我正在编写一个 xml 解析器,我需要将对象添加到一个类中,并打开对象的实际类型。问题是,我想保留一个简单的接口(interface) addElement(BaseClass*) 然后正确放置对象。
void E_TableType::addElement(Element *e)
{
QString label = e->getName();
if (label == "state") {
state = qobject_cast<E_TableEvent*>(e);
}
else if (label == "showPaytable") {
showPaytable = qobject_cast<E_VisibleType*>(e);
}
else if (label == "sessionTip") {
sessionTip = qobject_cast<E_SessionTip*>(e);
}
else if (label == "logoffmedia") {
logoffMedia = qobject_cast<E_UrlType*>(e);
}
else {
this->errorMessage(e);
}
}
这是调用类,一个对象工厂。 myElement 是 E_TableType 的实例。
F_TableTypeFactory::F_TableTypeFactory()
{
this->myElement = myTable = 0;
}
void F_TableTypeFactory::start(QString qname)
{
this->myElement = myTable = new E_TableType(qname);
}
void F_TableTypeFactory::fill(const QString& string)
{
// don't fill complex types.
}
void F_TableTypeFactory::addChild(Element* child)
{
myTable->addElement(child);
}
Element* F_TableTypeFactory::finish()
{
return myElement;
}
void F_TableTypeFactory::addAttributes(const QXmlAttributes &attribs) {
QString tName = attribs.value(QString("id"));
myTable->setTableName(tName);
}
最佳答案
你考虑过在这里使用多态吗?如果您的每个具体类都可以实现一个公共(public)接口(interface),那么所有这些代码都会消失,并且将来会变得简单且易于更改。例如:
class Camera {
public:
virtual void Init() = 0;
virtual void TakeSnapshot() = 0;
}
class KodakCamera : Camera {
public:
void Init() { /* initialize a Kodak camera */ };
void TakeSnapshot() { std::cout << "Kodak snapshot"; }
}
class SonyCamera : Camera {
public:
void Init() { /* initialize a Sony camera */ };
void TakeSnapshot() { std::cout << "Sony snapshot"; }
}
因此,假设我们有一个包含硬件设备的系统,在本例中为相机。每个设备需要不同的逻辑来拍照,但代码必须支持具有任何支持相机的系统,因此我们不希望 switch 语句散落在我们的代码中。所以,我们创建了一个抽象类Camera
。
每个具体类(即SonyCamera
、KodakCamera
)的实现都将包含不同的头文件、链接到不同的库等,但它们都共享一个公共(public)接口(interface);我们只需决定预先创建哪一个。所以……
std::unique_ptr<Camera> InitCamera(CameraType type) {
std::unique_ptr<Camera> ret;
Camera *cam;
switch(type) {
case Kodak:
cam = new KodakCamera();
break;
case Sony:
cam = new SonyCamera();
break;
default:
// throw an error, whatever
return;
}
ret.reset(cam);
ret->Init();
return ret;
}
int main(...) {
// get system camera type
std::unique_ptr<Camera> cam = InitCamera(cameraType);
// now we can call cam->TakeSnapshot
// and know that the correct version will be called.
}
现在我们有了一个实现Camera
的具体实例。我们可以调用 TakeSnapshot
而无需检查代码中任何地方的正确类型,因为这无关紧要;我们知道将调用正确硬件的正确版本。希望这对您有所帮助。
根据您的以下评论:
I've been trying to use polymorphism, but I think the elements differ too much. For example, E_SessionTip has an amount and status element where E_Url just has a url. I could unify this under a property system but then I lose all the nice typing entirely. If you know of a way this can work though, I'm open to suggestions.
我建议将编写 XML 数据的责任转移给共享公共(public)接口(interface)的类型。例如,而不是像这样:
void WriteXml(Entity *entity) {
switch(/* type of entity */) {
// get data from entity depending
// on its type and format
}
// write data to XML
}
做这样的事情:
class SomeEntity : EntityBase {
public:
void WriteToXml(XmlStream &stream) {
// write xml to the data stream.
// the entity knows how to do this,
// you don't have to worry about what data
// there is to be written from the outside
}
private:
// your internal data
}
void WriteXml(Entity *entity) {
XmlStream str = GetStream();
entity->WriteToXml(stream);
}
这对你有用吗?我以前做过这个,它对我有用。让我知道。
关于c++ - 打开对象的实际类型的正确方法是什么?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/11297752/