装饰模式

　　装饰模式：动态地给一个对象增加一些额外的职责。就增加功能而言，Decorator模式比生成子类更为灵活。（《设计模式》）

　　即是通过组合给对象增加功能，可以消除通过继承带来的代码重复和大量子类这些麻烦。通常要扩展对象的功能是用继承的方式实现，但是由于继承为类引入静态特征，一旦扩展的功能增多，子类也会随之增多。所以装饰模式主要用在想要动态扩展功能的情况下。

实现：

　　1、创建接口

class Stream
{
public:
    virtual void read() = 0;
    virtual void write() = 0;
    virtual void seek() = 0;
    virtual ~Stream() {};
};

　　2、创建接口的实体类

class FileStream :public Stream
{
    // 通过 Stream 继承   read write seek都是静态特征
    virtual void read() override;
    virtual void write() override;
    virtual void seek() override;
};

class NetworkStream : public Stream
{
    // 通过 Stream 继承
    virtual void read() override;
    virtual void write() override;
    virtual void seek() override;
};

void FileStream::read()
{
    cout << __FUNCTION__ << endl;
}

void FileStream::write()
{
    cout << __FUNCTION__ << endl;
}

void FileStream::seek()
{
    cout << __FUNCTION__ << endl;
}

void NetworkStream::read()
{
    cout << __FUNCTION__ << endl;
}

void NetworkStream::write()
{
    cout << __FUNCTION__ << endl;
}

void NetworkStream::seek()
{
    cout << __FUNCTION__ << endl;
}

　　3、创建装饰类接口

class DecoratorStream : public Stream
{
protected:
    Stream* stream;  //组合
public:
    virtual ~DecoratorStream() {};
};

　　4、通过继承装饰类接口扩展功能

class CryptoDecoratorStream : public DecoratorStream
{
public:
    CryptoDecoratorStream(Stream *stream);
    // 通过 DecoratorStream 继承
    virtual void read() override;
    virtual void write() override;
    virtual void seek() override;
    ~CryptoDecoratorStream();
};

class BufferDecoratorStream : public DecoratorStream
{
public:
    BufferDecoratorStream(Stream* stream);
    // 通过 DecoratorStream 继承
    virtual void read() override;
    virtual void write() override;
    virtual void seek() override;
    ~BufferDecoratorStream();
};

CryptoDecoratorStream::CryptoDecoratorStream(Stream* stream)
{
    this->stream = stream;
}

void CryptoDecoratorStream::read()
{
    stream->read();
    cout << "CryptoDecoratorStream" << endl;
}

void CryptoDecoratorStream::write()
{
    stream->write();
    cout << "CryptoDecoratorStream" << endl;
}

void CryptoDecoratorStream::seek()
{
    stream->seek();
    cout << "CryptoDecoratorStream" << endl;
}

CryptoDecoratorStream::~CryptoDecoratorStream()
{
    delete this->stream;
}

BufferDecoratorStream::BufferDecoratorStream(Stream* stream)
{
    this->stream = stream;
}

void BufferDecoratorStream::read()
{
    stream->read();
    cout << "BufferDecoratorStream" << endl;
}

void BufferDecoratorStream::write()
{
    stream->write();
    cout << "BufferDecoratorStream" << endl;
}

void BufferDecoratorStream::seek()
{
    stream->seek();
    cout << "BufferDecoratorStream" << endl;
}

BufferDecoratorStream::~BufferDecoratorStream()
{
    delete this->stream;
}

　　5、使用DecoratorStream来装饰Stream对象

int main()
{
    DecoratorStream* crytoFileStream = new CryptoDecoratorStream(new FileStream);
    crytoFileStream->read();

    DecoratorStream* bufferNetworkStream = new BufferDecoratorStream(new NetworkStream);
    bufferNetworkStream->write();

    delete crytoFileStream;
    delete bufferNetworkStream;
}

 　　DecoratorStream通过组合Stream类，使其子类能够不通过继承就可以为FileStream和NetworkStream扩展功能，减少了子类的个数，也避免了继承带来的大量代码重复。实现了动态扩展功能的需求。