Question

I’ve a class which is a wrapper class(serves as a common interface) around another class implementing the functionality required. So my code looks like this.

template<typename ImplemenationClass> class WrapperClass { // the code goes here } 

Now, how do I make sure that ImplementationClass can be derived from a set of classes only, similar to java’s generics

<? extends BaseClass> 

syntax?

Answer 1

It’s verbose, but you can do it like this:

#include <boost/utility/enable_if.hpp> #include <boost/type_traits/is_base_of.hpp>  struct base {};  template <typename ImplementationClass, class Enable = void> class WrapperClass;  template <typename ImplementationClass> class WrapperClass<ImplementationClass,       typename boost::enable_if<         boost::is_base_of<base,ImplementationClass> >::type> {};  struct derived : base {}; struct not_derived {};  int main() {     WrapperClass<derived> x;      // Compile error here:     WrapperClass<not_derived> y; } 

This requires a compiler with good support for the standard (most recent compilers should be fine but old versions of Visual C++ won’t be). For more information, see the Boost.Enable_If documentation.

As Ferruccio said, a simpler but less powerful implementation:

#include <boost/static_assert.hpp> #include <boost/type_traits/is_base_of.hpp>  struct base {};  template <typename ImplementationClass> class WrapperClass {     BOOST_STATIC_ASSERT((         boost::is_base_of<base, ImplementationClass>::value)); };