Re: visitor design pattern, loki and more
On Aug 12, 5:11 pm, aaragon <alejandro.ara...@gmail.com> wrote:
On Aug 12, 3:47 pm, mlimber <mlim...@gmail.com> wrote:
On Aug 12, 4:32 pm, aaragon <alejandro.ara...@gmail.com> wrote:
On Aug 12, 2:34 pm, mlimber <mlim...@gmail.com> wrote:
On Aug 12, 3:25 pm, aaragon <alejandro.ara...@gmail.com> wrote:
Hello everyone,
I've been trying to work with the visitor design pattern, and it =
works
fine except for the following.
Let's suppose that we have a fixed hierarchy of classes (many of =
them)
which I cannot modify. I decided to use the visitor design patter=
n
depending on the actual type of the classes because those classes
already support the loki visitor.
[...]
Now, what GenScatterHierarchy does, is to derive from
ClassComputerUnit<Class> where Class is each of those classes in =
the
ClassList. So now I'm supposed to have all 20 Visit functions in =
the
VisitorImpl class overide the one of the Visitor. However, this i=
s not
the case and I cannot compile my code because there are pure virt=
ual
functions. So in a sense the GenScatterHierarchy did not overide =
the
Visit function from the visitor as I thought it would.
Does anyone have a clue why is this happening? I'm using GCC4.3.
Thanks in advance for the time reading such a long post.
This is rather beyond the scope of this group, IMHO (cf.http://www.=
parashift.com/c++-faq-lite/how-to-post.html#faq-5.9). Have
you asked on the Loki help forum on Sourceforge? Or can you boil it
down to a standard C++ question?
Cheers! --M
It is a standard C++ question. It does not matter which kind of
visitor you use, all of them boil down to having a pure virtual
function in a base class, and have all subclasses override that
function. The GenScatterHiearchy class template is basically a
metaprogram, so it is a shorthand version of public Class1, public
Class2, and so on.
MFC, Qt, and many other libraries would also qualify under such a
definition of standard C++. Can you rephrase the question so that we
can answer it by looking at the C++ standard? One way to do that is to
boil it down to a minimal but complete program that demonstrates your
problem. (Note, you'll likely get more help if it doesn't require
downloading Loki.)
Cheers! --M
Ok, here we go, I thought that it was straightforward enough so it
would save me some coding. For simplicity let's assume that the return
type is void and that we only have three classes. Here it is:
#include <iostream>
using std::cout;
using std::endl;
class Base;
class Class1;
class Class2;
class Class3;
class ClassVisitor {
public:
void visit(Class1& ) {
cout<<"inside Class1"<<endl;
}
void visit(Class2& ) {
cout<<"inside Class2"<<endl;
}
void visit(Class3& ) {
cout<<"inside Class3"<<endl;
}
};
struct Base {
virtual void accept(ClassVisitor& vis) = 0;
virtual ~Base() {}
};
struct Class1 : public Base {
virtual void accept(ClassVisitor& vis) {
vis.visit(*this);
}};
struct Class2 : public Base {
virtual void accept(ClassVisitor& vis) {
vis.visit(*this);
}};
struct Class3 : public Base {
virtual void accept(ClassVisitor& vis) {
vis.visit(*this);
}
};
int main(int argc, char* argv[]) {
Class1 c1;
Class2 c2;
Class3 c3;
ClassVisitor vis;
c1.accept(vis);
c2.accept(vis);
c3.accept(vis);
return 0;
}
This prints:
inside Class1
inside Class2
inside Class3
Now, I want to change this in order to avoid having all those
declarations in the visitor class. So instead of having
class ClassVisitor {
public:
void visit(Class1& ) {
cout<<"inside Class1"<<endl;
}
void visit(Class2& ) {
cout<<"inside Class2"<<endl;
}
void visit(Class3& ) {
cout<<"inside Class3"<<endl;
}
};
I would like to have
template <class Class>
struct GenClass {
virtual void visit(Class& ) {
cout<<"inside class defined by template"<=
<endl;
}
};
class ClassVisitor :
public GenClass<Class1>, public GenClass<Class2>, public
GenClass<Class3> {
};
but when I try to compile this code I get ambiguous calls because of
course in the concrete classes, vis.visit(*this); this can be either
Class1, Class2 or Class3.
Now, I would like to solve this problem so I'm asking if anyone knows
how to do it. Thanks,
Now I see the problem. It's the one described in this FAQ:
http://www.parashift.com/c++-faq-lite/templates.html#faq-35.19
You can either modify ClassVisitor:
struct ClassVisitor
: public GenClass<Class1>
, public GenClass<Class2>
, public GenClass<Class3>
{
using GenClass<Class1>::visit;
using GenClass<Class2>::visit;
using GenClass<Class3>::visit;
};
Or you can change the classes themselves:
struct Class1 : Base
{
virtual void accept( ClassVisitor& vis )
{
vis.GenClass<Class1>::visit( *this );
}
};
Neither of these are ideal, but perhaps Loki has solved (or could
solve) this problem with macros and type lists so that ClassVisitor
looks something like:
typedef TypeList<Class1, Class2, Class3> MyTypeList;
struct ClassVisitor
: GenHierarchy< MyTypeList >
{
GEN_USINGS( MyTypeList, visit );
};
Cheers! --M