[boost] [iterator + lambda] transform_iterator and lambda expression

From: Vladimir Prus (ghost_at_[hidden])
Date: 2004-04-23 10:04:59

I'm again trying to use FP in C++, and having problems. What I wanted to do 
was:

   for_each(predecesor(v, g), /* BLL expression */ );

where predecessor(..) returns a pair of 'transform_iterator' instances. The 
simplest example which illustrates the problem is:

     struct functor {
         typedef int result_type;
        int operator()(int i) const { return i+1; };
     };

     transform_iterator<functor, vector<int>::iterator> it(v.begin());
     (cout << _1)(*it);

This does not compile, because operator* of transform_iterator return rvalue 
which can't be bound to non-const reference that operator() of the lambda 
expression accepts.

Of all workarounds suggsted on BLL docs, only break_const is applicable to my 
case (I want to pass non-const references to the lambda, so const_parameters 
won't do), and break_const is scary.

It looks like a serious problem with using FP, so I wonder what can be done. 
Maybe, transform_iterator should have yet another template parameter telling 
if it should store a value internally, and return reference (i.e. lvalue) in 
that case? It need not be enabled by default, but functions like my 
'predecessor' above would make use of that extra template parameter.

- Volodya