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使用标准C++的类型�{换符�Q�static_cast、dynamic_cast、reinterdivt_cast、和const_cast

blues — Tue, 23 Oct 2007 12:28:00 GMT

使用标准C++的类型�{换符�Q?strong style="color: black; background-color: #ffff66;">static_cast、dynamic_cast、reinterdivt_cast、和const_cast�?/p>

3.1 static_cast
用法�Q?strong style="color: black; background-color: #ffff66;">static_cast < type-id > ( exdivssion )
该运��符把exdivssion转换为type-id�c�d��Q�但没有�q�行时类型检查来保证转换的安全性。它主要有如下几�U�用法：
①用于类层次�l�构中基�c�d��子类之间指针或引用的转换�?br>　　�q�行上行转换�Q�把子类的指针或引用转换成基�c�表�C�）是安全的�Q?br>　　�q�行下行转换�Q�把基类指针或引用�{换成子类表示�Q�时�Q�由于没有动态类型检查，所以是不安全的�?br>②用于基本数据类型之间的转换�Q�如把int转换成char�Q�把int转换成enum。这�U��{换的安全性也要开发�h员来保证�?br>③把�I�指针�{换成目标�c�d��的空指针�?br>④把��M��c�d��的表辑ּ�转换成void�c�d��?/p>

注意�Q?strong style="color: black; background-color: #ffff66;">static_cast不能转换掉exdivssion的const、volitale、或者__unaligned属性�?/p>

3.2 dynamic_cast
用法�Q�dynamic_cast < type-id > ( exdivssion )
该运��符把exdivssion转换成type-id�c�d��的对象。Type-id必须是类的指针、类的引用或者void *�Q?br>如果type-id是类指针�c�d��Q�那么exdivssion也必��L��一个指针，如果type-id是一个引用，那么exdivssion也必��L��一个引用�?/p>

dynamic_cast主要用于�c�d��ơ间的上行�{换和下行转换�Q�还可以用于�c�M��间的交叉转换�?br>在类层次间进行上行�{换时�Q�dynamic_cast�?strong style="color: black; background-color: #ffff66;">static_cast的效果是一��L��Q?br>在进行下行�{换时�Q�dynamic_cast��h��c�d��查的功能�Q�比static_cast更安全�?br>class B{
public:
int m_iNum;
virtual void foo();
};

class D:public B{
public:
char *m_szName[100];
};

void func(B *pb){
D *pd1 = static_cast(pb);
D *pd2 = dynamic_cast(pb);
}

在上面的代码�D�中�Q�如果pb指向一个D�c�d��的对象，pd1和pd2是一��L��Q��ƈ且对�q�两个指针执行D�c�d��的�Q何操作都是安全的�Q?br>但是�Q�如果pb指向的是一个B�c�d��的对象，那么pd1��是一个指向该对象的指针，对它�q�行D�c�d��的操作将是不安全的（如访问m_szName�Q�，
而pd2��是一个空指针�?/p>

另外要注意：B要有虚函敎ͼ�否则会编译出错；static_cast则没有这个限制�?br>�q�是�׃��q�行时类型检查需要运行时�c�d��信息�Q�而这个信息存储在�cȝ��虚函数表�Q?br>关于虚函数表的概念，详细可见�Q�中�Q�只有定义了虚函数的�c�L��有虚函数表，
没有定义虚函数的�c�L��没有虚函数表的�?/p>

另外�Q�dynamic_cast�q�支持交叉�{换（cross cast�Q�。如下代码所�C��?br>class A{
public:
int m_iNum;
virtual void f(){}
};

class B:public A{
};

class D:public A{
};

void foo(){
B *pb = new B;
pb->m_iNum = 100;

D *pd1 = static_cast(pb); //compile error
D *pd2 = dynamic_cast(pb); //pd2 is NULL
delete pb;
}

在函数foo中，使用static_cast�q�行转换是不被允许的�Q�将在编译时出错�Q�而��?dynamic_cast的�{换则是允许的�Q�结果是�I�指针�?/p>

3.3 reindivter_cast
用法�Q�reindivter_cast (exdivssion)
type-id必须是一个指针、引用、算术类型、函数指针或者成员指针�?br>它可以把一个指针�{换成一个整敎ͼ�也可以把一个整数�{换成一个指针（先把一个指针�{换成一个整敎ͼ�
在把该整数�{换成原类型的指针�Q�还可以得到原先的指针��|��?/p>

