recursive template inheritance code duplication - c++

Question

#include <iostream>
template< class T, unsigned S >
struct my_iterator;
template< class T >
struct my_iterator< T, 1 >
{
T* p;
};
template< class T, unsigned S >
struct my_iterator : my_iterator< T, S / 2 >
{
static_assert ((S & (S - 1)) == 0, "S must be a power of 2");
using my_iterator< T, S / 2 >::p;
unsigned burp() {return (*p) + S;}
};
int main()
{
int v = 10;
my_iterator< int, 8 > a;
a.p = &v;
std::cout << a.burp() << std::endl;
my_iterator< int, 4 >& b = a;
std::cout << b.burp() << std::endl;
my_iterator< int, 1 > c;
c.p = &v;
std::cout << c.burp() << std::endl; // error: no member named 'burp'
return 0;
}
This will fix the error:
template< class T >
struct my_iterator< T, 1 >
{
unsigned burp() {return (*p) + 1;}
T* p;
};
but in my real code I have many methods, not just burp, all dependent on S and p, that would all need to be implemented twice, once in the general class and once in the specialization. Is there any way to avoid the duplicate code? I saw this similar question:
Avoiding code duplication in a specialized template
but the answer will not work in my case because I'll end up with many copies of p, one at each level of the recursive hierarchy.

Answer 1

Another option, if you have a recursion stop value of 1 which you want to still use, you can just move the stop value to 0...
This way my_iterator<T,1> still has the default implementation.
template< class T, unsigned S >
struct my_iterator;
template< class T >
struct my_iterator< T, 0 >
{
T* p;
};
template< class T, unsigned S >
struct my_iterator : my_iterator< T, S / 2 >
{
static_assert ((S & (S - 1)) == 0, "S must be a power of 2");
using my_iterator< T, S / 2 >::p;
unsigned burp() {return (*p) + S;}
};

Answer 2

An other way is to move specific stuff in an other structure:
template <typename T> struct opt_ptr { T* p; };
// to be able to use std::conditional with non instantiated type.
template <typename T> struct identity { using type = T; };
template<typename T, unsigned S>
struct my_iterator :
std::conditional<S == 1,
identity<opt_ptr<T>>,
identity<my_iterator<T, S / 2>>>::type::type
{
static_assert ((S & (S - 1)) == 0, "S must be a power of 2");
using opt_ptr<T>::p; // Note that you may use this->p to avoid this line
unsigned burp() {return *p + S;}
// Other methods using S and p.
};

Answer 3

You could pass down S to the base template, and move the method there.
template< class T, unsigned S, unsigned N >
struct my_iterator;
template< class T, unsigned S >
struct my_iterator< T,S, 1 >
{
unsigned burp() {return (*p) + S;}
T* p;
};
template< class T, unsigned S, unsigned N = S >
struct my_iterator : my_iterator< T, S, N / 2 >
{
static_assert ((S & (S - 1)) == 0, "S must be a power of 2");
using my_iterator< T,S, N / 2 >::p;
};