如果模板类型是可实例化的,则执行某些操作

//Only instantiable with types T for which T.x() is ok:
template <class T>
class TemplateClass
{
  T t;
public:
  void foo() { 
    t.x(); 
  }
}

template <class T>
class User
{
  void foo()
  { 
    "if TemplateClass<T> is ok then do something else do nothing" 
  }
}

我怎么能那样做?

谢谢!

template <class T>
struct TemplateClass
{
    T t;
    void foo() { t.x(); }

    static const bool value = true;

};
struct A {};
struct B { void x() {} };

template <class T>
struct User
{
    template<typename M>
    typename boost::enable_if<TemplateClass<M> >::type func( ) 
    { 
        std::cout << "enabled\n";
    }

    template<typename M>
    typename boost::disable_if<TemplateClass<M> >::type func( ) 
    { 
        std::cout << "disabled\n";
    }


    void foo()
    {
        func<TemplateClass<T> >();
    }

};

User<A> a;
a.foo();

User<B> b;
b.foo();

但这将返回“enabled enabled”。我错过了什么?