Re: [Boost-users] Iterators and maps [was Re: Where should STL suggestionsgo?]

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2004-12-13 14:33:05

Jim Lear wrote:
> The map iterators are really what tripped me up as an STL newbie.  One 
> must understand the iterators are implemented as pairs for maps, and 
> pointers for vectors and other things.  By providing an "T 
> operator[](iterator)" function for random access and associative 
> containers (and for sequences maybe), one could achieve a consistant 
> interface.

Container-independent code is an impossible and pointless goal.  maps 
have different semantics from other containers.  You may want to iterate 
over just the mapped values, but other people may want to iterate over 
the keys, or over the pairs (and indeed, so may you, in other part of 
your program).  Who's to say which of these is right?  What you can do 
is to write iterator-independent code, then use an iterator adapter to 
convert the iterators over pairs into iterators over the mapped values:

#include "boost/bind.hpp"
#include "boost/iterator/transform_iterator.hpp"

...

my_map_type map;
generic_function_on_iterators(
     boost::make_transform_iterator(
         map.begin(), boost::bind(&my_map_type::second, _1),
     boost::make_transform_iterator(
         map.end(), boost::bind(&my_map_type::second, _1));

(Unfortunately the return type of boost::bind is unspecified.)

It would perhaps be useful for map to have member functions like 
keys_begin(), keys_end(), etc.

<snip>
>     #include <map>
>     #include <string>
>     #include <iostream>
>    
>     void main() {

Should return int.

>       typedef map<string, int> si_map_t;
>       typedef map<string, si_map_t> ssi_map_t;
>       ssi_map_t table;
>       table["0"]["0"]=20;
>       table["0"]["1"]=21;
>       table["1"]["0"]=10;
>       table["1"]["1"]=11;
>    
>       for (ssi_map_t::iterator i=table.begin(); i!=table.end(); i++)
>         for (si_map_t::iterator j=i->second.begin(); j!=i->second.end(); 
> j++)
>           cout << j->second << endl;
>       // Blech! :-)  This is quite obscure for newbies.
<snip>

I realise that pairs don't have very helpful member names, but I really 
don't think it's that weird for iteration to yield both keys and mapped 
values.

Ben.