装饰器模式

装饰器模式简介

装饰器模式可以为已有类的方法增加新的行为，例如为一个文件数据读写类的写数据方法增加数据加密的行为。而实现的策略不是基于继承自这个已有的类。

装饰器模式通常代码结构

代码示例

//interface
class DataManipulable {
public:
    virtual size_t writeData(const char * data) = 0;
    virtual size_t readData(char * buffer, size_t len) = 0;
};

class BaseFileDataManipulator: public DataManipulable {
private:
    const std::string _filename;
public:
    BaseFileDataManipulator(const std::string & filename): _filename(filename) {}

    size_t writeData(const char * data) override {
        //write data to file
    }

    size_t readData(char * buffer, size_t len) {
        //read data from file
    }
};

//decorator interface
class DataManipulableDecorator: public DataManipulable {
private:
    DataManipulable & _dataManipulator;
public:
    DataManipulableDecorator(DataManipulable & dataManipulator): _dataManipulator(dataManipulator) {}
    DataManipulable & getManipulator() {
        return _dataManipulator;
    }
};

//concrete decorators
class EncryptDataManipulator: public DataManipulableDecorator {
public:
    EncryptDataManipulator(DataManipulable & dataManipulator): DataManipulable(dataManipulator) {}
    size_t writeData(const char  * data) override {
        //encrypt data first

        //then delegate to _dataManipulator
        return this->getManipulator().writeData(data);
    }

    size_t readData(char * buffer, size_t len) override {
        //delegate to _dataManipulator
        size_t readLen = this->getManipulator().readData(buffer, len);

        //do decrypt data operation

        return readLen;
    }
};

class CompressDataManipulator: public DataManipulableDecorator {
public:
    CompressDataManipulator(DataManipulable & dataManipulator): DataManipulableDecorator(dataManipulator) {}

    size_t writeData(const char * data) override {
        //compress data first

        //then delegate to _dataManipulator
        return this->getManipulator().writeData(data);
    }

    size_t readData(char * data, size_t len) {
        //delegate to _dataManipulator
        size_t readLen = this->getManipulator().readData(buffer, len);

        //do decompress operation

        return readLen
    }
};

在上例中，BaseFileDataManipulator是一个已有基本功能的数据读写类，而EncryptDataManipulator和CompressDataManipulator分别用于给已有的数据读写类增加加密数据和压缩数据的能力。EncryptDataManipulator和CompressDataManipulator并没有继承自BaseFileDataManipulator，而是继承自DataManipulableDecorator这个接口类。但是BaseFileDataManipulator和DataManipulableDecorator都继承自DataManipulable这个接口类。客户端只关心和DataManipulable中定义的接口即可。

和适配器模式的对比

• 适配器模式一般用于对已有对象的接口进行修改或者增加新的接口，而适配器模式一般不会改变已有对象的接口，而是对已有对象的接口的实现上添加新的行为。