Re: Is run-time switching on type traits bad?

"Daniel T." <>
Thu, 15 Apr 2010 16:32:12 CST
In article <4bc64413$0$275$>,
  DeMarcus <> wrote:


Is run-time switching on type traits as bad as switching on class type?
(Sutter & Alexandrescu, C++ Coding Standards, Item 90 - Avoid type

I.e. should type traits be for compile time only?

A run-time example:

class VisitableInterface
    virtual ~VisitableInterface() {}

    virtual void accept( Visitor& v ) const = 0;
    virtual bool isPOD() const = 0;

template<typename T>
class SomeVisitable : public VisitableInterface
    SomeVisitable( const T& element ) : element_(element) {}

    virtual void accept( Visitor& v ) const

    virtual bool isPOD() const
       return std::is_pod<T>::value;

    const T element_;

int main()
    std::vector<VisitableInferface*> vec;
    PodVisitor podVisitor;
    ClassVisitor classVisitor;

    for( auto i = vec.begin(); i != vec.end(); ++i )
       if( (*i)->isPOD() )
          (*i)->accept( podVisitor );
          (*i)->accept( classVisitor );

You are breaking "tell don't ask" here.

However, when dealing with PODs you don't have a lot of choices in the matter. Does it really need to be POD? Could you make two SomeVisitable classes, one for PODs and the other for classes, with compile time checking to make sure the correct one is
used? After all, when the programmer instantiates the template, he knows whether the type he is passing is POD or not.

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