将指向一个派生类的方法的指针转换为指向基类方法的指针 [英] Casting a pointer to a method of a derived class to a pointer to a method of a base class

问题描述

将指向某个派生类方法的指针转换为指向基类方法的指针是合法的，即使基类没有声明任何方法，特别是casted方法的类型通过类型基类的对象调用，遵循：

Is it legal to cast a pointer to a method of derived class to a pointer to a method of base class, even though the base class does not declare any methods, especially of the "casted" method is called through an object of type base class, as follows:

// works in VS 2008 and g++ 4.5.3
struct Base
{
};

struct Fuu : public Base
{
    void bar(){ std::cout << "Fuu::bar" << std::endl; }
    void bax(){ std::cout << "Fuu::bax" << std::endl; }
};

struct Foo : public Base
{
    void bar(){ std::cout << "Foo::bar" << std::endl; }
    void bax(){ std::cout << "Foo::bax" << std::endl; }
};

typedef void (Base::*PtrToMethod)();

int main()
{
    PtrToMethod ptr1 = (PtrToMethod) &Foo::bax;
    PtrToMethod ptr2 = (PtrToMethod) &Fuu::bax;

    Base *f1 = new Foo;
    Base *f2 = new Fuu;

    (f1->*ptr1)();
    (f2->*ptr2)();
}

推荐答案

目标对象是逆变的，因为这个实际上是传递给函数的参数，并且参数理论上是逆变的（如果函数可以使用 Base * ，它可以安全地插入任何只提供 Derived * 作为实际参数的算法。

It's worth noting that the reason the target object is contravariant is because this is effectively a parameter being passed to the function, and parameters are, in theory, contravariant (if the function can use a Base*, it can be safely plugged into any algorithm which provides only Derived* as the actual arguments).

但是，对于任意参数，如果基本子对象未放置在派生类布局的开头，则可能需要垫片来调整指针。对于指向成员的指针，此指针的指针调整内置到语言中。 （因此，指向成员的类与虚拟继承的指针可能会非常大）

However, for arbitrary parameters, a shim may be needed to adjust the pointer, if the base subobject is not placed at the beginning of the derived class layout. With pointer-to-members, pointer adjustment for the this pointer is built into the language. (And for this reason, pointer-to-member-of-class-with-virtual-inheritance can get quite large)

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