帮我看看这个头文件该怎么写？　看着简单却不好实现呢

donaroo 2008-07-06 12:57:29

目的：
　　　在头文件中定义函数和全局变量（同时赋初值），并且如果已经定义过了就不再定义，而只是声明。

我下面的做法不能成功：



#ifndef MYTEST_H

#define MYTEST_H

extern "C" int g_i = 1;

extern "C" int JustTest(){

return g_i;

}

#else

extern "C" int JustTest();

#endif

在被两个ｃｐｐ文件引用过这个头文件后会提示重复定义。　　
我知道把定义部分放在ｃｐｐ文件中，头文件只放声明可以解决这个问题。　　　能不能只通过一个头文件来解决？

各位高手出招

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donaroo 2008-07-19

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后来也想了想，问题是在这里，引起内存分配的东西不该放在头文件里

mymtom 2008-07-09

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[Quote=引用楼主 donaroo 的帖子:]
目的：
在头文件中定义函数和全局变量（同时赋初值），并且如果已经定义过了就不再定义，而只是声明。

我下面的做法不能成功：
C/C++ code#ifndef MYTEST_H#defineMYTEST_Hextern"C"intg_i=1;extern"C"intJustTest(){returng_i;
}#elseextern"C"intJustTest();#endif

在被两个ｃｐｐ文件引用过这个头文件后会提示重复定义。
我知道把定义部分放在ｃｐｐ文件中，头文件只放声明可以解决这个问题。　　　能不能只通过一…
[/Quote]
仔细看过才发现楼主的想法有问题。
头文件不是定义函数和全局变量的正确地方。
头文件的作用是声明函数和全局变量，以及定义宏和数据结构。
函数和全局变量定义应该在.c文件里。

简单的说：
.h文件不应该引起内存分配。
引起内存分配的语句都应该放在.c里。

mymtom 2008-07-09

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其实，变量的声明需要用extern, 函数的声明可以不用extern的。
比如getopt.h



/*      $NetBSD: getopt.h,v 1.4 2000/07/07 10:43:54 ad Exp $    */

/*      $FreeBSD: src/include/getopt.h,v 1.6 2004/02/24 08:09:20 ache Exp $ */



/*-

 * Copyright (c) 2000 The NetBSD Foundation, Inc.

 * All rights reserved.

 *

 * This code is derived from software contributed to The NetBSD Foundation

 * by Dieter Baron and Thomas Klausner.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the above copyright

 *    notice, this list of conditions and the following disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 * 3. All advertising materials mentioning features or use of this software

 *    must display the following acknowledgement:

 *        This product includes software developed by the NetBSD

 *        Foundation, Inc. and its contributors.

 * 4. Neither the name of The NetBSD Foundation nor the names of its

 *    contributors may be used to endorse or promote products derived

 *    from this software without specific prior written permission.

 *

 * THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS

 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED

 * TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR

 * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS

 * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR

 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF

 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS

 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN

 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)

 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE

 * POSSIBILITY OF SUCH DAMAGE.

 */



#ifndef _GETOPT_H_

#define _GETOPT_H_



#include <sys/cdefs.h>



/*

 * GNU-like getopt_long()/getopt_long_only() with 4.4BSD optreset extension.

 * getopt() is declared here too for GNU programs.

 */

#define no_argument        0

#define required_argument  1

#define optional_argument  2



struct option {

        /* name of long option */

        const char *name;

        /*

         * one of no_argument, required_argument, and optional_argument:

         * whether option takes an argument

         */

        int has_arg;

        /* if not NULL, set *flag to val when option found */

        int *flag;

        /* if flag not NULL, value to set *flag to; else return value */

        int val;

};



__BEGIN_DECLS

int     getopt_long(int, char * const *, const char *,

        const struct option *, int *);

int     getopt_long_only(int, char * const *, const char *,

        const struct option *, int *);

#ifndef _GETOPT_DECLARED

#define _GETOPT_DECLARED

int      getopt(int, char * const [], const char *);



extern char *optarg;                    /* getopt(3) external variables */

extern int optind, opterr, optopt;

#endif

#ifndef _OPTRESET_DECLARED

#define _OPTRESET_DECLARED

extern int optreset;                    /* getopt(3) external variable */

#endif

__END_DECLS

 

#endif /* !_GETOPT_H_ */

mymtom 2008-07-09

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楼主的想法很好，可以参考一些现成的库的写法，俺比较推荐这种写法。



/*-

 * Copyright (c) 1991, 1993

 *      The Regents of the University of California.  All rights reserved.

