On Apr 23, 2004, at 10:04 AM, Vladimir Prus wrote:
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.
Forwarding problem again :( For a limited number of placeholders (up until _3) or so, we could provide the extra overloads to make this problem go away. N-ary operator() requires 2^N overloads, so for current BLL it is feasible, for bind with placeholders up to _9 (or more?) not really. Jaakko