使用函数指针来代替switch语句

使用函数指针来代替Switch语句【以下是公司项目中部分的videoSwitchAgent】

使用一个单件的工程，将要创建的实体用不同的函数来表示，在这个工厂里面用一个map来保存所有创建实体的方法。由于map是可以链表结构没有大小限制，这样就可以做到便于扩充。

 

代码如下：

// EntityCreationFactory.h

namespace TA_IRS_App
{
    class EntityCreationFactory
    {
    public:
        typedef TA_Base_Bus::IEntity* ( *CreatorFunction )( TA_Base_Core::IEntityDataPtr entityData );
    static EntityCreationFactory& getInstance();

        void registerFunction( const std::string& type, CreatorFunction function );
        TA_Base_Bus::IEntity* createEntity( TA_Base_Core::IEntityDataPtr entityData );

    private:
        EntityCreationFactory();
        virtual ~EntityCreationFactory() {};

        EntityCreationFactory( const EntityCreationFactory& theEntityCreationFactory );
        EntityCreationFactory& operator=( const EntityCreationFactory& theEntityCreationFactory );

    static TA_Base_Bus::IEntity* createCamera( TA_Base_Core::IEntityDataPtr entityData );
        static TA_Base_Bus::IEntity* createVideoMonitor( TA_Base_Core::IEntityDataPtr entityData );
        static TA_Base_Bus::IEntity* createVideoOutputGroup( TA_Base_Core::IEntityDataPtr entityData );
        static TA_Base_Bus::IEntity* createQuad( TA_Base_Core::IEntityDataPtr entityData );
        static TA_Base_Bus::IEntity* createBVSStage( TA_Base_Core::IEntityDataPtr entityData );
        static TA_Base_Bus::IEntity* createSequence( TA_Base_Core::IEntityDataPtr entityData );
        static TA_Base_Bus::IEntity* createRecordingUnit( TA_Base_Core::IEntityDataPtr entityData );

    typedef std::map< std::string, CreatorFunction > FunctionMap;
        FunctionMap m_functions;

    static EntityCreationFactory*    s_singleton;
    static TA_Base_Core::ReEntrantThreadLockable    s_singletonLock
    }; // EntityCreationFactory
} // TA_IRS_App

//EntityCreeationFactory.cpp
namespace TA_IRS_App
{
    EntityCreationFactory* EntityCreationFactory::s_singleton = NULL;
    TA_Base_Core::ReEntrantThreadLockable EntityCreationFactory::s_singletonLock;

    EntityCreationFactory& EntityCreationFactory::getInstance()
    {
        if ( s_singleton == NULL )
        {
            TA_Base_Core::ThreadGuard guard( s_singletonLock );
            if ( s_singleton == NULL )
            {
                s_singleton = new EntityCreationFactory();
            }
        }
        return *s_singleton;

  }

    EntityCreationFactory::EntityCreationFactory()
    {
        registerFunction( TA_Base_Core::Camera::getStaticType(), createCamera );
        registerFunction( TA_Base_Core::VideoMonitor::getStaticType(), createVideoMonitor );
        registerFunction( TA_Base_Core::VideoOutputGroup::getStaticType(), createVideoOutputGroup );
        registerFunction( TA_Base_Core::Quad::getStaticType(), createQuad );
        registerFunction( TA_Base_Core::BVSStage::getStaticType(), createBVSStage );
        registerFunction( TA_Base_Core::Sequence::getStaticType(), createSequence );
        registerFunction( TA_Base_Core::RecordingUnit::getStaticType(), createRecordingUnit );

    }


    void EntityCreationFactory::registerFunction( const std::string& type, CreatorFunction function )
    {
        TA_ASSERT( m_functions.end() == m_functions.find( type ),
            "A function is already registered for this type." );
        m_functions[ type ] = function;
    }

    TA_Base_Bus::IEntity* EntityCreationFactory::createEntity( TA_Base_Core::IEntityDataPtr entityData )
    {
        std::string type = entityData->getType();
       
        LOG_GENERIC(SourceInfo, TA_Base_Core::DebugUtil::DebugInfo, "the type is %s", type.c_str());

        FunctionMap::iterator it = m_functions.find( type );
        if ( m_functions.end() == it )
        {
            LOG_GENERIC( SourceInfo, TA_Base_Core::DebugUtil::DebugInfo,
                "Entity could not be created because it is of an unrecognised type: %s", type.c_str() );
            return 0;
        }
        return it->second( entityData );
    }


    // Private methods
    TA_Base_Bus::IEntity* EntityCreationFactory::createCamera( TA_Base_Core::IEntityDataPtr entityData )
    {
        TA_Base_Bus::IEntity *pResult = NULL;
        return pResult;
    }

    // Private methods
    TA_Base_Bus::IEntity* EntityCreationFactory::createCamera( TA_Base_Core::IEntityDataPtr entityData )
    {
        TA_Base_Bus::IEntity *pResult = NULL;
        return pResult;
    }
} // end TA_IRS_App

// client
TA_Base_Bus::IEntity *pEntity = EntityAccessFactory::getInstance().createEntity();