 *

 * This code is derived from software contributed to Berkeley by

 * Berkeley Software Design, Inc.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the above copyright

 *    notice, this list of conditions and the following disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 * 4. Neither the name of the University nor the names of its contributors

 *    may be used to endorse or promote products derived from this software

 *    without specific prior written permission.

 *

 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND

 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE

 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

 * SUCH DAMAGE.

 *

 *      @(#)cdefs.h     8.8 (Berkeley) 1/9/95

 * $FreeBSD: src/sys/sys/cdefs.h,v 1.88.2.2 2006/01/31 17:57:16 stefanf Exp $

 */



#ifndef _SYS_CDEFS_H_

#define _SYS_CDEFS_H_



#if defined(__cplusplus)

#define __BEGIN_DECLS   extern "C" {

#define __END_DECLS     }

#else

#define __BEGIN_DECLS

#define __END_DECLS

#endif



/* ... */



#endif /* !_SYS_CDEFS_H_ */



/*-

 * Copyright (c) 1990, 1993

 *      The Regents of the University of California.  All rights reserved.

 * ...

 * ...

 *

 *      @(#)stdio.h     8.5 (Berkeley) 4/29/95

 * $FreeBSD: src/include/stdio.h,v 1.56.8.1 2006/01/31 17:57:17 stefanf Exp $

 */



#ifndef _STDIO_H_

#define _STDIO_H_



#include <sys/cdefs.h>

#include <sys/_null.h>

#include <sys/_types.h>



/* ... */



#define stdin   __stdinp

#define stdout  __stdoutp

#define stderr  __stderrp



__BEGIN_DECLS

/*

 * Functions defined in ANSI C standard.

 */

void     clearerr(FILE *);

int      fclose(FILE *);

int      feof(FILE *);          

int      ferror(FILE *);

int      fflush(FILE *);

int      fgetc(FILE *);



/* ... */

__END_DECLS

#endif /* !_STDIO_H_ */

tesling 2008-07-09

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楼主，就算找到了一种好的方法又有什么意义呢

头文件声明，实现文件定义，反过来能有什么好处呢

如果在只有几个文件的工程中，所有定义和声明都放在一个cpp文件中也可以搞定，但是这样根本没有意义

xkyx_cn 2008-07-09

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单独放在一个cpp中，在h中声明，不挺好的吗？难道这样很难实现？

晨星 2008-07-09

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不能，头文件天生是用来被人包含的，所以两个以上的cpp文件包含同一个头文件是很平常的事情。
因此，头文件中应该只放一些声明性的东西。
当然，也可以有类定义、模板、内联函数定义等等。说白了，就是只要编译器允许在多个编译单元（一个cpp以及它所直接或间接包含的头文件）中多次出现的，都适合放头文件，否则，就只能放cpp源文件。

donaroo 2008-07-08

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谢谢前边的回复~~ 可是就不能只用一个头文件实现么其他地方用这些函数或变量的时候仅引用一个头文件就可以

K行天下 2008-07-06

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shi8430419 2008-07-06

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头文件中只要申明就够了，具体的定义放到CPP中去

非完美主义者 2008-07-06

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2楼的方法好。
在头文件找那个将函数声明也extern吧

artman 2008-07-06

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头文件
#ifndef _MYTEST_H_
#define _MYTEST_H_
extern int g_i; //变量声明
int JustTest(); //函数声明

#endif //#ifndef _MYTEST_H_

在任意.CPP或新建一个cpp中实现它们
#include "mytest.h"

int g_i=1; //实现
int JustTest() //实现
{
return g_i;
}

其他CPP 可任意#include 该头文件。

楼上的做法，似乎越搞越乱！

du51 2008-07-06

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在当前CPP文件最顶部...定义
#ifndef XXX_CPP_
#define XXX_CPP_
#endif

在头文件里
#ifdef XXX_CPP_
void print_hello()
{
printf("Hello\n");
}
#else
void print_hello();
#endif

另建议EXTERN "C"不要一个一个的来...直接全括起来..看起来清晰点.