编译器添加的优化引起“最终”的不同行为。方法 [英] Compiler added optimization causes different behavior for "final" methods

问题描述

struct B { 
  virtual void foo ()
  { cout << "B::foo()\n"; }
};

struct D : B { 
  void foo () //final
  { cout << "D::foo()\n"; }
};

int main ()
{
  B *pB = new B;
  D *pD = static_cast<D*>(pB);
  pB->foo();
  pD->foo();
}

输出预期行为：

B::foo()
B::foo()

如果我们使 D :: foo（） final，那么输出是令人愉快的：

If we make the D::foo() final, then the output is pleasantly different:

B::foo()
D::foo()

$ b b

这意味着 virtual 功能不会在使用指针/引用调用该方法时被踢入，该类的方法声明为 final 。

也就是说， final 不仅仅是一个编译时检查，也有助于运行时行为。

Which means that virtual functionality is not kicked-in when the method is invoked with pointer/reference of a class which has that method declared as final.
Also it means that, final isn't just a compile-time check but also contributes to runtime behavior.

这是所有编译器的标准行为。我已经用g ++ 4.7测试。

Is it a standard behavior for all compilers. I have tested with g++4.7.

编辑：

生成一个新问题。关闭此问题。

推荐答案

D *pD = static_cast<D*>(pB);

使用此语句，您放弃了获得健全程序行为的权利。如果给出 static_cast 实际上不是类型 D c>或 D 的派生类之一（它不是）。

With this statement, you gave up the right to having sane program behavior. C++ does not require this operation to work if what static_cast is given is not actually of type D or one of D's derived classes (which it isn't).

有一个原因为什么 dynamic_cast 存在;一个适当的 dynamic_cast 会很快失败，返回 nullptr 非法投射。

There's a reason why dynamic_cast exists; a proper dynamic_cast would have quickly failed on this, returning nullptr for an illegal cast.

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