Declaring member or not depending on template parameter

Question

Is it possible to declare or not a member variable depending on template condition without using dummy empty type?

Example:

struct empty{};
struct real_type{};

template<bool condition>
struct foo
{
    typename std::conditional<condition, real_type, empty>::type _member;
};

Answer 1

You can derive from a template that has a specialization:

struct real_type { };

template<bool c>
struct foo_base { };

template<>
struct foo_base<true>
{
    real_type _member;
};

template<bool condition>
struct foo : foo_base<condition>
{
};

As a small test:

int main()
{
    foo<true> t;
    t._member.x = 42; // OK

    foo<false> f;
    f._member.x = 42; // ERROR! No _member exists
}