11.}
12.typedef struct {
13. int a;
14. int b;
15. int c[R+1];
16.} s_t;
17.s_t s;
18.int main(int argc, char ** argv)
19.{
20. int i=0;
21. M(51);
22. M(i);
23. return 0;
24.}
25.
26./* compile time error message:
27.$ gcc -o a a.c
28.a.c: In function ‘main’:
29.a.c:21:1: error: size of array ‘static_assertion_in_M_A_out_of_range’ is negative
31.不完美，變量i作參數沒有報錯，原因是C99支援
32.
33.Z:\>cl a.c
34.Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
35.Copyright (C) Microsoft Corporation. All rights reserved.
36.
37.a.c
38.a.c(21) : error C2118: negative subscript
39.a.c(22) : error C2057: expected constant expression
40.a.c(22) : error C2466: cannot allocate an array of constant size 0
41.
42.Z:\>
43.msvc由于不支援C99，反而能找到第二個問題。
44.*/
複制代碼3. 利用gcc擴充的實作
檔案:n.c
01./* GCC version */
03.#define CTC(X) ({ extern int __attribute__((error("assertion failure: '" #X "' not true"))) compile_time_check(); ((X)?0:compile_time_check()),0; })
07.CTC( A>=0 && A<=R ); \
27.$ gcc -o n n.c
28.n.c: In function ‘main’:
29.n.c:21:2: error: call to ‘compile_time_check’ declared with attribute error: assertion failure: 'i>=0 && i<=R' not true
30.n.c:22:2: error: call to ‘compile_time_check’ declared with attribute error: assertion failure: 'i>=0 && i<=R' not true
31.$
32.*/
複制代碼4.利用位域的C實作，但用了__COUNTER__
01./* ANSI C version 2 */
03.#define CTASTR2(pre,post) pre ## post
04.#define CTASTR(pre,post) CTASTR2(pre,post)
05.#define STATIC_ASSERT(cond,msg) \
06. typedef struct { int CTASTR(static_assertion_failed_,msg) : !!(cond); } \
07. CTASTR(static_assertion_failed_,__COUNTER__)
08.#define R 50
09.#define M(A) \
10.{ \
11.STATIC_ASSERT( A>=0 && A<=R, in_M_A_out_of_range ); \
12. s.a = A; \
13. s.b = A+1; \
14. s.c[A] = 0; \
15.}
16.typedef struct {
17. int a;
18. int b;
19. int c[R+1];
20.} s_t;
21.s_t s;
22.int main(int argc, char ** argv)
23.{
24. int i=0;
25. M(51);
26. M(i);
27. return 0;
28.}
29.
30./* compile time error message:
31.$ gcc -o b b.c
32.b.c: In function ‘main’:
33.b.c:25:2: error: zero width for bit-field ‘static_assertion_failed_in_M_A_out_of_range’
34.b.c:26:2: error: bit-field ‘static_assertion_failed_in_M_A_out_of_range’ width not an integer constant
35.$
37.Z:\>cl b.c
38.Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
39.Copyright (C) Microsoft Corporation. All rights reserved.
40.
41.b.c
42.b.c(25) : error C2149: 'static_assertion_failed_in_M_A_out_of_range' : named bit
43.field cannot have zero width
44.b.c(26) : error C2057: expected constant expression
45.b.c(26) : error C2149: 'static_assertion_failed_in_M_A_out_of_range' : named bit
46.field cannot have zero width
47.
48.Z:\>
49.*/
複制代碼
本文轉自 wws5201985 51CTO部落格，原文連結：http://blog.51cto.com/wws5201985/772303，如需轉載請自行聯系原作者
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