c++ - 将指针从派生类方法转换为基类

Question

假设我想创建一个层次结构来响应以字符串编码的特定事件。例如来自网络的命令。想法是有一个 Base 类处理网络连接，接收缓冲区，拆分它等，命令 react 的处理在派生类中(派生类也可以添加一些新词来处理)。所以我的解决方案是:

class Base {
public:
    typedef void (Base::*Method)();
    typedef std::unordered_map<std::string, Method> Methods;

    void reactToA();
    void reactToB();

    Base() :
       methods{ 
          { "A", &Base::reactToA },
          { "B", &Base::reactToB }
       }
    {
    }
    void action( const std::string &s )
    {
        auto f = methods.find( s );
        if( f != methods.end() )
           (*this.*)(f->second)();
    }

protected:
    Methods methods;
};

class Child : public Base {
public:
    void reactToB();
    void reactToC();

    Child() {
        methods[ "B" ] = static_cast<Method>( &Child::reactToB );
        methods[ "C" ] = static_cast<Method>( &Child::reactToC );
    }
};

所以我必须将指向 Child 方法的指针转换为指向 Base 方法的指针。类型转换定义明确吗？是否有更优雅(或正确，如果这导致 UB)解决方案？

Answer 1

来自 [expr.static.cast]:

A prvalue of type “pointer to member of D of type cv1 T” can be converted to a prvalue of type “pointer to member of B” of type cv2 T, where B is a base class (Clause 10) of D, if a valid standard conversion from “pointer to member of B of type T” to “pointer to member of D of type T” exists (4.11), and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1. [...] If class B contains the original member, or is a base or derived class of the class containing the original member, the resulting pointer to member points to the original member. Otherwise, the behavior is undefined.

在我们的例子中，&Base::reactToB可以转换为&Child::reactToB , 但自 Base 不 包含原始成员，则行为未定义。

您必须存储类似 std::function<void(Base*)> 的内容或 void(*)(Base*) .

如果是前者，您可以向Base 添加一个成员函数。喜欢:

template <typename C>
void addMethod(std::string const& name, void (C::*method)()) {
    methods[name] = [method](Base* b){
        (static_cast<C*>(b)->*method)();
    };
}

addMethod("B", &Child::reactToB);

如果是后者，你可以这样做:

methods[ "B" ] = +[](Base* b){ 
   static_cast<Child*>(b)->reactToB();
};