策略模式与命令模式比较及实现

定义

Strategy策略模式 把易于变化的行为分别封装起来，让它们之间可以互相替换， 让这些行为的变化独立于拥有这些行为的客户。
GoF《设计模式》中说道：定义一系列算法，把它们一个个封装起来，并且使它们可以相互替换。该模式使得算法可独立于它们的客户变化。

Command命令模式是一种对象行为型模式，它主要解决的问题是：在软件构建过程中，“行为请求者”与“行为实现者”通常呈现一种“紧耦合”的问题。
GoF《设计模式》中说道：将一个请求封装为一个对象，从而使你可用不同的请求对客户进行参数化；对请求排队或记录请求日志，以及支持可撤销的操作。

区别

命令模式是含有不同的命令（含有接收者的请求）：做不同的事情；隐藏接收者执行细节。常见菜单事件，
而策略模式含有不同的算法，做相同的事情；
区别在于是否含有接收者。命令模式含有，策略模式不含有。

c++代码

策略模式

#include <iostream.h>
#include <fstream.h>
#include <string.h>

class Strategy;

class TestBed
{
  public:
    enum StrategyType
    {
        Dummy, Left, Right, Center
    };
    TestBed()
    {
        strategy_ = NULL;
    }
    void setStrategy(int type, int width);
    void doIt();
  private:
    Strategy *strategy_;
};

class Strategy
{
  public:
    Strategy(int width): width_(width){}
    void format()
    {
        char line[80], word[30];
        ifstream inFile("quote.txt", ios::in);
        line[0] = '\0';

        inFile >> word;
        strcat(line, word);
        while (inFile >> word)
        {
            if (strlen(line) + strlen(word) + 1 > width_)
              justify(line);
            else
              strcat(line, " ");
            strcat(line, word);
        }
        justify(line);
    }
  protected:
    int width_;
  private:
    virtual void justify(char *line) = 0;
};

class LeftStrategy: public Strategy
{
  public:
    LeftStrategy(int width): Strategy(width){}
  private:
     /* virtual */void justify(char *line)
    {
        cout << line << endl;
        line[0] = '\0';
    }
};

class RightStrategy: public Strategy
{
  public:
    RightStrategy(int width): Strategy(width){}
  private:
     /* virtual */void justify(char *line)
    {
        char buf[80];
        int offset = width_ - strlen(line);
        memset(buf, ' ', 80);
        strcpy(&(buf[offset]), line);
        cout << buf << endl;
        line[0] = '\0';
    }
};

class CenterStrategy: public Strategy
{
  public:
    CenterStrategy(int width): Strategy(width){}
  private:
     /* virtual */void justify(char *line)
    {
        char buf[80];
        int offset = (width_ - strlen(line)) / 2;
        memset(buf, ' ', 80);
        strcpy(&(buf[offset]), line);
        cout << buf << endl;
        line[0] = '\0';
    }
};

void TestBed::setStrategy(int type, int width)
{
  delete strategy_;
  if (type == Left)
    strategy_ = new LeftStrategy(width);
  else if (type == Right)
    strategy_ = new RightStrategy(width);
  else if (type == Center)
    strategy_ = new CenterStrategy(width);
}

void TestBed::doIt()
{
  strategy_->format();
}

int main()
{
  TestBed test;
  int answer, width;
  cout << "Exit(0) Left(1) Right(2) Center(3): ";
  cin >> answer;
  while (answer)
  {
    cout << "Width: ";
    cin >> width;
    test.setStrategy(answer, width);
    test.doIt();
    cout << "Exit(0) Left(1) Right(2) Center(3): ";
    cin >> answer;
  }
  return 0;
}

命令模式

#include <iostream>
using namespace std;

#define SAFE_DELETE(p) if (p) { delete p; p = NULL; }

class Receiver
{
public:
     void Action()
     {
          cout<<"Receiver->Action"<<endl;
     }
};

class Command
{
public:
     virtual void Execute() = 0;
};

class ConcreteCommand : public Command
{
public:
     ConcreteCommand(Receiver *pReceiver) : m_pReceiver(pReceiver){}
     void Execute()
     {
          m_pReceiver->Action();
     }
private:
     Receiver *m_pReceiver;
};

class Invoker
{
public:
     Invoker(Command *pCommand) : m_pCommand(pCommand){}
     void Invoke()
     {
          m_pCommand->Execute();
     }
private:
     Command *m_pCommand;
};

int main()
{
     Receiver *pReceiver = new Receiver();
     Command *pCommand = new ConcreteCommand(pReceiver);
     Invoker *pInvoker = new Invoker(pCommand);
     pInvoker->Invoke();
     SAFE_DELETE(pInvoker);
     SAFE_DELETE(pCommand);
     SAFE_DELETE(pReceiver);
     return 0;
}