Re: Parameter List / Parameter Block / Anything patterns...

"=?iso-8859-1?q?Erik_Wikstr=F6m?=" <>
29 Mar 2007 04:19:03 -0700
On 29 Mar, 12:39, "" <> wrote:

Hi everyone

I need to come up with an efficient solution to add/remove/access
parameters to an object. Lets say I create an object "Cart" and at run
time based on the user input, lets say we add to this "Cart" object 3
Oranges 4 Apples and 1 Salad (stupid example I know). Whenever we do
something with this cast object we want to be able to have access to
all the data that it holds (3 oranges, 4 apples, 1 salad).

in pseudo code it would do something like that

Cart *cart = new Cart;
Orange orange[4] = { orange1, orange2, orange3, orange4 };
Cart.SetParameter( "SomeOrange", orange ); // copy the 4 orange in
Orange *iterOrange = Cart.GetParameter( "SomeOrange" );
for ( unsigned i = 0; i < 4; ++i )
  Orange *o = iterOrange[i];

To get something like that to work the Cart class would have a member
variable that would look like that.

std::map<const char *, Data> dict;

So a unique name is associated with each data, such that we can look
for that parameter name later on when we call GetParameter.

I looked a little bit on the net and found a few documents on C++
patterns that seem to do what I need. They seem to be called Parameter
List, Parameter Block or Anything Patterns. But I have a hard time
making to work or find a good implementation example. It sounds like
some API are using these patterns. I have heard of the 3DMaya API &
the nVidia Gelato API. It is used for these 3D renderers because it
allows to add any data of any type at run time for example to lets say
geometric primitives (and I want to use it for that purpose).

I am not sure what is the best way of doing that so was wondering if
someone knows a good implementation of these patterns or could put me
on the right track to write my own. I thought of simply doing
something like that :

enum ParamType { kFloat, kInt, kString };

class Param
  ParamType type;
  std::vector<float> fdefault;
  std::vector<const char *> sdefault;
  std::vector<int> idefault;
  Param() {}


std::map<const char *, Param> paramList;

but if the param is a float array i carry in the Param object useless
data like sdefault & idefault. Therefore Param is "heavier" than it
should. I am not sure this is very elegant (although simple).

I thought of having some sort of Data class that would hold a void*
variable pointer to an array of data. But of course using std::vector
is better since having a void * data type of situation makes have to
deal with allocating/deallocating memory.

class Data
  void *data;
  Data( ParamType type, void *d, unsigned n )
     switch( type )
       case kFloat: data = new float[n];
memcpy(data,d,sizoef(float)*n); break;
       case kInt: data = new int[n]; memcpy(data, d, sizeof(int)*n);
       case kString: /* etc */
  ~Data() { /* release mem */ }


Could anybody advise me of the best way of doing this please (in terms
of code robustness, simplicity of use, better c++ coding, memory &
speed efficiency, etc).

If you have a limited set of items that can be placed in the Cart and
if they all have some operations in common it would probably be
easiest to simply let them inherit from some base-type. Failing that
you need a heterogeneous container, it's covered slightly in the FAQ: and you
can find some implementations by googling, Boost.Any is one such

Erik Wikstr=F6m

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