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沒本

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發(fā)表于 2012-02-01 11:42 |只看該作者 |倒序瀏覽

本帖最后由沒本于 2012-02-01 18:45 編輯

定義一個帶參數(shù)的宏

#define MACRO_P(A) \
{ \
some.a = A; \
some.b = A+1; \
some.c[A] = 0; \
}
/* 用法，參數(shù)約定為只能是整型常量，范圍在0..50 */
MACRO_P(0);
MACRO_P(33);

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其中參數(shù)A使用時必為常量，且在0..50范圍內(nèi)。那么如何在編譯時判斷越界并報錯停止編譯，而不是在運行時？
有可能實現(xiàn)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語言最早官方支持，當(dāng)然C和C++也都實現(xiàn)了，能保證程序的正確性并且沒有運行時開銷：
1. C_1x標(biāo)準(zhǔn)提供了_Static_assert()。gcc 4.6以后開始支持。
文件s.c

/* C_1x standard _Static_assert version */
#include <stdio.h>
#define R 50
#define M(A) \
{ \
_Static_assert( A>=0 && A<=R, "in M(A) A out of range" ); \
s.a = A; \
s.b = A+1; \
s.c[A] = 0; \
}
typedef struct {
int a;
int b;
int c[R+1];
} s_t;
s_t s;
int main(int argc, char ** argv)
{
int i=0;
M(51);
M(i);
return 0;
}
/* compile time error message:
$ gcc -o s s.c
s.c: In function ‘main’:
s.c:20:2: error: static assertion failed: "in M(A) A out of range"
s.c:21:2: error: expression in static assertion is not constant
$
*/

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2.一個不完美的數(shù)組下標(biāo)不能為負(fù)的方案，ANSI C實現(xiàn)。
文件:a.c

/* ANSI C version */
#include <stdio.h>
#define STATIC_ASSERT(COND,MSG) typedef char static_assertion_##MSG[(COND)?1:-1]
#define R 50
#define M(A) \
{ \
STATIC_ASSERT( A>=0 && A<=R, in_M_A_out_of_range ); \
s.a = A; \
s.b = A+1; \
s.c[A] = 0; \
}
typedef struct {
int a;
int b;
int c[R+1];
} s_t;
s_t s;
int main(int argc, char ** argv)
{
int i=0;
M(51);
M(i);
return 0;
}
/* compile time error message:
$ gcc -o a a.c
a.c: In function ‘main’:
a.c:21:1: error: size of array ‘static_assertion_in_M_A_out_of_range’ is negative
$
不完美，變量i作參數(shù)沒有報錯，原因是C99支持
Z:\>cl a.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
Copyright (C) Microsoft Corporation. All rights reserved.
a.c
a.c(21) : error C2118: negative subscript
a.c(22) : error C2057: expected constant expression
a.c(22) : error C2466: cannot allocate an array of constant size 0
Z:\>
msvc由于不支持C99，反而能找到第二個問題。
*/

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3. 利用gcc擴展的實現(xiàn)
文件:n.c

/* GCC version */
#include <stdio.h>
#define CTC(X) ({ extern int __attribute__((error("assertion failure: '" #X "' not true"))) compile_time_check(); ((X)?0:compile_time_check()),0; })
#define R 50
#define M(A) \
{ \
CTC( A>=0 && A<=R ); \
s.a = A; \
s.b = A+1; \
s.c[A] = 0; \
}
typedef struct {
int a;
int b;
int c[R+1];
} s_t;
s_t s;
int main(int argc, char ** argv)
{
int i=0;
M(51);
M(i);
return 0;
}
/* compile time error message:
$ gcc -o n n.c
n.c: In function ‘main’:
n.c:21:2: error: call to ‘compile_time_check’ declared with attribute error: assertion failure: 'i>=0 && i<=R' not true
n.c:22:2: error: call to ‘compile_time_check’ declared with attribute error: assertion failure: 'i>=0 && i<=R' not true
$
*/

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4.利用位域的C實現(xiàn)，但用了__COUNTER__

/* ANSI C version 2 */
#include <stdio.h>
#define CTASTR2(pre,post) pre ## post
#define CTASTR(pre,post) CTASTR2(pre,post)
#define STATIC_ASSERT(cond,msg) \
typedef struct { int CTASTR(static_assertion_failed_,msg) : !!(cond); } \
CTASTR(static_assertion_failed_,__COUNTER__)
#define R 50
#define M(A) \
{ \
STATIC_ASSERT( A>=0 && A<=R, in_M_A_out_of_range ); \
s.a = A; \
s.b = A+1; \
s.c[A] = 0; \
}
typedef struct {
int a;
int b;
int c[R+1];
} s_t;
s_t s;
int main(int argc, char ** argv)
{
int i=0;
M(51);
M(i);
return 0;
}
/* compile time error message:
$ gcc -o b b.c
b.c: In function ‘main’:
b.c:25:2: error: zero width for bit-field ‘static_assertion_failed_in_M_A_out_of_range’
b.c:26:2: error: bit-field ‘static_assertion_failed_in_M_A_out_of_range’ width not an integer constant
$
Z:\>cl b.c
Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 16.00.40219.01 for 80x86
Copyright (C) Microsoft Corporation. All rights reserved.
b.c
b.c(25) : error C2149: 'static_assertion_failed_in_M_A_out_of_range' : named bit
field cannot have zero width
b.c(26) : error C2057: expected constant expression
b.c(26) : error C2149: 'static_assertion_failed_in_M_A_out_of_range' : named bit
field cannot have zero width
Z:\>
*/