指向类成员的指针

本文内容来源：《C＋＋必知必会》

      A a;
    a.a = 5;
    //int A::*pMemberA = &a.a; //Error! cannot convert 'int *' to 'int A::*' 
    int A::*pMemberA = &A::a;
    int B::*pMemberB = pMemberA; //implicitly convert 'int A::*' to 'int B::*'
    //int A::*pMemberA2 = pMemberB; //Error! cannot convert 'int B::*' to 'int A::*' 
    B b;
    b.a = 2011;
    printf("b.*pMemberB = %d \r\n", b.*pMemberB); // output: b.*pMemberB = 2011

上例中这种 'int A::*' to 'int B::*' 的单向转换现象被称为"Contravariance",即“逆变性”。