Re: [Boost-users] FOREACH with maps

From: Ben Hutchings (ben.hutchings_at_[hidden])
Date: 2005-05-10 17:02:02

Dave Steffen wrote:
> 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!)
<snip>
> 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.

The first (key) member of the value_type for a map is const, so it can't 
have a default assignment operator.  This issue doesn't arise if you 
declare the loop variable inside the loop because it's constructed and 
destroyed on each pass rather than being assigned to.

 > Anybody else run into this?  Any thoughts?

You could make i a reference to value_type if you don't need to copy the 
map elements, or you could declare it as pair<int, int> (without the 
const) instead.

Ben.