Using CRTP with partial class specialization? - c++

Question

I'm trying to mix-in an operator [] with a class. My problem is I've partially specialized the class, and the compiler doesn't like me not specifying the template parameters to the derived class:
#include <iostream>
#include <type_traits>
using namespace std;
template <typename T>
struct mixin {
template <typename U>
void operator[](U u) {
cout << u;
}
};
template <typename T, typename = void>
struct derived : mixin<derived> {};
template <typename T>
struct derived<T,
typename enable_if<
is_same<T, int>{}
>::type> : mixin<derived> {};
int main() {
derived<int> d;
d[3.14];
}
With clang this gives:
test.cc:16:24: error: use of class template 'derived' requires template arguments
struct derived : mixin<derived> {};
^~~~~~~
test.cc:16:8: note: template is declared here
struct derived : mixin<derived> {};
^
test.cc:23:22: error: use of class template 'derived' requires template arguments
>::type> : mixin<derived> {};
^~~~~~~
test.cc:16:8: note: template is declared here
struct derived : mixin<derived> {};
^
gcc is even less helpful:
test.cc:16:31: error: type/value mismatch at argument 1 in template parameter list for ‘template<class T> struct mixin’
struct derived : mixin<derived> {};
^
test.cc:16:31: note: expected a type, got ‘derived’
test.cc:23:29: error: type/value mismatch at argument 1 in template parameter list for ‘template<class T> struct mixin’
>::type> : mixin<derived> {};
^
test.cc:23:29: note: expected a type, got ‘derived’
test.cc: In function ‘int main()’:
Is my only option to re-specify the template parameters inside of the mixin clause?

Answer 1

Well, try this:
#include <iostream>
#include <type_traits>
using namespace std;
template <typename T>
struct mixin {
template <typename U>
void operator[](U u) {
cout << u;
}
};
template <typename T, typename = void>
struct derived : mixin<derived<T>> {};
template <typename T>
struct derived<T,
typename enable_if<
is_same<T, int>::value
>::type> : mixin<derived<T>> {};
int main() {
derived<int> d;
d[3.14];
}
It does work...
What I changed:
Using is_same<foo,bar>::value, not is_same<foo,bar>{} edit: Hmm, it seems you don't need to change that after all. Neat!
Not trying to get the compiler to deduce the template parameter for derived when using mixin<derived>. You were being way too optimistic there...