c++ - 如何使用C++中的装饰器模式在一行代码中装饰对象？

Question

我正在从 Head First Design Patterns book 学习装饰器模式 ，下面是我编写的 (C++) 代码来让模式工作:

#include <iostream>


class AbstractType
{
public: 
    virtual double value() const = 0;
};


class FirstConcreteType
    :
    public AbstractType
{
public: 
    double value() const 
    {
        return 1; 
    }
};

class SecondConcreteType
    : 
    public AbstractType
{
public:
    double value() const
    {
        return 2;
    }
};

class DecoratorType
    :
    public AbstractType
{
    const AbstractType* decoratedObject_; 

public:

    DecoratorType(const AbstractType& abstractObject)
        :
    decoratedObject_(&abstractObject)
    {}

    DecoratorType(const DecoratorType& decoratorObject)
        :
    decoratedObject_(&decoratorObject)
    {}

    virtual double value() const = 0; 

    const AbstractType& getObject() const
    {
        return *decoratedObject_; 
    }
};

class FirstDecoratorType
    :
    public DecoratorType
{
public:
    FirstDecoratorType(const AbstractType& abstractObject)
        :
    DecoratorType(abstractObject)
    {}

    FirstDecoratorType(const DecoratorType& decoratorObject)
        :
    DecoratorType(decoratorObject)
    {}

    double value() const
    {
        const AbstractType& object = getObject(); 

        return 1 + object.value(); 
    }
};

class SecondDecoratorType
    :
    public DecoratorType
{
public:
    SecondDecoratorType(const AbstractType& abstractObject)
        :
    DecoratorType(abstractObject)
    {}

    SecondDecoratorType(const DecoratorType& decoratorObject)
        :
    DecoratorType(decoratorObject)
    {}

    double value() const
    {
        const AbstractType& object = getObject(); 

        return 2 + object.value(); 
    }
};

using namespace std;

int main()
{
    // When I decorate sequentially, it works fine

    SecondConcreteType secondConc;

    FirstDecoratorType firstDec(secondConc); 
    cout << firstDec.value() << endl;

    SecondDecoratorType secondDec(firstDec); 
    cout << secondDec.value() << endl;

    FirstDecoratorType firstDecSecond (secondDec); 
    cout << firstDecSecond.value() << endl; 

    // Decorating in a single line, messes things up, since there is no
    // constructor taking the value argument defined.  
    //FirstDecoratorType firstDynamicDec (SecondConcreteType()); 
    //cout << firstDynamicDec.value() << endl;

    return 0;
};

在主程序中，必须首先创建 ConcreteType 的对象，然后使用指向 AbstractType 的指针(在 DecoratorType 内)的组合对其进行装饰。如果我创建具体对象，并一个接一个地创建新的装饰对象，它工作正常。

为了使 DecoratorType 能够在单行代码(示例代码中的注释行)中使用组合来装饰对象，我需要做什么？这样的东西在“现实世界”中有用吗？我(显然)在使用设计模式方面没有太多经验.. 所以我很难看出我应该瞄准什么功能。

编辑:

这是一个使用基本指针的版本(valgrind 显示没有内存泄漏，并声明没有内存泄漏是可能的):

#include <iostream>


class AbstractType
{
    public: 
        virtual double value() const = 0;

        virtual ~AbstractType() {}; 
};

class FirstConcreteType
:
    public AbstractType
{
    public: 
        double value() const 
        {
            return 1; 
        }
};

class SecondConcreteType
: 
    public AbstractType
{
    public:
        double value() const
        {
            return 2;
        }
};

class DecoratorType
:
    public AbstractType
{
    const AbstractType* decoratedObject_; 
    bool own_;

    public:

        DecoratorType(const AbstractType& abstractObject)
            :
                decoratedObject_(&abstractObject), 
                own_(false)
        {}

        DecoratorType(const DecoratorType& decoratorObject)
            :
                decoratedObject_(&decoratorObject), 
                own_(false)

        {}

        DecoratorType (AbstractType* abstractPtr)
            : 
                decoratedObject_(abstractPtr), 
                own_(true)
        {}

        DecoratorType (DecoratorType* decoratorPtr)
            :
                decoratedObject_(decoratorPtr), 
                own_(true)
        {}

        virtual ~DecoratorType()
        {
            if (own_)
            {
                delete decoratedObject_; 
                decoratedObject_ = 0;
            }
        }

        virtual double value() const = 0; 

        const AbstractType& getObject() const
        {
            return *decoratedObject_; 
        }
};

class FirstDecoratorType
:
    public DecoratorType
{
    public:
        FirstDecoratorType(const AbstractType& abstractObject)
            :
                DecoratorType(abstractObject)
        {}

        FirstDecoratorType(const DecoratorType& decoratorObject)
            :
                DecoratorType(decoratorObject)
        {}

        FirstDecoratorType (AbstractType* abstractPtr)
            :
                DecoratorType(abstractPtr)
        {}

        FirstDecoratorType (FirstDecoratorType* decoratorPtr)
            :
                DecoratorType(decoratorPtr)
        {}


        double value() const
        {
            const AbstractType& object = getObject(); 

            return 1 + object.value(); 
        }
};

class SecondDecoratorType
:
    public DecoratorType
{
    public:
        SecondDecoratorType(const AbstractType& abstractObject)
            :
                DecoratorType(abstractObject)
        {}

        SecondDecoratorType(const DecoratorType& decoratorObject)
            :
                DecoratorType(decoratorObject)
        {}

        SecondDecoratorType (AbstractType* abstractPtr)
            :
                DecoratorType(abstractPtr)
        {}

        SecondDecoratorType (SecondDecoratorType* decoratorPtr)
            :
                DecoratorType(decoratorPtr)
        {}

        double value() const
        {
            const AbstractType& object = getObject(); 

            return 2 + object.value(); 
        }
};

using namespace std;

int main()
{
    // When I decorate sequentially, it works fine

    SecondConcreteType secondConc;

    FirstDecoratorType firstDec(secondConc); 
    cout << firstDec.value() << endl;

    SecondDecoratorType secondDec(firstDec); 
    cout << secondDec.value() << endl;

    FirstDecoratorType firstDecSecond (secondDec); 
    cout << firstDecSecond.value() << endl; 

    // Decorating in a single line, messes things up, since there is no
    // constructor taking the value argument defined.  
    FirstDecoratorType firstDynamicDec (new SecondDecoratorType (
           new FirstDecoratorType (new SecondConcreteType()))); 

    cout << firstDynamicDec.value() << endl;

    return 0;
};

Answer 1

FirstDecoratorType firstDynamicDec (SecondConcreteType()); 

问题在于它没有定义对象。相反，它声明一个函数。在此站点上查找 C++ 中的 most-vexing-parse，您将获得很多关于它的主题。

简短说明:函数名称是 firstDynamicDec，其返回类型是 FirstDecoratorType，它采用的参数也是返回 SecondConcreteType 并采用没有争论。