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Tip of the Day
Language: C++
Expertise: Intermediate
Apr 6, 1999



Building the Right Environment to Support AI, Machine Learning and Deep Learning

Forward Declarations and Templates

It is illegal to use forward declarations with typedef names, as in:
   class string;
   void f(string& s); //illegal, string is a typedef name
Even a typename won't do here:
   typename  std::string; //still illegal
   void f(std::string& s);  
The problem with these forward declarations is that std::string is not a class, but a typedef name defined like this:
   typedef basic_string<char, char_traits<char>, allocator<char> >string;
In order to generate the correct mangled name for the function f, the compiler has to see the non-typedef'd form of its argument. In other words, there is no escape from #including the header <string> in this case.
Danny Kalev
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