Hello,
The range metafunctions currently do not work on range reference types.
For example, the following:
#include <vector>
#include
typedef boost::range_iterator::type iter_t;
Fails with the errors:
In file included from lib/boost/boost/range/iterator.hpp:23:0,
from lib/boost/boost/range/metafunctions.hpp:18,
from test4.cpp:2:
lib/boost/boost/mpl/eval_if.hpp: In instantiation of 'boost::mpl::eval_if_c, boost::range_mutable_iterator >':
lib/boost/boost/range/iterator.hpp:72:63: instantiated from 'boost::range_iterator'
test4.cpp:4:49: instantiated from here
lib/boost/boost/mpl/eval_if.hpp:60:31: error: no type named 'type' in 'boost::mpl::eval_if_c, boost::range_mutable_iterator >::f_ {aka struct boost::range_mutable_iterator}'
This behaviour is OK when you are doing things like this:
template <typename Range>
typename range_iterator<Range>::type some_function(const Range& r) { ... }
because when called with a "const vector<T>&" argument, template argument deduction
will deduce "Range" to be just "vector<T>", and this is what we call the metafunction
on.
Likewise, if some_function has a non-const version that takes a "Range&" argument,
it works fine too.
But in C++0x, rvalue references allow us to kill two birds with one stone and have
just one function that accepts both const and non-const references:
template <typename Range>
typename range_iterator<Range>::type some_function(Range&& r) { ... }
However, the template argument deduction rules for rvalue references are different:
if the function is called with a "const vector<T>&" parameter, "Range" will be
deduced as "const vector<T>&", i.e. the reference is preserved.
But now range_iterator gives an error as indicated above, so we have to do something
awkward and verbose like this:
template <typename Range>
typename range_iterator::type some_function(Range&& r) { ... }
Could the range metafunctions be modified to accept range reference parameters and
remove the reference themselves?
Thanks,
Nate.
