I have the following type traits to distinguish between fundamental and container types:
template <typename T>
using enable_if_fundamental_t = std::enable_if_t<std::is_fundamental_v<T>>;
template <typename T, typename = void>
struct is_container : std::false_type {};
template <typename T>
struct is_container<
T
, std::void_t<
typename T::value_type
, typename T::size_type
, typename T::allocator_type
, typename T::iterator
, typename T::const_iterator
, decltype(std::declval<T>().size())
, decltype(std::declval<T>().begin())
, decltype(std::declval<T>().end())
, decltype(std::declval<T>().cbegin())
, decltype(std::declval<T>().cend())
>
> : std::true_type {};
template <typename T>
constexpr bool is_container_v = is_container<T>::value;
template <typename T>
using enable_if_container_t = std::enable_if_t<is_container_v<T>>;
and they are used with the following functions:
template <typename T, typename = enable_if_fundamental_t<T>>
void foo(T)
{
std::cout << "This is a fundamental type" << std::endl;
}
template <typename C, typename = enable_if_container_t<C>>
void foo(const C& c)
{
std::cout << "This is a container type" << std::endl;
}
with the following arguments:
std::list<std::uint32_t> l;
std::vector<std::uint32_t> v;
std::map<std::string, std::uint32_t> m;
std::unordered_map<std::string, std::uint32_t> um;
std::uint32_t i = 42;
foo(l);
foo(v);
foo(m);
foo(um);
foo(i);
and they work fine.
Now I want to distinguish containers which has overloaded operator[]
from the others. I tried the following code:
template <typename T, typename = void>
struct is_container_with_index_operator_with_size_type : std::false_type {};
template <typename T>
struct is_container_with_index_operator_with_size_type<
T
, std::void_t<
enable_if_container_t<T>
, decltype(std::declval<T>().operator[](std::declval<typename T::size_type>()))
>
> : std::true_type {};
template <typename T>
constexpr bool is_container_with_index_operator_with_size_type_v =
is_container_with_index_operator_with_size_type<T>::value;
template <typename T, typename = void>
struct is_container_with_index_operator_with_key_type : std::false_type {};
template <typename T>
struct is_container_with_index_operator_with_key_type<
T
, std::void_t<
enable_if_container_t<T>
, typename T::key_type
, decltype(std::declval<T>().operator[](std::declval<typename T::key_type>()))
>
> : std::true_type {};
template <typename T>
constexpr bool is_container_with_index_operator_with_key_type_v =
is_container_with_index_operator_with_key_type<T>::value;
template <typename T>
constexpr bool is_container_with_index_operator_v =
is_container_with_index_operator_with_size_type_v<T> ||
is_container_with_index_operator_with_key_type_v<T>;
template <typename T>
constexpr bool is_container_without_index_operator_v =
is_container_v<T> &&
!is_container_with_index_operator_v<T>;
template <class T>
using enable_if_container_with_index_operator_t =
std::enable_if_t<is_container_with_index_operator_v<T>>;
template <class T>
using enable_if_container_without_index_operator_t =
std::enable_if_t<is_container_without_index_operator_v<T>>;
with the following overloads:
template <typename T, typename = enable_if_fundamental_t<T>>
void foo(T)
{
std::cout << "This is a fundamental type" << std::endl;
}
template <typename C, typename = enable_if_container_without_index_operator_t<C>>
void foo(const C&)
{
std::cout << "This is a container type without index operator" << std::endl;
}
template <typename C, typename = enable_if_container_with_index_operator_t<C>>
void foo(const C&)
{
std::cout << "This is a container type with index operator" << std::endl;
}
with the same arguments as above, bit it produced me the error:
error C2995: 'void foo(const C &)': function template has already been defined
I tried several variations of the code above, but I didn't manage to do it the right way.
How to do this the right way and is it possible to achieve a simpler code? For example without separate meta functions for containers which use size_type
and key_type
for the operator[]
?
I'm using Visual Studio 2017 version 15.7.2 with v141 toolset and enabled /std:c++17
.
That's because you can't overload functions based on default template parameter values alone. You can reproduce that with:
template <typename T, typename = std::enable_if_t<(sizeof(T) > 2)>> void foo() {}
template <typename T, typename = std::enable_if_t<!(sizeof(T) > 2)>> void foo() {}
// error: redefinition of 'template<class T, class> void foo()'
A possible solution is to use enable_if_t
in a type of a template parameter:
template <typename T, std::enable_if_t<(sizeof(T) > 2), int> = 0> void foo() {}
template <typename T, std::enable_if_t<!(sizeof(T) > 2), int> = 0> void foo() {}
Or in return type:
template <typename T> std::enable_if_t<(sizeof(T) > 2)/*,void*/> foo() {}
template <typename T> std::enable_if_t<!(sizeof(T) > 2)/*,void*/> foo() {}
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