Item 5. Typename

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Difficulty: 7

"What's in a (type) name ?" Here's an exercise that demonstrates why and how to use typename , using an idiom that's common in the standard library.

1. What is typename , and what does it do?

2. What, if anything, is wrong with the code below?

    template<typename T> class X_base { public:   typedef T instantiated_type; }; template<typename A, typename B> class X : public X_base<B> { public:   bool operator()( const instantiated_type& i ) const   {     return i != instantiated_type();   }   // ... more stuff ... }; 

I l @ ve RuBoard