DECLARE_HANDLE 宏，#,## 预处理指令

在WINNT.H中有这样一段代码:

#define DECLARE_HANDLE(name) struct name##__ { int unused; }; typedef struct name##__ *name

#define sabc(val) #val
#define glue(a,b) a##b

#define MY_DECLARE_HANDLE(name) struct name##__ { int unused; }; typedef struct name##__ *name

struct HTYPE2__ {int unused;}; typedef struct HTYPE2__ * HTYPE2;

MY_DECLARE_HANDLE(HTYPE);
void CTestGetModuleHandleDlg::OnBtnSendmsg() 
{	
	//MessageBox(sabc(中国));			//相当于 MessageBox("中国");
	/*
	int c=20,ab=0;
	glue(a,b) = ++c;	//相当于 ab = ++c;
	CString str;
	str.Format("%d",ab);
	MessageBox(str);
	*/
	
	HTYPE__ type;
	type.unused = 10;
	HTYPE pType = &type;
	CString str;

	HTYPE2 pT = pType;	//error C2440: 'initializing' : cannot convert from 'struct HTYPE2__ *' to 'struct HTYPE__ *'
	str.Format("%d,%d",type.unused,pType->unused);
	MessageBox(str);
}

 

 

Function macro definitions accept two special operators (# and ##) in the replacement sequence:
If the operator # is used before a parameter is used in the replacement sequence, that parameter is replaced by a string literal (as if it were enclosed between double quotes)

1	#define str(x) #x cout << str(test);

This would be translated into:

cout << "test";

The operator ## concatenates two arguments leaving no blank spaces between them:

1	#define glue(a,b) a ## b glue(c,out) << "test";

This would also be translated into:

cout << "test";

 

现在明白了，之所以定义这个宏去声明 结构体与结构体指针，就是为了让句柄之间转化赋值时能在编绎时检测到不匹配的赋值。

从使用者的角度上看，也非常简练。不得不佩服微软的人，确实是个好点子。