我将此代码作为使用双重调度的示例,但我并不真正理解代码的一部分。创建“抽象类”打印机,为什么我需要添加:
virtual void print(PDFDoc *d)=0;
virtual void print(DocDoc *d)=0;
据我了解,在运行时 p.print(docA);
会将我发送到virtual void print(Document *d)
myPrinter,然后d->printMe(this)
将我发送到 PDFDoc 的 printMe,然后它会在运行时调用virtual void print(PDFDoc *d)
我的打印机?
那么为什么要定义
virtual void print(PDFDoc *d)=0;
virtual void print(DocDoc *d)=0;
抽象类是必需的吗?
class Document{
public:
//this is the accept function
virtual void printMe(Printer *p)=0;
};
class Printer{
public:
virtual void print(Document *d)=0;
//the visitors
virtual void print(PDFDoc *d)=0;
virtual void print(DocDoc *d)=0;
};
class PDFDoc : public virtual Document{
public:
virtual void printMe(Printer *p){
std::cout << "PDFDoc accepting a print call" << std::endl;
p->print(this);
}
};
class DocDoc : public virtual Document{
public:
virtual void printMe(Printer *p){
std::cout << "DocDoc accepting a print call" << std::endl;
p->print(this);
}
};
class MyPrinter : public virtual Printer{
public:
virtual void print(Document *d){
std::cout << "dispatching function <print> called" << std::endl;
d->printMe(this);
}
virtual void print(PDFDoc *d){
std::cout << "printing a PDF doc" << std::endl;
}
virtual void print(DocDoc *d){
std::cout << "printing a Doc doc" << std::endl;
}
};
int main(){
MyPrinter p;
Document *docA = new PDFDoc();
Document *docB = new DocDoc();
p.print(docA);
p.print(docB);
delete docA;
delete docB;
return 0;
}