Re: Not getting CRTP right.
On Oct 5, 3:23 am, iCoder <iFi...@gmail.com> wrote:
I am trying out this simple recurring template pattern and i am
getting compilation issues. Corresponding code is:
#include <iostream>
template <typename T>
class Crtp
{
public:
void DefCall(T& aT)
{
aT.DerCall();
}
};
template < template<typename> class CrtpPolicy >
class MyClass : public CrtpPolicy<MyClass>
{
friend class Crtp<MyClass>;
....
};
As the compiler reports, "MyClass" in "CrtpPolicy<MyClass>" names a
template - not a type. So CrtpPolicy cannot be instantiated with
"MyClass" (but could be instantiated with a particular MyClass
specialization.)
What confuses matters here, is that a template name like "MyClass" -
within its own template definition - often does refer to a type:
namely, the type of the current template specialization. As an
example, in the friend declaration above, "MyClass" does not refer to
the MyClass template, but is in fact shorthand for the
MyClass<CrtpPolicy> specialization.
In the base class declaration, however, this shorthand notation is not
recognized - so "MyClass" can refer only to the template. The solution
therefore is to replace "MyClass" with the name of the current MyClass
template specialization:
template <template <typename C> class CrtpPolicy >
class MyClass : public CrtpPolicy< MyClass<CrtpPolicy> >
{
friend class Crtp<MyClass>;
...
};
Greg
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