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From: Ruben Perez (rubenperez038_at_[hidden])
Date: 2025-05-06 13:51:21
<CC-ing the mailing list, since you just replied me, and answering inline>
On Tue, 6 May 2025 at 01:35, Jean-Louis Leroy <jl_at_leroy.nyc> wrote:
>
> Your example is very similar to
> https://jll63.github.io/Boost.OpenMethod/#tutorials_custom_rtti.
>
> virtual_ptr does not try to use boost_openmethod_vptr in its constructors. That
> is an oversight. It is a recent invention. YOMM2 tests for the presence of a
> boost_openmethod_vptr instance variable in the object directly. In that context,
> the function is very simple and inlinable, but if it is user-defined, there is
> no reason to assume that. So I will make virtual_ptr consider it as well.
>
> Regardless, the easiest way to handle your example is to put the vptr
> acquisition in the policy. dynamic_type returns a type_id, not a vptr. The
> function for that is dynamic_vptr. It is not part of a facet, eventhough facets
> can provide one. The dispatch mechanism is described here:
> https://jll63.github.io/Boost.OpenMethod/#ref_description_17
>
> WARNING: if you try that code with the "review" branch, make sure to pull the
> latest changes, because they fix a bug I introduced while re-establishing checks
> for non-polymorphic classes.
>
> Thus:
>
>
> // ------------------------
> // you
>
> #include <cstdint>
>
> enum class kind : std::uintptr_t { unknown, n1, n2 };
>
> class base_node {
> kind k_;
>
> protected:
> base_node(kind k) noexcept : k_(k) {
> }
>
> public:
> kind getKind() const {
> return k_;
> }
> };
>
> class node1 : public base_node {
> public:
> node1() noexcept : base_node(kind::n1) {
> }
> };
>
> class node2 : public base_node {
> public:
> node2() noexcept : base_node(kind::n2) {
> }
> };
>
> // ------------------------
> // me
>
> // Get kind from static type, as you RTTI system does not provide it as a
> // static member.
>
> template<typename T>
> struct kind_of {
> static constexpr kind value = kind::unknown;
> };
>
> template<>
> struct kind_of<node1> {
> static constexpr kind value = kind::n1;
> };
>
> template<>
> struct kind_of<node2> {
> static constexpr kind value = kind::n2;
> };
>
> #include <boost/openmethod/policies/basic_policy.hpp>
>
> struct custom_rtti : boost::openmethod::policies::rtti {
> template<class Node>
> static constexpr bool is_polymorphic =
> std::is_base_of_v<base_node, Node>;
>
> using type_id = boost::openmethod::type_id;
>
> template<typename T>
> static auto static_type() {
> return type_id(kind_of<T>::value);
> }
>
> template<typename T>
> static auto dynamic_type(const T& obj) {
> if constexpr (is_polymorphic<T>) {
> return type_id(obj.getKind());
> } else {
> return kind::unknown;
> }
> }
> };
>
> struct custom_policy
> : boost::openmethod::policies::basic_policy<custom_policy,
> custom_rtti> {
>
> static auto dynamic_vptr(const base_node& b) {
> switch (b.getKind()) {
> case kind::n1:
> return custom_policy::static_vptr<node1>;
> case kind::n2:
> return custom_policy::static_vptr<node2>;
> default:
> return custom_policy::static_vptr<base_node>;
> }
> }
> };
>
> #define BOOST_OPENMETHOD_DEFAULT_POLICY custom_policy
>
> #include <iostream>
>
> #include <boost/openmethod.hpp>
> #include <boost/openmethod/compiler.hpp>
>
> BOOST_OPENMETHOD(process, (std::ostream&, virtual_ptr<base_node>), void);
>
> BOOST_OPENMETHOD_OVERRIDE(
> process, (std::ostream & os, virtual_ptr<node1> node1), void) {
> os << "process node1\n";
> }
>
> BOOST_OPENMETHOD_OVERRIDE(
> process, (std::ostream & os, virtual_ptr<node2> node2), void) {
> os << "process node2\n";
> }
>
> BOOST_OPENMETHOD_CLASSES(base_node, node1, node2);
>
> auto main() -> int {
> boost::openmethod::initialize();
>
> auto a = std::make_unique<node1>();
> auto b = std::make_unique<node2>();
>
> process(std::cout, *a);
> process(std::cout, *b);
>
> return 0;
> }
>
> We could also take advantage of the fact that type_ids fall in a short, compact
> range. We can use the vptr_vector facet, but there is no need to hash the
> type_ids, we can use them as straight indexes. So we don't use a type_hash
> facet:
>
> struct custom_policy
> : boost::openmethod::policies::basic_policy<
> custom_policy, custom_rtti,
> boost::openmethod::policies::vptr_vector<custom_policy>> {};
>
> This time dynamic_vptr() is provided by a facet. Everything else stays the same.
