From: casey_at_carter_dot_net@nospam.invalid   
      
   On 2012-09-05 02:48, Ivan Godard wrote:   
      
   > I have a template class taking a typename T argument, and need to   
   > define a differenceType representing the abstract "distance" between   
   > any pair of values. The template is instantiated with enums,   
   > built-in types, and random user classes that define   
   > operator-(T, T). This difference type was initially declared as:   
   >      typedef typeof(T() - T()) differenceType;   
   >   
   > I recently converted an existing enum to a C++11 "enum class", and   
   > the above broke because operator- was not defined for enum classes   
   > and there is no conversion to something that defines it.   
      
   This is exactly the use case for std::declval(): "creating" an   
   expression of type T for use in non-evaluated contexts - e.g.,   
   decltype(), sizeof() - without needing a valid constructor   
   expression. Try:   
      
       typedef   
         decltype(std::declval() - std::declval()) differenceType;   
      
      
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