The first thing I need help with is to resolve the ambiguity below. But once the ambiguity is gone, I still need to know if there is a more concise and elegant way to implement the 8 specializations.
#include <iostream>
template <typename>
using void_t = void;
template <typename T, typename U, typename = void, typename = void, typename = void>
struct Foo {
static void call() {std::cout << "Case 1\n";}
};
template <typename T, typename U>
struct Foo<T, U,
void_t<decltype(std::declval<T>().foo(int()))>,
void_t<decltype(std::declval<U>().bar(bool(), char()))>,
void_t<decltype(execute(std::declval<const T&>(), std::declval<const U&>()))>> {
static void call() {std::cout << "Case 2\n";}
};
template <typename T, typename U>
struct Foo<T, U,
void_t<decltype(std::declval<T>().foo(int()))>,
void, void> {
static void call() {std::cout << "Case 3\n";}
};
// etc... for the remaining 5 specializations.
struct Thing {
void foo(int) {}
};
struct Uber {
int bar(bool, char) {return 2;}
};
void execute (const Thing&, const Uber&) {}
int main() {
Foo<Thing, int>::call(); // Case 3
// Foo<Thing, Uber>::call(); // Ambiguous. Want this to be "Case 2" instead of "Case 3".
}
So first, I need to know why Foo<Thing, Uber>::call();
is ambiguous. All 3 void_t's are fulfilled, so isn't Case 2 more specialized than Case 3? Also, I intend to have 5 more specializations for the 2x2x2 possibilities of the 3 void_t's being fulfilled or not fulfilled. What is the most elegant way to handle 2^n such specializations if there were n void_t's being used?
As an analogy, for the case of handling 3 std::enable_if_t
calls like
#include <iostream>
#include <type_traits>
template <bool B> using bool_constant = std::integral_constant<bool, B>;
template <std::size_t N>
struct is_even : bool_constant<N % 2 == 0> {};
template <std::size_t N, std::size_t A>
struct add_to_odd : bool_constant<(N + A) % 2 == 1> {};
template <std::size_t N, std::size_t A, std::size_t B>
struct is_one_of_these : bool_constant<N == A || N == B> {};
template <std::size_t N, std::size_t A, std::size_t B, typename = void, typename = void, typename = void>
struct Foo {
static void call() {std::cout << "Case 1\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct Foo<N,A,B,
std::enable_if_t<is_even<N>::value>,
std::enable_if_t<!add_to_odd<N,A>::value>,
std::enable_if_t<!is_one_of_these<N,A,B>::value>> {
static void call() {std::cout << "Case 2\n";}
};
// etc... for the other combinations of the 3 enable_if conditions being true/false.
int main() {
Foo<1,2,3>::call();
Foo<8,2,3>::call();
}
I figured out
#include <iostream>
#include <type_traits>
template <bool B> using bool_constant = std::integral_constant<bool, B>;
template <std::size_t N>
struct is_even : bool_constant<N % 2 == 0> {};
template <std::size_t N, std::size_t A>
struct add_to_odd : bool_constant<(N + A) % 2 == 1> {};
template <std::size_t N, std::size_t A, std::size_t B>
struct is_one_of_these : bool_constant<N == A || N == B> {};
template <std::size_t N, std::size_t A, std::size_t B, bool, bool, bool>
struct FooHelper {
static void call() {std::cout << "Case 1\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct FooHelper<N, A, B, true, false, false> {
static void call() {std::cout << "Case 2\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct FooHelper<N, A, B, true, true, false> {
static void call() {std::cout << "Case 3\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct FooHelper<N, A, B, true, true, true> {
static void call() {std::cout << "Case 4\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct FooHelper<N, A, B, false, true, true> {
static void call() {std::cout << "Case 5\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct FooHelper<N, A, B, false, false, true> {
static void call() {std::cout << "Case 6\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct FooHelper<N, A, B, true, false, true> {
static void call() {std::cout << "Case 7\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct FooHelper<N, A, B, false, true, false> {
static void call() {std::cout << "Case 8\n";}
};
template <std::size_t N, std::size_t A, std::size_t B>
struct Foo : FooHelper<N, A, B, is_even<N>::value, add_to_odd<N,A>::value, is_one_of_these<N,A,B>::value> {};
int main() {
Foo<1,2,3>::call();
Foo<8,2,3>::call();
// etc...
}
to make the 8 template specializations more concise, maintainable, and easier to read. But the analogy for n void_t's is not so obvious to me (well, the above ambiguity doesn't really help either).
In your case, both case #2 and case #3 are equally specialized, you are just writing it differently void_t<decltype(std::declval<U>().bar(bool(), char()))>
is void
if decltype does not fail.
You could add additional flag to specify, which specialization you want to use. Alternativelly, you can resolve ambiguities with conversion sequence ranking.
#include <iostream>
template <typename>
using void_t = void;
template <int P>
struct Rank : Rank<P - 1> {};
template <>
struct Rank<0> : std::integral_constant<int, 0> {};
// Helper functions
template <typename T>
static auto has_foo_impl(int)
-> decltype(std::declval<T>().foo(int()), std::true_type());
template <typename T>
static auto has_foo_impl(long) -> std::false_type;
template <typename T>
constexpr bool has_foo() {
return std::is_same<decltype(has_foo_impl<T>(0)), std::true_type>::value;
};
template <typename T>
static auto has_bar_impl(int)
-> decltype(std::declval<T>().bar(bool(), char()), std::true_type());
template <typename T>
static auto has_bar_impl(long) -> std::false_type;
template <typename T>
constexpr bool has_bar() {
return std::is_same<decltype(has_bar_impl<T>(0)), std::true_type>::value;
};
template <typename T, typename U>
static auto has_execute_impl(int)
-> decltype(execute(std::declval<T&>(), std::declval<U&>()),
std::true_type());
template <typename T, typename U>
static auto has_execute_impl(long) -> std::false_type;
template <typename T, typename U>
constexpr bool has_execute() {
return std::is_same<decltype(has_execute_impl<T, U>(0)),
std::true_type>::value;
};
// Call overloads
template <typename T, typename U>
static void call_impl(Rank<0>) {
std::cout << "Case 1\n";
}
template <typename T, typename U>
static auto call_impl(Rank<5>)
-> std::enable_if_t<has_foo<T>() && has_bar<U>() && has_execute<T, U>()> {
std::cout << "Case 2\n";
}
template <typename T, typename U>
static auto call_impl(Rank<4>) -> std::enable_if_t<has_foo<T>()> {
std::cout << "Case 3\n";
}
template <typename T, typename U>
struct Foo {
static void call() { call_impl<T, U>(Rank<10>()); }
};
struct Thing {
void foo(int) {}
};
struct Uber {
int bar(bool, char) { return 2; }
};
void execute(const Thing&, const Uber&) {}
int main() {
Foo<Thing, int>::call(); // Case 3
Foo<Thing, Uber>::call(); // Ambiguous. Want this to be "Case 2" instead of
// "Case 3".
}
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