meta: deprecate meta properties in favor of custom data

TODO

@@ -32,7 +32,6 @@ TODO:
 * cleanup common view from tricks to handle single swap-only and in-place, if constexpr branches
 * entity based component_traits
 * review cmake warning about FetchContent_Populate (need .28 and EXCLUDE_FROM_ALL for FetchContent)
-* deprecate meta properties in favor of custom data
 * make meta objects safe to use with null nodes
 * suppress -Wself-move on CI with g++13
 * view and view iterator specializations for multi, single and filtered elements

docs/md/meta.md

@@ -16,7 +16,6 @@
   * [Policies: the more, the less](#policies-the-more-the-less)
   * [Named constants and enums](#named-constants-and-enums)
   * [User defined data](#user-defined-data)
-    * [Properties and meta objects](#properties-and-meta-objects)
     * [Traits](#traits)
     * [Custom data](#custom-data)
   * [Unregister types](#unregister-types)
@@ -841,15 +840,10 @@ object optimization performed by the `meta_any` class.
 ## User defined data
 
 Sometimes (for example, when it comes to creating an editor) it might be useful
-to attach _properties_, _traits_ or arbitrary _custom data_ to the meta objects
-created.
+to attach _traits_ or arbitrary _custom data_ to the meta objects created.
 
 The main difference between them is that:
 
-* Properties are usually key-only or key-value pairs with lower access
-  performance. They are deprecated today but have long been the only way to
-  bind user data with meta objects.
-
 * Traits are simple user-defined flags with much higher access performance. The
   library reserves up to 16 bits for traits, that is 16 flags for a bitmask or
   2^16 values otherwise.
@@ -860,51 +854,6 @@ The main difference between them is that:
 In all cases, this support is currently available only for meta types, meta data
 and meta functions.
 
-### Properties and meta objects
-
-Properties are set via a meta factory and are not editable once created:
-
-```cpp
-entt::meta<my_type>().prop("tooltip"_hs, "message");
-```
-
-They are always in the key/value form. The key is a numeric identifier, mostly
-similar to the identifier used to register meta objects. There are no
-restrictions on the type of the value instead, as long as it's movable.<br/>
-Key only properties are also supported out of the box:
-
-```cpp
-entt::meta<my_type>().prop(my_enum::key_only);
-```
-
-To attach multiple properties to a meta object, just invoke `prop` more than
-once.<br/>
-It's also possible to call `prop` at different times, as long as the factory is
-reset to the meta object of interest:
-
-```cpp
-entt::meta<my_type>()
-    .data<&my_type::data_member, entt::as_ref_t>("member"_hs)
-    .prop("key"_hs, value);
-```
-
-Once created, all meta objects offer a couple of member functions named `prop`
-to iterate all properties at once or to search a specific property by key:
-
-```cpp
-// iterate all properties of a meta type
-for(auto &&[id, prop]: entt::resolve<my_type>().prop()) {
-    // ...
-}
-
-// search for a given property by name
-auto prop = entt::resolve<my_type>().prop("tooltip"_hs);
-```
-
-Meta properties are objects having a fairly poor interface, all in all. They
-only provide the `value` member function to retrieve the contained value in the
-form of a `meta_any` object.
-
 ### Traits
 
 User-defined traits are set via a meta factory:

src/entt/meta/factory.hpp

@@ -498,7 +498,7 @@ public:
      * @return A meta factory for the parent type.
      */
     template<typename... Value>
-    meta_factory prop(id_type id, [[maybe_unused]] Value &&...value) {
+    [[deprecated("use ::custom() instead")]] meta_factory prop(id_type id, [[maybe_unused]] Value &&...value) {
         if constexpr(sizeof...(Value) == 0u) {
             base_type::prop(internal::meta_prop_node{id, &internal::resolve<void>});
         } else {

src/entt/meta/meta.hpp

@@ -765,7 +765,7 @@ private:
 };
 
 /*! @brief Opaque wrapper for properties of any type. */
-struct meta_prop {
+struct [[deprecated("use meta_custom instead")]] meta_prop {
     /*! @brief Default constructor. */
     meta_prop() noexcept = default;
 
@@ -938,7 +938,7 @@ struct meta_data {
      * @brief Returns a range to visit registered meta properties.
      * @return An iterable range to visit registered meta properties.
      */
-    [[nodiscard]] meta_range<meta_prop, typename decltype(internal::meta_data_node::prop)::const_iterator> prop() const noexcept {
+    [[nodiscard]] [[deprecated("use ::custom() instead")]] meta_range<meta_prop, typename decltype(internal::meta_data_node::prop)::const_iterator> prop() const noexcept {
         return {{*ctx, node->prop.cbegin()}, {*ctx, node->prop.cend()}};
     }
 
@@ -947,7 +947,7 @@ struct meta_data {
      * @param key The key to use to search for a property.
      * @return The registered meta property for the given key, if any.
      */
-    [[nodiscard]] meta_prop prop(const id_type key) const {
+    [[nodiscard]] [[deprecated("use ::custom() instead")]] meta_prop prop(const id_type key) const {
         for(auto &&elem: node->prop) {
             if(elem.id == key) {
                 return meta_prop{*ctx, elem};
@@ -1083,7 +1083,7 @@ struct meta_func {
     }
 
     /*! @copydoc meta_data::prop */
-    [[nodiscard]] meta_range<meta_prop, typename decltype(internal::meta_func_node::prop)::const_iterator> prop() const noexcept {
+    [[nodiscard]] [[deprecated("use ::custom() instead")]] meta_range<meta_prop, typename decltype(internal::meta_func_node::prop)::const_iterator> prop() const noexcept {
         return {{*ctx, node->prop.cbegin()}, {*ctx, node->prop.cend()}};
     }
 
@@ -1092,7 +1092,7 @@ struct meta_func {
      * @param key The key to use to search for a property.
      * @return The registered meta property for the given key, if any.
      */
-    [[nodiscard]] meta_prop prop(const id_type key) const {
+    [[nodiscard]] [[deprecated("use ::custom() instead")]] meta_prop prop(const id_type key) const {
         for(auto &&elem: node->prop) {
             if(elem.id == key) {
                 return meta_prop{*ctx, elem};
@@ -1562,7 +1562,7 @@ public:
      * @brief Returns a range to visit registered top-level meta properties.
      * @return An iterable range to visit registered top-level meta properties.
      */
-    [[nodiscard]] meta_range<meta_prop, typename decltype(internal::meta_type_descriptor::prop)::const_iterator> prop() const noexcept {
+    [[nodiscard]] [[deprecated("use ::custom() instead")]] meta_range<meta_prop, typename decltype(internal::meta_type_descriptor::prop)::const_iterator> prop() const noexcept {
         using range_type = meta_range<meta_prop, typename decltype(internal::meta_type_descriptor::prop)::const_iterator>;
         return node.details ? range_type{{*ctx, node.details->prop.cbegin()}, {*ctx, node.details->prop.cend()}} : range_type{};
     }
@@ -1572,7 +1572,7 @@ public:
      * @param key The key to use to search for a property.
      * @return The registered meta property for the given key, if any.
      */
-    [[nodiscard]] meta_prop prop(const id_type key) const {
+    [[nodiscard]] [[deprecated("use ::custom() instead")]] meta_prop prop(const id_type key) const {
         const auto *elem = internal::look_for<&internal::meta_type_descriptor::prop>(internal::meta_context::from(*ctx), node, key);
         return elem ? meta_prop{*ctx, *elem} : meta_prop{};
     }