mardi 28 juillet 2015

Specializing variadic template class for template template parameters

I have the following meta struct templates, which are intended to check whether a specific type (the KeyType) is part of a parameter pack.

#include <type_traits>
#include <utility>

template<typename ...>
struct find_type : std::false_type {};

//specialization used if queried type is found
template<typename QueriedType>
struct find_type<QueriedType, QueriedType> : std::true_type {};

//specialization used if KeyType and ValueType not wrapped
template<typename QueriedType,
         typename KeyType,
         typename ValueType,
         typename... Remaining>
struct find_type<QueriedType, KeyType, ValueType, Remaining...>
{
    static const bool value =
        std::conditional<find_type<QueriedType, KeyType>::value,
                         std::true_type,
                         find_type<QueriedType, Remaining...>>::type::value;
};

//specialization used if KeyType and ValueType are wrapped inside e.g. std::pair
template<typename QueriedType,
         template<typename, typename> class C,
         typename KeyType,
         typename ValueType,
         typename ...Remaining>
struct find_type<QueriedType, C<KeyType, ValueType>, Remaining...> : 
    find_type<QueriedType, KeyType, ValueType, Remaining...>{};

//entry point
template<typename ...>
struct entry_find_type;

template<typename QueriedType,
         typename ...Remaining>
struct entry_find_type<QueriedType, std::tuple<Remaining...>> :
    find_type<QueriedType, Remaining...> {};

There are probably better ways to do this, but I got an interesting behavior when using the structs. Consider the following code snippet. In the first scope I trigger the find_type first passing a key-value-pair without a wrapper followed by passing the second key-value-pair using std::pair. This compiles fine. In the second scope I trigger the find_type with the same key-value-pairs, just switching std::pair with the unwrapped key-value-pair. This yields a compiler error due to ambiguous specializations (see below).

class IFirstType
{
};

class ISecondType
{
};

class IDerived : public IFirstType, public ISecondType
{
};

int main(int argc, char*argv[])
{
  { //compiles fine
    typedef std::tuple<IFirstType, IDerived,
                       std::pair<ISecondType, IFirstType>>  tuple_type;
    typedef entry_find_type<ISecondType, tuple_type>        query_type;

    static const bool bFound = query_type::value;
  }

  { //compile error
    typedef std::tuple<std::pair<ISecondType, IFirstType>,
                       IFirstType, IDerived>                tuple_type;
    typedef entry_find_type<ISecondType, tuple_type>        query_type;

    //static const bool bFound = query_type::value; //<--- compiler error here
  }

  return 0;
}

The compiler output (gcc 4.9):

/tmp/gcc-explorer-compiler115628-35-8rkb3f/example.cpp: In instantiation of 'struct entry_find_type<ISecondType, std::tuple<std::pair<ISecondType, IFirstType>, IFirstType, IDerived> >':
69 : required from here
39 : error: ambiguous class template instantiation for 'struct find_type, IFirstType, IDerived>'
struct entry_find_type<QueriedType, std::tuple<Remaining...>> :
^
16 : error: candidates are: struct find_type
struct find_type<QueriedType, KeyType, ValueType, Remaining...>
^
30 : error: struct find_type, Remaining ...>
struct find_type<QueriedType, C<KeyType, ValueType>, Remaining...> :
^
39 : error: invalid use of incomplete type 'struct find_type, IFirstType, IDerived>'
struct entry_find_type<QueriedType, std::tuple<Remaining...>> :
^
5 : error: declaration of 'struct find_type, IFirstType, IDerived>'
struct find_type : std::false_type {};
^
/tmp/gcc-explorer-compiler115628-35-8rkb3f/example.cpp: In function 'int main(int, char**)':
69 : error: 'value' is not a member of 'query_type {aka entry_find_type, IFirstType, IDerived> >}'
static const bool bFound = query_type::value; //<--- compiler error here
^
Compilation failed

live example at gcc.godbolt.org

If I understand correctly, the compiler should choose the same template specializations in both cases and yet the second case does not compile. I want the compiler to pick the specialized template template parameter version for both usages of std::pair. Can someone tell me what I'm doing wrong?

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