C++编程常见错误解决方案：IntelliSense修复与外部链接问题

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1. IntelliSense: #error directive: Please use the /MD switch for _AFXDLL builds.


There's a bug(漏洞) where the settings of a generated MFC project would show "Multi-threaded Debug DLL (/MDd)"; however, it dosn't pass that argument to the intellisense(智能感应) engine. To workaround this problem, please try the following:

Right-click the Project. ------------- Go to Config Properties->C/C++-> Code Gen ->. Double-click "Runtime Library" and set to "Multi-threaded Debug DLL (/MDd)" . If this value already appears to be set, make sure it is by selecting it again (it should then appear as bold).------------------ Click OK
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2. ra.obj   :   error   LNK2001:   unresolved   external   symbol   "public:   void   __thiscall   CEraApp::OnAppAbout(void) "   (?OnAppAbout@CEraApp@@QAEXXZ) 

有把cpp文件加进去。 ---------------这是最常见的原因。你可以在类视图中单击这几个函数，可以发现跳出对话框说找不到实现文件，这就是最明显的原因了。你可以用project-> add   to   project-> files   加入CustomMenu.cpp等几个缺少的文件。先试一试这个，不行在看是否有别的原因，这个是可能性最大的原因。 
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3.error C2664: “BOOL CDC::TextOutW(int,int,const CString &)”: 不能将参数 3 从“const char [18]”转换为“const CString &” 
问题相关分析：


一、 在字符串前加一个L作用: 

   如  L"我的字符串"表示将ANSI字符串转换成unicode的字符串，就是每个字符占用两个字节。

  strlen("asd")=3;   

  strlen(L"asd")=6;

  二、  _T宏可以把一个引号引起来的字符串，根据你的环境设置，使得编译器会根据编译目标环境选择合适的（Unicode还是ANSI）字符处理方式

   如果你定义了UNICODE，那么_T宏会把字符串前面加一个L。这时 _T("ABCD") 相当于 L"ABCD" ，这是宽字符串。 

   如果没有定义，那么_T宏不会在字符串前面加那个L，_T("ABCD") 就等价于 "ABCD"