For the reference, this is what I ended up doing:
enum class kind : std::uintptr_t { unknown, n1, n2 };
class base_node {
kind k_;
protected:
base_node(kind k) noexcept : k_(k) {
}
public:
kind getKind() const {
return k_;
}
};
class node1 : public base_node {
public:
static constexpr kind node_kind = kind::n1;
node1() noexcept : base_node(node_kind) {
}
};
class node2 : public base_node {
public:
static constexpr kind node_kind = kind::n2;
node2() noexcept : base_node(node_kind) {
}
};
constexpr const char* kind_to_string(kind k) {
switch (k) {
case kind::n1:
return "node1";
case kind::n2:
return "node2";
default:
return "<unknown node type>";
}
}
struct custom_rtti : boost::openmethod::policies::rtti {
template<class T>
static constexpr bool is_polymorphic = std::is_base_of_v<base_node, T>;
template<typename T>
static auto static_type() -> std::uintptr_t {
if constexpr (
std::is_base_of_v<base_node, T> && !std::is_same_v<base_node, T>) {
return static_cast<std::uintptr_t>(T::node_kind);
} else {
return static_cast<std::uintptr_t>(kind::unknown);
}
}
template<typename T>
static auto dynamic_type(const T& obj) -> std::uintptr_t {
if constexpr (is_polymorphic<T>) {
return static_cast<std::uintptr_t>(obj.getKind());
} else {
return static_cast<std::uintptr_t>(kind::unknown);
}
}
template<class Stream>
static void type_name(std::uintptr_t type, Stream& stream) {
stream << kind_to_string(static_cast<kind>(type));
}
};
struct custom_policy
: boost::openmethod::policies::basic_policy<
custom_policy, custom_rtti,
boost::openmethod::policies::runtime_checks,
boost::openmethod::policies::basic_error_output<custom_policy>,
boost::openmethod::policies::vptr_vector<custom_policy>> {};
BOOST_OPENMETHOD_CLASSES(base_node, node1, node2, custom_policy)
BOOST_OPENMETHOD(print, (virtual_<base_node>), void, custom_policy);
BOOST_OPENMETHOD_OVERRIDE(print, (base_node&), void) { /* whatever */ }
I've just realized that I've disabled hashing by accident, and this
happened to work by accident. I think that this implicit dependency
between the vptr_vector and the hashing policy is a problem - I prefer
explicit dependencies.
>
> As you see in this example, we don't have to fork policies, we can just
> construct them from scratch. Forking was introduced with the idea of tuning
> default_policy, without having to replicate it from scratch. Also the
> with/without setters are less verbose than the add/replace/remove ones.
>
> Elsewhere I said that facets categories are a closed set. That is not correct.
> The dispatcher and the compiler look only at a closed set of facets
> (using if constexpr (Policy::has_facet>)). But some facets, like vptr_vector and
> error_handler, look for other facets in the policy, which are not necessarily
> referenced directly by the compiler or the dispatcher. In that sense, the set of
> facets is actually open.
>
> I am going to think about your suggested alternative to the facet system.
> Seriously, thus it will take me a few days. In the meantime, how would you see
> the two example policies above, expressed with your system?
This is what it'd look like with my proposal:
struct custom_rtti : openmethod::policies::rtti {
template<class T>
static constexpr bool is_polymorphic = std::is_base_of_v<base_node, T>;
template<typename T>
auto static_type() const -> std::uintptr_t { /* same definition */ }
template<typename T>
auto dynamic_type(const T& obj) const -> std::uintptr_t { /* same
definition */ }
template<class Stream>
void type_name(std::uintptr_t type, Stream& stream) const { /*
same definition */ }
};
struct custom_policy : boost::openmethod::policies::abstract_policy {
// enable runtime checks
openmethod::policies::runtime_checks runtime_checks;
// enable the default error output
openmethod::policies::basic_error_output<> error_output;
// pass the hashing policy to vptr_vector explicitly, rather than
implicitly via the entire Policy
// disable_hashing should be either a marker type to tell
vptr_vector "I don't have any hashing available"
// or implement the type_hash facet and be a no-op (I guess the
1st option is better)
openmethod::policies::vptr_vector<openmethod::policies::disable_hashing>
extern_vptr;
};
// Now register the classes. See below for the definition of registry
BOOST_OPENMETHOD_CLASSES(base_node, node1, node2,
openmethod::registry<custom_policy>)
BOOST_OPENMETHOD(print, (virtual_<base_node>), void,
openmethod::registry<custom_policy>);
BOOST_OPENMETHOD_OVERRIDE(print, (base_node&), void) { /* whatever */ }
// This type needs to be provided by the library, and is in charge of
creating the required static storage
namespace boost::openmethod {
template <class Policy>
struct registry {
using policy_type = Policy;
static Policy policy;
};
}
There are likely things I'm not seeing now that will probably
complicate this. If the review is favorable, I can write a PR with the
proposed changes and see if it's viable.
>
> I would love to ship a fully functional clang RTTI policy right off the bat; it
> is an important use-case.
>
> J-L
Thanks,
Ruben.
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