该运��符的用法比较多�?/p>

3.4 const_cast
用法�Q�const_cast (exdivssion)
该运��符用来修改�c�d��的const或volatile属性。除了const 或volatile修饰之外�Q?type_id和exdivssion的类型是一��L��?br>帔R��指针被�{化成非常量指针，�q�且仍然指向原来的对象；
帔R��引用被�{换成非常量引用，�q�且仍然指向原来的对象；帔R��对象被�{换成非常量对象�?/p>

Voiatile和const�c�试。�D如下一例：
class B{
public:
int m_iNum;
}
void foo(){
const B b1;
b1.m_iNum = 100; //comile error
B b2 = const_cast(b1);
b2. m_iNum = 200; //fine
}
上面的代码编译时会报错，因�ؓb1是一个常量对象，不能对它�q�行改变�Q?br>使用const_cast把它转换成一个常量对象，��可以对它的数据成员��L��改变。注意：b1和b2是两个不同的对象�?br>

== ===========================================

== dynamic_cast .vs. static_cast
== ===========================================

class B { ... };
class D : public B { ... };

void f(B* pb)
{

D* pd1 = dynamic_cast(pb);

D* pd2 = static_cast(pb);
}

If pb really points to an object of type D, then pd1 and pd2 will get the same value. They will also get the same value if pb == 0.

If pb points to an object of type B and not to the complete D class, then dynamic_cast will know enough to return zero. However, static_cast relies on the programmer’s assertion that pb points to an object of type D and simply returns a pointer to that supposed D object.

�?/span>dynamic_cast可用于��承体�p�M��的向下�{型，卛_��基类指针转换为派生类指针�Q�比static_cast更严格更安全�?/span>dynamic_cast在执行效率上�?/span>static_cast要差一些，�?/span>static_cast在更宽上范围内可以完成映��，�q�种不加限制的映��伴随着不安全性�?/span>static_cast覆盖的变换类型除�c�d��ơ的静态导航以外，�q�包括无映射变换、窄化变�?/span>(�q�种变换会导致对象切�?/span>,丢失信息)、用VOID*的强制变换、隐式类型变换等...

== ===========================================
== static_cast .vs. reinterdivt_cast
== ================================================

reinterdivt_cast是�ؓ了映��到一个完全不同类型的意思，�q�个关键词在我们需要把�c�d��映射回原有类型时用到它。我们映��到的类型仅仅是��Z��故弄玄虚和其他目的，�q�是所有映��中最危险的�?/span>(�q�句话是C++�~�程思想中的原话)

static_cast �?/span> reinterdivt_cast 操作�W�修改了操作数类型。它们不是互逆的�Q?/span> static_cast 在编译时使用�c�d��信息执行转换�Q�在转换执行必要的检��?/span>(诸如指针��界计算, �c�d��?/span>). 其操作数相对是安全的。另一斚w��Q?/span>reinterdivt_cast 仅仅是重新解释了�l�出的对象的比特模型而没有进行二�q�制转换�Q?/span> 例子如下�Q?/span>

int n=9; double d=static_cast < double > (n);

上面的例子中, 我们��一个变量从 int 转换�?/span> double�?/span> �q�些�c�d��的二�q�制表达式是不同的�?/span> 要将整数 9 转换�?/span> 双精度整�?/span> 9�Q?/span>static_cast 需要正��地为双�_�ֺ�整数 d 补��比特位。其�l�果�?/span> 9.0。�?/span>reinterdivt_cast 的行为却不同:

int n=9;

double d=reinterdivt_cast (n);

�q�次, �l�果有所不同. 在进行计��以�?/span>, d 包含无用�?/span>. �q�是因�ؓ reinterdivt_cast 仅仅是复�?/span> n 的比特位�?/span> d, 没有�q�行必要的分�?/span>.

因此, 你需要�}慎��?/span> reinterdivt_cast.

