工厂方法实现 - C ++ [英] Factory method implementation - C++

问题描述

我有factory设计模式实现的以下代码。

I have the following code for "factory" design pattern implementation.

class Pen{
public:
     virtual void Draw() = 0;
};

class RedPen : public Pen{
public:
     virtual void Draw(){
         cout << "Drawing with red pen" << endl;
     }
};

class BluePen : public Pen{
public:
     virtual void Draw(){
         cout << "Drawing with blue pen" << endl;
     }
};

auto_ptr<Pen> createPen(const std::string color){
     if(color == "red")
         return auto_ptr<Pen>(new RedPen);
     else if(color == "blue")
         return auto_ptr<Pen>(new BluePen);
}

但我听说可以使用C ++ templates 。

But I heard that it can be done in a better way using "C++ templates". Can anyone help how it is done and how template approach is better than this?

任何想法

推荐答案

在您发布的示例中，工厂或模板方法对我来说都没有意义。
我的解决方案涉及Pen类中的数据成员。

In the example you posted, neither a factory or a template approach makes sense to me. My solution involves a data member in the Pen class.

class Pen {
public:
    Pen() : m_color(0,0,0,0) /* the default colour is black */
    {            
    }

    Pen(const Color& c) : m_color(c)
    {
    }

    Pen(const Pen& other) : m_color(other.color())
    {
    }

    virtual void Draw()
    {
        cout << "Drawing with a pen of color " << m_color.hex();
    }
    void setColor(const Color& c) { m_color = c; }
    const Color& color() const { return m_color; }
private:
    Color m_color;
};

class Color {
public:
    Color(int r, int g, int b, int a = 0) :
        m_red(r), m_green(g), m_blue(other.blue()), m_alpha(a)  
    {
    }

    Color(const Color& other) : 
        m_red(other.red()), m_green(other.green()), 
        m_blue(other.blue()), m_alpha(other.alpha())
    {
    }

    int red() const { return m_red; }
    int green() const  { return m_green; }
    int blue() const { return m_blue; }
    int alpha() const { return m_alpha; }

    std::string hex() const
    {
        std::ostringstream os;
        char buf[3];
        os << "#";

        sprintf(buf, "%2X", red());
        os << buf;

        sprintf(buf, "%2X", green());
        os << buf;

        sprintf(buf, "%2X", blue());
        os << buf;

        sprintf(buf, "%2X", alpha());
        os << buf;

        return os.str();
    }

private:
    int m_red;
    int m_green;
    int m_blue;
    int m_alpha;
}

当然，颜色类必须调整为绘图API

Of course, the color class would have to be adjusted to the drawing API you use -- and perhaps be way more advanced than this one (different color spaces, etc).

使用模板没有意义的原因是（大概）不同绘图操作之间的唯一区别是颜色变量。所以，通过使用模板（或手动声明不同的类，如你所做的），你将复制类似的代码。

The reason it does not make sense to use templates, is that (presumably) the only difference between the different drawing operations is the color variable. So, by using templates (or manually declaring different classes, as you did), you will duplicate similar code. This will make your program large, and slow it down.

因此，绘制函数应该将颜色作为参数，或者（在我的例子中）有color作为类数据成员。

So, the draw function should either take the color as an argument, or (as in my example) have the color as a class data member.

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