Using iterator_facade from base class with sub classes
Hello!
I have implemented the iterator_facade for a base class called DataObject.
So now I am
able to write things like this:
boost::shared_ptr<PxyFileData> file(...);
const DataObject& points = file->GetPoints();
int num_points = std::distance(points.begin(), points.end()); //! random
access iterator
A DataObject acts like a tree structure. Each DataObject holds a vector of
pointers to
children, which are also DataObjects.
Now to my problem: A PxyPoint is a sub class to DataObject, and
PxyFileData::GetPoints()
returns a DataObject where all the children are PxyPoint's. Now I would like
to call a
member function, PxyPoint::PutSpCode(const char*), on each of the children,
like this:
void put_sp_code(DataObject&, const char*);
void test()
{
shared_ptr<PxyFileData> file(...);
DataObject& points = file->GetPoints();
put_sp_code(points, "spcode");
}
void put_sp_code(DataObject& points, const char* code)
{
std::for_each(points.begin(), points.end(),
std::bind2nd(std::mem_fun(&PxyPoint::PutSpCode), code));
}
This does not compile (VC 7.1):
c:\Program Files\Microsoft Visual Studio .NET
2003\Vc7\include\algorithm(21): error C2664: 'binder2nd<_Fn2>::result_type
binder2nd<_Fn2>::operator ()(binder2nd<_Fn2>::argument_type & ) const':
cannot convert parameter 1 from 'DataObject' to
'binder2nd<_Fn2>::argument_type & '
with
[
_Fn2=mem_fun1_t
Daniel Lidström
Now to my problem: A PxyPoint is a sub class to DataObject, and PxyFileData::GetPoints() returns a DataObject where all the children are PxyPoint's. Now I would like to call a member function, PxyPoint::PutSpCode(const char*), on each of the children, like this:
void put_sp_code(DataObject&, const char*);
void test() { shared_ptr<PxyFileData> file(...);
DataObject& points = file->GetPoints(); put_sp_code(points, "spcode"); }
void put_sp_code(DataObject& points, const char* code) { std::for_each(points.begin(), points.end(), std::bind2nd(std::mem_fun(&PxyPoint::PutSpCode), code));
If PutSpCode is a virtual function on DataObjects, it's easy.
// Using Boost.Bind because the standard binders are too tricky for
// me to get right (untested);
std::for_each(points.begin(), points.end(),
boost::bind(&DataObject::PutSpCode, _1, code));
Otherwise I don't think you could reasonably expect anything like this
to work without a cast. You'd need to create a function object with a
cast in it:
// using Boost.Lambda (untested)
std::for_each(points.begin(), points.end(),
(&boost::ll_dynamic_cast
Is this possible with our design? I would like to be able to do this without having to create a PxyPointList, that derives from DataObject and only stores PxyPoint's (my guess is that something like that would work with this code).
I can't tell for sure, but this has all the markings of a design that's trying to use runtime polymorphism where it should be using compile-time polymorphism, in some form or other. BTW, what does any of this have to do with iterator_facade? -- Dave Abrahams Boost Consulting www.boost-consulting.com
David Abrahams wrote:
I can't tell for sure, but this has all the markings of a design that's trying to use runtime polymorphism where it should be using compile-time polymorphism, in some form or other.
Thanks for the interesting tips! If this is possible to solve without explicitly creating a functor, that would be really cool. I'll have to study your examples carefully.
BTW, what does any of this have to do with iterator_facade?
You're right, it is totally unrelated. Sorry I referred to iterator_facade. Regards, Daniel Lidström
participants (3)
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Daniel Lidstrom -
Daniel Lidström
-
David Abrahams