学习笔记:候捷深入浅出MFC第二章
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这段代码的输出是:
#1
CDocment::func()
CMyDoc::Serialize()
#2 testing
CDocment::func()
CMyDoc::Serialize()
#3 testing
CDocment::func()
CMyDoc::Serialize()
#4 testing
CDocment::func()
CDocument::Serialize() ------>注意这里
其它的调用都符合多态的规则,注意到#2和#4都是类型的强制转换.但#2是对指针类型的转换,#4是对实例的转换.候捷在书上是这样说的:
所谓的"向上类型转换"将会造成对象的内容被切割:当我们调用这个方法时,mydoc已经是半条命对象了,而func内部调用的虚函数Serialize后者将使用"mydoc"的虚函数指针,虽然存在,但值是什么呢?幸运的是,由于 ((CDocument)mydoc).func();是转值而非传址操作,编译器以所谓的拷贝构造函数把Cdocument对象内容复制了一份,使得mydoc的vtable(虚函数表)与CDocument对象的相同.例子中没有使用拷贝构造函数,但编译器会自动合成一个.这相对于 ((CDocument)mydoc)实际上是返回了一个CDocument的新对象.
一点说明:为什么在标题中要嵌入英文?原因是为了能够让国外的网友能查询到这篇文章。平常在Google上查资料的时候,经常参考国外网友的博客,帮助我解决了很多问题,所以我也想让他们能够参考我写的内容。当然文中我不可能全部译为英文,所以我尽量把代码粘全,靠代码说话吧。