blues 2007-10-23 20:28 发表评论

offsetof 解析

blues — Mon, 22 Oct 2007 08:02:00 GMT

1offsetof(s,m)解析 offsetof(s,m)解析
2
3今天看代码时�Q�发��C��个有用的东东�Q�offsetof(s,m)�Q�这是一个宏�Q�定义如下：
4
5 #define offsetof(s,m) (size_t)&(((s *)0)->m)
6
7 然后到网上查了一下，发现�q�真的是很有用，附带一位大侠的解说�Q?br> 8
9  struct   AAA
10  {
11  int   i;
12  int   j;
13  };
14
15  struct AAA *pAAA;
16  pAAA=new AAA;
17  �q�时�Q�pAAA实际上是一个Pointer, 指向某一��定的内存地址�Q�比�?x1234;
18  �?nbsp;pAAA->i 整体是一个int型变量，其地址�?/span>&(pAAA->i) ,'&'为取址�q�算�W�；
19  那么&(pAAA->i)一定等�?x1234,因�ؓi是结构体AAA的第一个元素�?nbsp;
20  �?/span>&(pAAA->j)一定是0x1234 + 0x4 = 0x1238; 因�ؓsizeof(int) = 4;
21
22  �q�个做法的��y妙之处就是：它把“0”作�ؓ上例中的pAAA,那么 &(pAAA->j)��是j�?nbsp;
23  offset�?br>24
25  解析�l�果是：
26  (s   *)0 ,��?nbsp;0 强制转换为Pointer to   "s"
27  可以�?nbsp;pS = (s *)0 �Q�pS是指向s的指针，它的值是0;
28  那么pS->m��是m�q�个元素了，�?/span>&(pS->m)��是m的地址�Q�而在本例中就是offset�?nbsp;
29
30  再把�l�果强制转换为size_t型的��OK 了，size_t其实也就是int啦！�Q?nbsp;
31
32
33

blues 2007-10-22 16:02 发表评论

blues — Fri, 26 Jan 2007 06:55:00 GMT

http://blog.cnetnews.com.cn/hand/keji/3658/

blues 2007-01-26 14:55 发表评论

blues — Mon, 11 Dec 2006 07:39:00 GMT

1 通过VARIANT;
2   VARIANT   varData;(出参)
3
4   MYSTRUCT    * pMyData    =    NULL;
5
6   pMyData    =    (MYSTRUCT * )CoTaskMemAlloc( sizeof (MYSTRUCT));
7   .
8
9   varData.byref    =    (LPVOID)pMyData;
10
11   在调用文仉��Q�同样定义  �?br /> 12   VARIANT   varData(入参�Q�  �?br /> 13
14   MYSTRUCT    * pMyData    =    NULL;
15
16   pMyData    =    (MYSTRUCT * )varData.byref;
17
18   CoTaskMemFree((LPVOID)pMyData);

blues 2006-12-11 15:39 发表评论

Getting an (ATL) ActiveX control to print from Office Applications

blues — Tue, 07 Nov 2006 09:38:00 GMT

13 votes for this article.

Popularity: 4.73. Rating: 4.25 out of 5.

Introduction

Seeing as this is my first ever post to CodeProject, let me do a quick introduction as to who I am and what I do. I have been working in one form of C and C++ or another for as long as I can remember (among the other myriad of languages that I've run into). Nowadays, most of my development is focused on Microsoft Windows platforms, and is done in VC6, VC2002.NET. I am heavily entrenched in BI (Business Intelligence) development, and in my spare time develop little ActiveX controls and games etc...

The plot

Having learnt COM a while ago, I made the obvious progression to ATL to ease the development of boilerplate code, and to leverage off Microsoft's template library. As my experience grew, I ventured into creating ActiveX controls using the ATL framework ... and life was good. I could spew out a fairly useful (albeit not overly complex) control within a short period of time. Recently, I was asked to create a KPI (Key Performance Indicator) control that could be embedded in a web page and an Excel document. Obviously based on my experience (which was obviously not vast) I thought that this would be no problem and off I went, creating code that would meet the functional spec (we all work to these don't we :)).

A couple of days later the control was finished and the final tests were being run when someone asked me to print a hardcopy of an example spreadsheet with the embedded control. This is where my nightmares began. Not only did my control not print, but there was no clear indication as to why it didn't print. And so my exploration into this apparent mystery began.

Have you ever tried to include 3rd party ActiveX controls into an Office document? They sure seem to work fine, but most (apart from the Microsoft controls) don't seem to render themselves when you request a Print Preview or a simple Print of the worksheet or document. So, if any of you have ever had this problem, or have never dabbled with this, but think that you may be heading this way, take note of this, cos it might save you hours of frustration and frantic searching on MSDN and Google.

So what now?

The first thing one needs to realize is that even though we have been blessed with Office 2000 and Office XP, the printing architecture still uses the old Windows-format metafile for its printing operations. This metafile format was used in 16-bit Windows-based applications (thinks back to Win3.1). Now, this becomes a major problem for ActiveX developers who wish their controls to be printable from within Office applications, because this old metafile format only supported a limited set of GDI functionality. The list of supported GDI functions can be found here.

