Re: [suse-programming-e] meaning of "typename"

Verdi March wrote:

Hi,

What is the meaning of "typename" in C++? I never realized there's such a keyword until now.

typename tells the compiler to interpret what follows as a type rather than as a function or member. So the compiler would expect map::const_iterator to be a static member function or static member variable of the class map and generate an error in this case. g++ is clever enough to work out that in this case it cannot be a static member and there is in map a typdef const_iterator and so just warns you that you should put in typename. To see why typename is essential, consider the following template<class T> void function( T* t ){ T::const_iterator it = t->begin(); ... } Since this can be compiled without any knowledge of a specific T, the compiler has no way to tell whether T::const_iterator should be a type or a static member. The rule is that it assumes it is a static member and produces a syntax error. If you want to change the interpretation, use typename.

For the following function:

template<class T> void clearMap(map m) { map::const_iterator it = m.begin(); /* X */ for (int i = m.size(); i > 0; i--, ++it) { const string* key = &(it->first); m.erase(*key); delete(key); } }

g++ will warn that the declaration of iterator it (line marked with /* X */) is deprecated. But using: typename map::const_iterator it = ... will remove the warning.

TIA.

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