http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58191
--- Comment #5 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
(In reply to Paolo Carlini from comment #2)
> Francois, did we change anything in the library for 4.8.x?
I think that Francois added more iterator concept checking and this one looks
correct. Unfortunately I would say, this is correct, because the standard says
so, but from a practical point of view, std::upper_bound "just works" for any
forward traversal iterator and it does not require that the return type of
operator* *really* is a reference type. We also cannot blame boost here,
because it is not their fault that the standard requires for forward iterators
to have a real reference return type for iterator dereference. My
recommendation to the issue reporter would be to provide a function object that
returns a real reference. This can be easily done by a unary functor adaptor
such as the following one:
template<class UnaryFunction, class ArgumentType>
struct LValueUnaryFunctionAdaptor
{
typedef typename std::result_of<const UnaryFunction(ArgumentType)>::type
value_type;
LValueUnaryFunctionAdaptor(UnaryFunction func) : func(func) {}
typedef const value_type& result_type;
result_type operator()(ArgumentType arg) const
{
return res = func(arg);
}
private:
UnaryFunction func;
mutable value_type res;
};
template<class ArgumentType, class UnaryFunction>
inline
LValueUnaryFunctionAdaptor<UnaryFunction, ArgumentType>
make_LValueUnaryFunction(UnaryFunction func)
{
return LValueUnaryFunctionAdaptor<UnaryFunction, ArgumentType>(func);
}
So instead of
calc_value
he could use
make_LValueUnaryFunction<int>(calc_value)
This trick doesn't make the transform_iterator fully conforming, but it should
convince the compile-time test machinery.
