Re: [Boost-users] FOREACH with maps

From: Stefan Strasser (sstrasser_at_[hidden])
Date: 2005-05-10 17:39:37

Dave Steffen schrieb:
> Another question about the to-be-released FOREACH macro, which I find
> I am using heavily... (BTW - Thanks Eric for your quick responses, and
> for the cool macro!)
> 
> The issue is declaring the 'loop variable' for maps.  For example,
> 
> 
>  vector<int> v;
>  ...
>  FOREACH (int a, v) { ... }   // this compiles
>  int b;
>  FOREACH (b, v) {...}  // this also compiles
> 
> However, for maps, the second variation (declaring outside the macro)
> doesn't work:
> 
>  map<int,int> m;
>  ...
>  FOREACH (map<int,int>::value_type i, m) {...}  // compiles
>  map<int,int>::value_type i;
>  FOREACH (i, m) {...}                     // COMPILE ERROR
> 
> 
> Specifically, the compiler says 
> 
> error: non-static const member 
> ` const int std::pair<const int,int>::first', 
> can't use default assignment operator
> 
> (from GCC 3.3.1).  Despite my best efforts, I can't figure out what
> this error means, or what it's talking about.  Anybody else run into
> this?  Any thoughts?
> 

this also doesn't work:

map<int,int>::value_type i;
i=std::pair<int,int>(1,2); // error

that's because map<Key,Value>::value_type is defined to
std::pair<Key /**/const/**/,Value>.

and that's probably because you're not allowed to change the key of a 
map entry when you're e.g. iterating over the map.

just use std::pair<int,int> instead and your example compiles.

-- 
Stefan Strasser