Now that you are armed with your limited function set, you cringe with the realization that you can no longer create memory DC's, you can no longer use your lovely DrawText() functions and you can definitely no longer call GetTextExtentPoint32() function. However, those realizations only hold true for the instance of when your control is being rendered to an old format metafile. So how do we empower our control to know that its being rendered to an old format metafile? Simple, we use the GetObjectType() function and check if the result is equal to OBJ_METADC (old metafile format):

HRESULT Cxxxxx::OnDraw(ATL_DRAWINFO& di)
{
    HDC hdc = di.hdcDraw;
    bool bMetaFile = false;

    //// lets check if we're drawing to an old// metafile format.. (like Office printing)//if ( GetObjectType(hdc) == OBJ_METADC )
        bOldMetaFile = true;

    //// the rest of your code...//
}

For interest, the opposite of OBJ_METADC is OBJ_ENHMETADC (refer to this MSDN document).

Now that we know if we're drawing to an old metafile format or not, we can write adaptive code to cater for each instance or we can just write all our drawing logic using the limited set of functionality that is supported by the old metafile DC.

What about fonts and text extents?

As any ATL ActiveX developer knows, using fonts in AX controls provides for limited amount of fun. The typical piece of code would probably look something like this:

																//
																// ... some code
																//
    CComQIPtr pFont(m_pFont);
    TEXTMETRICOLE tm;if ( pFont != NULL )
    {
        pFont->get_hFont(&newFont);
        pFont->AddRefHfont(newFont);
        pFont->QueryTextMetrics(&tm);
        oldFont = (HFONT) SelectObject(dc, newFont);
    }

The Bolded lines of code are ones that I didn't use regularly, due to the fact that I didn't really need to know about the breakdown of my font's details because I had access to GetTextExtentPoint32() function. Unfortunately, in this scenario, we don't have access to that function to determine how wide (in pixels) our text is going to be. But there is another way to calculate this fairly accurately, as is demonstrated in the code below:

																//
																// assume that we have called QueryTextMetrics() and
																// have a filled TEXTMETRICOLE structure called tm
																//
CComBSTR strText(_T("Hello, world"));
SIZE sz;

sz.cx = strText.Length() * tm.tmAveCharWidth;
sz.cy = tm.tmHeight;

Having said this, there are many other functions that I use a lot that I can't use if I want my ActiveX control to be printable by Office, but as with GetTextExtentPoint32() and its respective replacement, there is always a way to replace these functions using Old-Metafile-Safe-Drawing-Code (OMSDC). *maybe that acronym will catch on*

Conclusion

When creating an ActiveX control that you know will be used inside Office applications, and will most probably be printed, remember to stick to these guidelines when developing your drawing logic. I was fairly shocked by how little information was available in the MSDN and online in general, while I was searching for information on how to enable my ActiveX control to print from within an Office application. There are hundreds of documents on ActiveX controls being printed from within Internet Explorer, but none address this particular issue. Perhaps I was looking in the wrong places. Hopefully this article will help one or more of you one day ;)

Acknowledgment

Many thanks to Igor Tandetnik for pointing me in the right direction on this.

About Peter Mares

blues 2006-11-07 17:38 发表评论

blues — Tue, 07 Nov 2006 08:53:00 GMT

http://www.czvc.com/down.asp?id=105

blues 2006-11-07 16:53 发表评论

blues — Tue, 12 Sep 2006 06:08:00 GMT

http://www.ibm.com/developerworks/cn/downloads/im/udbexp/

blues 2006-09-12 14:08 发表评论

blues — Fri, 25 Aug 2006 09:53:00 GMT

�q�美国还有这�{�喷饭的法律�Q�实属大开眼界�Q�看来有��Z��要好好善加利用了�Q�！��C��各位一起分享：
��国联邦法律规定�Q��?

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2�Q�每周四�?:00以后不得放P。（以后�q�真要小心了�Q�别一不留��坐牢了�q�不知�ؓ
啥）
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无论��M��时候，��冰�Ȁ淋卷攑֜�口袋里是�q�法的。（有病丫的�Q��?

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��h��可以合法殴打其配偶�Q�但每月最多一�ơ。（估计很多东北的兄弟知道了一定想
�U�L��阿肯色了�Q�可也有例外呀�Q�克林顿��是阿肯色的前州长，咋老被喜莱莉扁呀�Q��?

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袜，那将��N��|�。（兄弟们千万注意了呀�Q�别全脱了）
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晚期患者的��音�Q��?

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不得在�ʎ室唱歌。（难怪在宑֤�商学院的同胞都不会K歌）

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闷ING�Q��?

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奶就拉肚子的朋友从犹大�{到纽�U�了�Q�保命要紧呀。）
2�Q�不得在正在执行急救��d��的救护�R后��上Make Love。（�q�好理解�Q�怕病人看见血��?
爆裂么！哈哈�Q��?/div>

blues 2006-08-25 17:53 发表评论

上�v印象

blues — Wed, 23 Aug 2006 07:21:00 GMT

blues 2006-08-23 15:21 发表评论