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 注意：先做类的声明和抽象基类的声明

抽象基类的函数方法中引入类，具体方法在类的实现后面声明。

在抽象基类的子类的函数中可以调用类的成员函数。

#include <iostream>using namespace std;class Contex;class state {
public:virtual void Handel( Contex* contex) = 0;
};class Contex {
public:Contex(state* _state) :State(_state) {};void changeState( state* _state){State = _state;}void showState(){if (State != nullptr){State->Handel(this);}}void showNum(){cout << num << endl;}
private:state *State = nullptr;int num = 10;};class state1 :public state {
public:void Handel(Contex* contex) {cout << "状态1" << endl;}
};class state2 :public state {
public:void Handel(Contex* contex) {cout << "状态2" << endl;}
};class state3 :public state {
public:void Handel(Contex* contex) {cout << "状态3" << endl;contex->showNum();}
};int main()
{state *myState1 = new state1();state *myState2 = new state2();Contex *contex = new Contex(myState1);contex->showState();contex->changeState(myState2);contex->showState();state* myState3 = new state3();contex->changeState(myState3);contex->showState();return 0;
}