enum 做索引

enum可以做索引， 配上虚函数，或者函数指针，可以实现上层的统一封装和快速索引。

MoTbl.cpp
#include <stdio.h>
#include        <stdlib.h>

enum {
    eA=0,
    eB,
    eC,
    eD,
    eMax
};

class Bs{
    public:
        virtual void say(){ printf("Bs\n"); }
};
class A: public Bs{
    int aa[1];
    public:
    void say(){printf("A\n");}
    void call(){printf("A::call\n");}

};
class B: public Bs{
    int aa[3];
    public:
    void say(){printf("B\n");}
    void sing(){printf("B::sing\n");}
};
class C: public Bs{
    int aa[10];
    public:
    void say(){printf("C\n");}
};
class D: public Bs{
    int aa[100];
    public:
    void say(){printf("D\n");}
};

class  MoTbl{
    public:
        union{
            struct{
                A *a;
                B *b;
                C *c;
                D *d;
            }t;
            void *tv[eMax];
        };
};

    int
main ( int argc, char *argv[] )
{
    int i = 0;
    MoTbl mt;

    A a;
    B b;
    C c;
    D d;

    printf("A:%d\n",sizeof(MoTbl));

    for(i=0; i<eMax; i++)
        printf("%p\n",mt.tv[i]);
    printf("--------------\n");

    mt.tv[eA] = &a;
    mt.tv[eB] = &b;
    mt.tv[eC] = &c;
    mt.tv[eD] = &d;

    for(i=0; i<eMax; i++)
        printf("%p\n",mt.tv[i]);
    printf("--------------\n");

    mt.t.a->say();
    printf("--------------\n");

    for(i=0; i<eMax; i++){
        switch(i){
            case eA: mt.t.a->say(); break;
            case eB: mt.t.b->say(); break;
            case eC: mt.t.c->say(); break;
            case eD: mt.t.d->say(); break;
        }
    }
    printf("--------------\n");

    for(i=0; i<eMax; i++)
        ((Bs *)mt.tv[i])->say();
    printf("--------------\n");

    for(i=0; i<eMax; i++){
        switch(i){
            case eA: mt.t.a->call(); break;
            case eB: mt.t.b->sing(); break;
        }

    }
    printf("--------------\n");

}

输出

A:16
0xb7726ff4
0xb77238e4
(nil)
0xb7615ff4
--------------
0xbff0cd98
0xbff0cd88
0xbff0cd5c
0xbff0cbc8
--------------
A
--------------
A
B
C
D
--------------
A
B
C
D
--------------
A::call
B::sing
--------------