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"));
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"
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 * value(&dummy);
data_type & _value(*value);
if (it != _map.end())
value = &it->second;
> 1. exception... throwing the exception removes the complexity of
> writing but introduces the complexity of exception (writing code +
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)
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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