-> arrow operator

参考资料

• stackoverflow
• Thinking in C++ 12.3.4.2 ~ 12.3.4.4

stackoverflow 最高票翻译

• ->
  它必须被定义为非静态成员函数(原因是需要this指针), 且没有参数. 它的返回值 被用来 调用 该返回值的成员
  如果该返回值是另一个类类型, 而非指针, 那么就会继续调用该类类型的operator->函数. 这种行为被称为"drill-down behavior". 直到最后一个返回的是指针为止.

struct client
    { int a; };

struct proxy {
    client *target;
    client *operator->() const
        { return target; }
};

struct proxy2 {
    proxy *target;
    proxy &operator->() const
        { return * target; }
};

void f() {
    client x = { 3 };
    proxy y = { & x };
    proxy2 z = { & y };

    std::cout << x.a << y->a << z->a; // print "333"
}

->* .* 和 .

• ->*
  这个运算符并不特殊,和普通的二目运算符相似, 也没有必须为非静态成员的限制.
  若为非静态成员, 参考Thinking in c++ , 专门为类设计了存放函数的类. 该类是一个类函数, ->* 在返回处 调用它的构造函数, 执行时会把目标函数的参数传给该类的迭代器的operator()

• .* 和 .
  这两个不能重载

#include <iostream>
using namespace std;
class AB {
public:
    int a;
    int b;
    typedef void (AB::*action)() const;
    AB():a(1),b(2){}
    AB(const int&x, const int &y):a(x),b(y){}
    void func_1() const {
        cout <<"AB::func_1()"<<endl;
    }
    void func_2() const {
        cout <<"AB::func_2()"<<endl;
    }
};

class ABset {
    typedef AB* ABP;
    AB ab[20];
public:
    friend class absiterator;
    typedef class absiterator iterator;
    ABset(){
        for(int i=0;i<20;i++){
            ab[i].a = i;
            ab[i].b = i*2;
        }
    }
    ABP begin(){
        return &(ab[0]);
    }
};
class absiterator {
    typedef AB& ABRef;
    typedef AB* ABP;
    typedef AB::action ABACT;
    typedef ABP & ABPRef;
    typedef absiterator __Self;
    ABP abptr;
public:
    
    absiterator(ABP abp):abptr(abp){}
    absiterator():abptr(NULL){}
    
    ABRef operator*(){
        return *abptr;
    }
    ABP operator->(){
        return abptr;
    }
    ABPRef operator=(AB *abp){
        abptr = abp;
        return abptr;
    }
    ABPRef operator+(int n){
        abptr += n;
        return abptr;
    }
};
struct Dog{
    Dog(){cout <<"\nDog()"<<endl;}
    int run(int a){
        cout <<"run "<<a<<endl;
        return a;
    }
    int sleep(int b){
        cout <<"sleep "<<b<<endl;
        return b;
    }
    int eat(int c){
        cout <<"eat "<<c<<endl;
        return c;
    }
    typedef int (Dog::*action)(int); 
    class FuncIter {
        Dog* ptr;
        Dog::action act;
    public:
        FuncIter(Dog* wp, action ai): ptr(wp), act(ai) {
            cout <<"construction FuncIter"<<endl;
        } 
        int operator()(int a){
            cout <<"operator()"<<endl;
            return (ptr->*act)(a);
        }
     };
    class FuncIter operator->*(action ai){
        cout <<"operator->*"<<endl;
        return FuncIter(this,ai);
    }
};

int main ()
{
    ABset ax;
    ABset::iterator ai = ax.begin();
    //AB::action act = &AB::func_2;
    ai = ai+2;
    cout <<ai->a<<endl;
    cout <<(*ai).b<<endl;
    ai->func_1();
        
    Dog dg;
    Dog::action dact = &Dog::run;
    (dg->*dact)(3);
    return 0;
}