subject_23231.htm

一些关于vc的问答

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序号：23231 发表者：符号 发表日期：2002-12-02 16:56:42
<br>主题：相同的源文件，在TC3和VC6下的结果为什么不同？
<br>内容：相同的源文件，在TC3和VC6下的结果为什么不同？<BR>下面是源文件：<BR>#include&lt;stdio.h&gt;<BR>void copy_string(char *from,char *to)<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;while((*from)!='\0')<BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*to=*from;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from++;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;to++;<BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR>&nbsp;&nbsp;&nbsp;&nbsp;*to='\0';<BR>}<BR><BR>void main()<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;char *a="I am a teacher.";<BR>&nbsp;&nbsp;&nbsp;&nbsp;char *b="you are a student.";<BR>&nbsp;&nbsp;&nbsp;&nbsp;printf("string_a=%s\n string_b=%s\n",a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;copy_string(a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;printf("\nstring_a=%s\nstring_b=%s",a,b);<BR>}<BR>在TC3下的结果<BR>string_a=I am a teacher.<BR>string_b=you are a student.<BR><BR>string_a=I am a teacher.<BR>string_b=I am a teacher.<BR>在VC6下的结果<BR><BR><BR>
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<font color=red>答案被接受</font><br>回复者：孟德II 回复日期：2002-12-02 18:23:25
<br>内容：最重要的是由于Windows 2000的内存管理机制不同于Dos模式的内存管理机制；其次是编译器认为下属定义中的a,b是字符串常量：<BR>char *a="I am a teacher.";<BR>char *b="you are a student.";<BR>作为字符串常量的指针，a,b只能读写字符，不能改写字符的内容。<BR>所以执行 *to=*from；时编译器认为这是错误的。<BR>如果你把上述两句改为<BR>char a[]="I am a teacher.";<BR>char b[]="you are a student.";<BR>再次编译时就会正常了。附件是我编译的结果
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回复者：tjhe 回复日期：2002-12-02 20:10:52
<br>内容：字符指针变量a与b被字符串常量（常量指针）初始化，即a与b当前指向的空间作为常量空间，不能修改，即VC下的*a=*b编译没错，有运行错误。（虽然a与被声明为指针变量，即其可以向其它空间，当指向非常量空间时，可以修改它们指向的空间）。另外还要保证复制的目标有足够的空间，否则也可能出现类似错误。<BR><BR>#include&lt;stdio.h&gt;<BR>#define MAXCHARS&nbsp;&nbsp; 256&nbsp;&nbsp;&nbsp;&nbsp;/*定义目标空间大小*/<BR>void copy_string(char *from,char *to)<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;while((*from)!='\0')<BR>&nbsp;&nbsp;&nbsp;&nbsp;{<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*to=*from;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;from++;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;to++;<BR>&nbsp;&nbsp;&nbsp;&nbsp;}<BR>&nbsp;&nbsp;&nbsp;&nbsp;*to='\0';<BR>}<BR><BR>void main()<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;char a[]="I am a teacher.";<BR>&nbsp;&nbsp;&nbsp;&nbsp;char b[MAXCHARS]="you are a student.";&nbsp;&nbsp;/*应确保目标地有足够的空间*/<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;printf("string_a=%s\n string_b=%s\n",a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;copy_string(a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;printf("\nstring_a=%s\nstring_b=%s",a,b);<BR>}<BR><BR>主函数也可写为：<BR>void main()<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;char *a="I am a teacher.";<BR>&nbsp;&nbsp;&nbsp;&nbsp;char b[MAXCHARS]="you are a student.";&nbsp;&nbsp;/*应确保目标地有足够的空间*/<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;printf("string_a=%s\n string_b=%s\n",a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;copy_string(a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;printf("\nstring_a=%s\nstring_b=%s",a,b);<BR>}<BR>或<BR>void main()<BR>{<BR>&nbsp;&nbsp;&nbsp;&nbsp;char *a="I am a teacher.";<BR>&nbsp;&nbsp;&nbsp;&nbsp;char *b=（char*)malloc(MAXCHARS); /*应确保目标地有足够的空间*/<BR><BR>&nbsp;&nbsp;&nbsp;&nbsp;printf("string_a=%s\n string_b=%s\n",a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;copy_string(a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;printf("\nstring_a=%s\nstring_b=%s",a,b);<BR>&nbsp;&nbsp;&nbsp;&nbsp;free(b);<BR>}<BR><BR>这种字符串的拷贝函数也最好同标准函数相同的习惯，即目标参数在前，源串在后，如：<BR>char* stringcopy(char* dest,const char* src);<BR>2002-12-2 20:17:35