Code duplication and template specialization (when the specialized function has different return types)

Question

I am creating a templated class D<N>, with a method (operator(), in this case) that returns different types, depending on the value of N.

I could only make this work by creating two separate class declarations, but this came at the cost of a lot of code duplication. I also tried to create a common base class to throw the common stuff into, but I couldn't get the constructor to inherit right and don't know how idiomatic that would be as well...

#include <cstdio>

template <int N>
struct D{
    int s;
    D(int x):s(x){}

    int yell(int x){
        printf("N=%d, %d\n", N, s+x);
        return s+x;
    }

    D<N-1> operator()(int x){
        D<N-1> d(yell(x));
        return d;
    }
};

template <>
struct D<1>{
    int s;
    D(int x): s(x){}

    int yell(int x){
        printf("N=%d, %d\n", 1, s+x);
        return s+x;
    }

    int operator()(int x){
        return yell(x);
    }
};


int main()
{
    D<2> f(42);
    printf("%d\n", f(1)(2));
    return 0;
}

How can I make my code better looking?

Answer 1

You can use the Curiously Recurring Template Pattern.

template<int N, template<int> typename D> struct d_inner {
    D<N-1> operator()(int x) {
        return D<N-1>(static_cast<D<N>*>(this)->yell(x));
    }
};
template<template<int> typename D> struct d_inner<1, D> {
    int operator()(int x) {
        return static_cast<D<1>*>(this)->yell(x);
    }
};

template <int N> struct D : public d_inner<N, D> {
    int s;
    D(int x):s(x){}

    int yell(int x){
        printf("N=%d, %d\n", N, s+x);
        return s+x;
    }
};

Not that I see the utility- or purpose- of this particular object being templated, it could easily not be.