[Boost-users] Re: FOREACH with maps

From: Pablo Aguilar (pablo.aguilar_at_[hidden])
Date: 2005-05-10 17:08:44

> 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?

A map's value_type is defined as:
std::pair<const key, value>

Since the .first member is const, and pair doesn't have an overloaded 
assignment operator, you can't assign to it, as the compiler can't generate 
the operator automatically because it can't assign to .first.

You'll have better luck using std::pair<int, int> in place of 
map<int,int>::value_type, hence
pair<int,int> i;
FOREACH (i, m) {...}

should work.

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HTH

Pablo Aguilar