Question

I have ran into yet another problem I do not understand.

The following does not instantiate (argument instantiation fails), why?

template<class E>
void operator[](typename boost::mpl::identity<E>::type e) const;

thank you for your help

Answer 1

identity can be used to force you to specify the template argument explicitly. It effectively prevents that function parameter from partaking in template argument deduction.

A qualified type name is one of the non deduced contexts; that is, identity<E>::type will not be used to deduce the template parameter for E.

For example, if you have:

template<class E>
void f(typename boost::mpl::identity<E>::type e) { }

f(42);      // won't work
f<int>(42); // works