相关疑难解决方法(0)

CAT *p;
...
p->speak();
...

有些书说编译器会将p-> speak()翻译为:

(*p->vptr[i])(p); //i is the idx of speak in the vtbl

我的问题是:因为在编译时,不可能知道p的实际类型,这意味着不可能知道要使用哪个vptr或vtbl.那么,编译器如何生成正确的代码呢？

[改性]

例如:

void foo(CAT* c)
{
    c->speak();
    //if c point to SmallCat
    // should translate to (*c->vptr[i])(p); //use vtbl at 0x1234   
    //if c point to CAT
    // should translate to (*c->vptr[i])(p); //use vtbl at 0x5678  

    //since ps,pc all are CAT*, why does compiler can generate different code for them 
    //in compiler time?
}

...
CAT *ps,*pc;
ps = new SmallCat;  //suppose SmallCat's …

我想知道vtable何时创建？它是在main()之前的启动代码还是在其他时间点？