In C++14, std::set<Key>::find
is a template
function if Compare::is_transparent
exists. The type you pass in does not need to be Key
, just equivalent under your comparator.
So write a comparator:
template<class T>
struct pointer_comp {
typedef std::true_type is_transparent;
// helper does some magic in order to reduce the number of
// pairs of types we need to know how to compare: it turns
// everything into a pointer, and then uses `std::less<T*>`
// to do the comparison:
struct helper {
T* ptr;
helper():ptr(nullptr) {}
helper(helper const&) = default;
helper(T* p):ptr(p) {}
template<class U, class...Ts>
helper( std::shared_ptr<U,Ts...> const& sp ):ptr(sp.get()) {}
template<class U, class...Ts>
helper( std::unique_ptr<U, Ts...> const& up ):ptr(up.get()) {}
// && optional: enforces rvalue use only
bool operator<( helper o ) const {
return std::less<T*>()( ptr, o.ptr );
}
};
// without helper, we would need 2^n different overloads, where
// n is the number of types we want to support (so, 8 with
// raw pointers, unique pointers, and shared pointers). That
// seems silly:
// && helps enforce rvalue use only
bool operator()( helper const&& lhs, helper const&& rhs ) const {
return lhs < rhs;
}
};
then use it:
typedef std::set< std::unique_ptr<Foo>, pointer_comp<Foo> > owning_foo_set;
now, owning_foo_set::find
will accept unique_ptr<Foo>
or Foo*
or shared_ptr<Foo>
(or any derived class of Foo
) and find the correct element.
Outside of C++14, you are forced to use the map
to unique_ptr
approach, or something equivalent, as the signature of find
is overly restrictive. Or write your own set
equivalent.