f2b07020   
   From: daniel.kruegler@googlemail.com   
      
   Am 12.01.2012 20:48, schrieb kelvSYC:   
   > I'm kind of confused on how enable_if and virtual functions work   
   > together.  Suppose you have this:   
   >   
   > (using your favourite implementation of enable_if here)   
   >   
   > template   
   > struct DerivedProperty;   
   >   
   > template   
   > class AbstractBase {    // CRTP, as you can't mix "virtual" and   
   > "function template", it would appear... or can you?   
      
   Yes, you cannot declare a function template that is a virtual function.   
      
   >     virtual enable_if, int>::type doSomething() =   
   > 0;   
   > };   
      
   This is an absolutely useless application of enable_if - and this has nothing   
   to do with virtual functions. In your example the declaration of doSomething()   
   will be instantiated once AbstractBase will be instantiated for some type U   
   making the    
   instantiation of AbstractBase potentially ill-formed (depending on U). The   
   only reason for such a construction would be to act as some convoluted form of   
   static assertion that would better be written as   
      
   template   
   class AbstractBase {   
    static_assert(DerivedProperty::value, "DerivedProperty not satisfied");   
    virtual int doSomething() = 0;   
   };   
      
   instead. If you want to declare a virtual function depending on some   
   constraints, it is better to do that via a base-class switch, e.g.   
      
   template::value>   
   class AbstractBase {   
    virtual int doSomething() = 0;   
   };   
      
   template   
   class AbstractBase {};   
      
   > Also, would something similar apply if the enable_if was instead a   
   > parameter instead of the return type?   
      
   This would not change anything, because in either situation the declaration of   
   the function could be ill-formed.   
      
   HTH & Greetings from Bremen,   
      
   Daniel Krügler   
      
      
      
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