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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!)
> 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.


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