Tがvalue_typeを持っていればvalue_typeを、持っていなければTを返すメタ関数。　参考https://twitter.com/kester44/status/348712647203618816

value_type_oridentity.cpp

#include <type_traits>

template <bool C, class T, class F>
struct eval_if_c;

template <class T, class F>
struct eval_if_c<true, T, F>
{
    typedef typename T::type type;
};

template <class T, class F>
struct eval_if_c<false, T, F>
{
    typedef typename F::type type;
};

template <class C, class T, class F>
struct eval_if : eval_if_c<C::value, T, F>
{};

template <class T>
struct value_type
{
    typedef typename T::value_type type;
};

template <class T>
struct identity
{
    typedef T type;
};

template <class T, class U = typename T::value_type>
std::true_type check(int);

template <class T>
std::false_type check(long);

template <class T>
struct has_value_type : decltype(check<T>(0)) {};

template <class T>
struct value_type_or_identity
: eval_if<has_value_type<T>, value_type<T>, identity<T>>
{};

#include <vector>
typedef std::vector<int> vec_type;

int main()
{
    static_assert(std::is_same<value_type_or_identity<vec_type>::type,
    vec_type::value_type>::value, "");
    
    static_assert(std::is_same<value_type_or_identity<int>::type,
    int>::value, "");
}