Re: [boost] Re : BOOST_FOREACH like macro for map

Subject: Re: [boost] Re : BOOST_FOREACH like macro for map
From: Stewart, Robert (Robert.Stewart_at_[hidden])
Date: 2009-10-21 09:03:35

mobi phil wrote:
>
> I forgot to put in my question that I am looking for a macro and not
> template if possible, and for a solution as elegant as BOOST_FOREACH,
> where you have to code as less as possible. It not (only :) ) that I

How does a macro reduce the code you write?

Your example is:

   BOOST_CONSTMAPFIND(aStringMap, "key", data)
   {
      processResult( data );
   }

My version was:

   data_type & data(get_value(aStringMap, "key"));
   processResult(data);

Looking at your example here, it seems that you want this logic:

   if (find "key" in aStringMap)
   {
      // initialize data to reference the value for "key"
      processResult(data);
   }

I had thought you wanted to assume that the value was always found.

Given what I now understand, your macro would need to be something like this (ignoring necessary encapsulation details and not implementing the std::map to type mappings):

#define BOOST_CONSTMAPFIND(_map, _key, _value) \
   const_iterator_type_from(_map) const it(_map.find(_key));
   typedef data_type_from(_map) data_type;
   data_type dummy;
   data_type * value(&dummy);
   data_type & _value(*value);
   if (it != _map.end())
   {
      value = &it->second;
   }

> furthermore:
> 1. exception... throwing the exception removes the complexity of
> writing but introduces the complexity of exception (writing code +
> runtime).

I had understood your initial use case to imply that you assumed the value was found.  I didn't recognize that you meant the macro to be a conditional controlling the following block.

> I was thinking all the day about this little "handicap" of the
> std::map. One way would be to have the concept of invalid object of
> the map data type that could be returned to satisfy the constraint of

Not all types have an unused value to indicate invalidity.

> with the form I proposed
>
> BOOST_CONSTMAPFIND(aStringMap, "key", data)
> {
>      processResult( data );
> }
>
> the problem is that I do not know how to detect the type of data
> knowing in the map object. I thought for the moment that BOOST_FOREACH

That's the unimplemented part in my macro above.

> would "automatically" declare the type in the container (using
> non-standard typeof for eg), but revisiting again BOOST_FOREACH
> documentation i see  that it declares the type.
> vector<int> v;
> BOOST_FOREACH( int i, v) etc.
>
> so it seems that if I declare the type of the data, my job becomes
> easy, a sort of "problem solved"
>
> const map<string, Mytype> & mymap = getMap();
> //I changed the order of variables
> BOOST_CONSTMAPFIND(const Mytype &data, "key", mymap)
> {
>     processResult(data);
> }

You need the iterator type, too.

_____
Rob Stewart                           robert.stewart_at_[hidden]
Software Engineer, Core Software      using std::disclaimer;
Susquehanna International Group, LLP  http://www.sig.com

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