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();