[mpl] Newbie question - working with sets
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I've been trying to use MPL to create a system on an embedded platform
(where memory is at a premium and dynamic allocations are expensive)
that allows for multiple components or properties to be combined into a
single object at both compile and run-time.
The basic idea is, say I have the following properties for a 3D model
(each its own independent class; there are more than the examples I list
here):
ModelBase
Configuration
JointAnimation
TextureAnimation
LightingConfig
I know that a box doesn't need JointAnimation, so when I instantiate a
Model for it I'd like it to not have that "property".
(I could easily accomplish all of this using a std::map and a dynamic
allocation for each property, but again, it's prohibitive on the
platform and I'd rather use metaprogramming if I can.)
So what I'd like to do, in theory, is something like:
// Declare a generic class for a property-based object that templates
off of the set of properties it can contain
template<class PropertiesSet>
class ObjectWithProperties
{
public:
// Create an instance of ObjectWithProperties, using some subset of
PropertiesSet
template<class PropertiesSubset>
static ObjectWithProperties *create();
// Attempt to retrieve a Property, return null on failure
template<class PropertyType>
PropertyType *getProperty();
private:
SomeStaticListManager< size<PropertiesSet>::value >
which_properties_are_used;
void *data_buffer_for_properties;
};
// Define the set of properties that a model can consist of
typedef set
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on Fri Sep 07 2007, Dan Posluns
The Create() method would iterate over the subset, tally up the sizeof() for each class, and then perform a single allocation for data_buffer_for_properties and proceed to use placement new to instantiate each class in the subset. (The destructor would then clean up all of that ugliness.)
This sounds like a good job for Boost.Fusion
The which_properties_are_used member would be an efficient list manager that can be used to lookup the existence of properties, indexing the types in the set using the order<> operator.
Unfortunately, I haven't been able to get very far with set<>. I tried the following:
typedef fold< PropertiesSubset, int_<0>, plus< _1, sizeof_<_2> > >::type TotalSize;
cout << TotalSize::value;
... and it gives me nothing like what I'm looking for.
Huh... That won't give you anything useful anyway, because it doesn't account for the alignment requirement of the different types. I suggest using Fusion to build a tuple of the properties you're interested in. -- Dave Abrahams Boost Consulting http://www.boost-consulting.com The Astoria Seminar ==> http://www.astoriaseminar.com
participants (2)
-
Dan Posluns
-
David Abrahams