C++设计模式 ==> 简单工厂模式

简述

简单工厂模式的核心在于使用一个工厂类根据需求动态生成子类对象。以计算器程序作为例子，加减乘除都分别作为运算类的子类，然后让工厂类根据要求动态实例化子类。这种设计模式遵守了“开闭原则”，极大地方便了未来对算法进行维护和扩展，例如未来要添加乘方算法，只需要继承Operation类就可以了，避免了频繁修改Operation类，造成维护困难。

图示

代码实现

/////////////////////////////
//
// @ FileName : Operation.h
// @ Date : 2018-08-06 18:04
//
/////////////////////////////

#pragma once

#include <cstdio>
#include <iostream>
#include <windows.h>

class Operation
{
private:
    double _numberA;
    double _numberB;
public:
    Operation() :_numberA(0), _numberB(0) {}
    void setNumberA(double&);
    void setNumberB(double&);
    double getNumberA() const;
    double getNumberB() const;
    double virtual getResult()
    {
        return 0;
    }
};

void Operation::setNumberA(double& numberA)
{
    this->_numberA = numberA;
    return;
}

void Operation::setNumberB(double& numberB)
{
    this->_numberB = numberB;
    return;
}

double Operation::getNumberA() const
{
    return this->_numberA;
}

double Operation::getNumberB() const
{
    return this->_numberB;
}

class OperationAdd : public Operation
{
    double getResult()
    {
        return getNumberA() + getNumberB();
    }
};

class OperationSub : public Operation
{
    double getResult()
    {
        return getNumberA() - getNumberB();
    }
};

class OperationMul : public Operation
{
    double getResult()
    {
        return getNumberA() * getNumberB();
    }
};

class OperationDiv : public Operation
{
    double getResult()
    {
        if (getNumberB() > 0.0)
        {
            return getNumberA() / getNumberB();
        }
        else
        {
            MessageBox(NULL, "Div Args-2 Should Be Bigger than Zero!", "Error", MB_OK);
            ExitProcess(0);
        }
    }
};

class OperaionFactory
{
public:
    static Operation* createOperate(char&);
};

Operation* OperaionFactory::createOperate(char& operate)
{
    Operation *oper = NULL;
    switch (operate)
    {
    case '+':
        oper = new OperationAdd();
        break;
    case '-':
        oper = new OperationSub();
        break;
    case '*':
        oper = new OperationMul();
        break;
    case '/':
        oper = new OperationDiv();
        break;
    default:
        MessageBox(NULL, "Symbol Error!", "Error", MB_OK);
        ExitProcess(0);
    }
    return oper;
}

/////////////////////////////
//
// @ FileName : SFM.cpp
// @ Date : 2018-08-06 18:04
//
/////////////////////////////

#include "Operation.h"

using namespace std;

int main(void)
{
    Operation *oper;
    double _numberA = 0;
    double _numberB = 0;
    char symbol = '';
    scanf("%lf %c %lf", &_numberA, &symbol, &_numberB);
    oper = OperaionFactory::createOperate(symbol);
    oper->setNumberA(_numberA);
    oper->setNumberB(_numberB);
    double result = oper->getResult();
    cout << result << endl;
    system("pause");
    return 0;
}