這個(gè)模式也叫責(zé)任鏈模式。這種模式解決的是一個(gè)請求可能由多個(gè)類處理的問題，可以處理的多個(gè)類可以按照優(yōu)先級先后決定自己能否處理請求。

以下代碼定義了可以處理請求的接收者。以處理整型數(shù)字為例，兩個(gè)子類分別處理正數(shù)和負(fù)數(shù)。

#include <iostream>

using namespace std;

class Receiver
{
public:
    virtual ~Receiver(){}
    virtual bool Receive(int value) = 0;
};

class PositiveReceiver:public Receiver
{
private:
    Receiver* NextReceiver;
public:
    PositiveReceiver(Receiver* rcvr):NextReceiver(rcvr){}
    bool Receive(int value)
    {
        if (value > 0)
        {
            cout<<__PRETTY_FUNCTION__<<endl;
            return true;
        }
        else
        {
            if (NextReceiver)
            {
                return NextReceiver->Receive(value);
            }
        }
        return false;
    }
};

class NegativeReceiver:public Receiver
{
private:
    Receiver* NextReceiver;
public:
    NegativeReceiver(Receiver* rcvr):NextReceiver(rcvr){}
    bool Receive(int value)
    {
        if (value < 0)
        {
            cout<<__PRETTY_FUNCTION__<<endl;
            return true;
        }
        else
        {
            if (NextReceiver)
            {
                return NextReceiver->Receive(value);
            }
        }
        return false;
    }
};

以下代碼定義了一個(gè)請求發(fā)送者。它可以發(fā)送一個(gè)處理數(shù)字的請求。

class Requester
{
private:
    Receiver* receiver;
public:
    Requester(Receiver* rcvr):receiver(rcvr){}
    void Request(int value)
    {
        cout<<__FUNCTION__<<" value="<<value<<endl;
        bool bResult = receiver->Receive(value);
        cout<<__FUNCTION__<<" result="<<bResult<<endl<<endl;
    }
};

以下代碼演示了職責(zé)鏈模式。首先建立負(fù)數(shù)處理對象，再用它建立正數(shù)處理對象，最后請求者將請求發(fā)給正數(shù)處理對象。正數(shù)處理對象會優(yōu)先處理，遇到不大于0的數(shù)，就會將數(shù)字給下一個(gè)處理者——負(fù)數(shù)處理對象。
在這個(gè)處理鏈中有一個(gè)漏洞，就是數(shù)字0沒人處理，這在實(shí)踐中要注意避免。

int main(void){
    Receiver* NR = new NegativeReceiver(nullptr);
    Receiver* PR = new PositiveReceiver(NR);
    Requester R(PR);
    R.Request(1);
    R.Request(0);
    R.Request(-1);
    delete NR;
    delete PR;
    return 0;
}

輸出

Request value=1
virtual bool PositiveReceiver::Receive(int)
Request result=1

Request value=0
Request result=0

Request value=-1
virtual bool NegativeReceiver::Receive(int)
Request result=1