# Crash Course: runtime reflection system # Table of Contents * [Introduction](#introduction) * [Identifiers](#identifiers) * [Reflection in a nutshell](#reflection-in-a-nutshell) * [Any to the rescue](#any-to-the-rescue) * [Enjoy the runtime](#enjoy-the-runtime) * [Tell me your name](#tell-me-your-name) * [Container support](#container-support) * [Pointer-like types](#pointer-like-types) * [Template information](#template-information) * [Automatic conversions](#automatic-conversions) * [Implicitly generated default constructor](#implicitly-generated-default-constructor) * [From void to any](#from-void-to-any) * [Policies: the more, the less](#policies-the-more-the-less) * [Named constants and enums](#named-constants-and-enums) * [User defined data](#user-defined-data) * [Traits](#traits) * [Custom data](#custom-data) * [Unregister types](#unregister-types) * [Meta context](#meta-context) # Introduction Reflection (or rather, its lack) is a trending topic in the C++ world and a tool that can unlock a lot of interesting features in the specific case of `EnTT`. I looked for a third-party library that met my needs on the subject, but I always came across some details that I did not like: macros, being intrusive, too many allocations, and so on.
I finally decided to write a built-in, non-intrusive and macro-free runtime reflection system for `EnTT`. Maybe I did not do better than others or maybe yes, time will tell me, but at least I can model this tool around the library to which it belongs and not the opposite. # Identifiers The meta system does not force users to rely on the tools provided by the library when it comes to working with identifiers. It does this by offering an API that works with integral values that may or may not be generated by means of a hashed string.
This means that users can assign any type of identifier to the meta objects, as long as they are numeric. It does not matter if they are generated at runtime, at compile-time or with custom functions. That being said, some of the examples in the following sections are based on the `hashed_string` class as provided by this library. Therefore, where an integral identifier is provided, it is likely that a user defined literal is used as follows: ```cpp entt::meta_factory{}.type("reflected_type"_hs); ``` For what it is worth, this is completely equivalent to: ```cpp entt::meta_factory{}.type(42u); ``` Obviously, human-readable identifiers are more convenient to use and highly recommended. # Reflection in a nutshell Reflection always starts from actual C++ types. Users cannot reflect _imaginary_ types.
The `meta_factory` class is where it all starts: ```cpp entt::meta_factory factory{}; ``` The returned value is a _factory object_ to use to continue building the meta type. By default, a meta type is associated with the identifier returned by the runtime type identification system built-in in `EnTT`.
However, it is also possible to assign custom identifiers to meta types: ```cpp entt::meta_factory{}.type("reflected_type"_hs); ``` Identifiers are used instead of the type to _retrieve_ meta types at runtime, if necessary.
However, users can be interested in adding features to a reflected type so that the reflection system can use it correctly under the hood, while they do not want to also make the type _searchable_. In this case, it is sufficient not to invoke `type`. A factory is such that all its member functions return the factory itself. It is generally used to create the following: * _Constructors_. A constructor is assigned to a reflected type by specifying its _list of arguments_. Free functions are also accepted if the return type is the expected one. From a client perspective, nothing changes between a free function or an actual constructor: ```cpp entt::meta_factory{}.ctor().ctor<&factory>(); ``` Meta default constructors are implicitly generated, if possible. * _Data members_. Meta data members are actual data members of the underlying type but also static and global variables or constants of any kind. From the point of view of the client, all the variables associated with the reflected type appear as if they were part of the type itself: ```cpp entt::meta_factory{} .data<&my_type::static_variable>("static"_hs) .data<&my_type::data_member>("member"_hs) .data<&global_variable>("global"_hs); ``` The `data` function requires the identifier to use for the meta data member. This is then used for runtime access.
Data members are also defined by means of a setter and getter pair. These are either free functions, class members or a mix of them. This approach is also convenient to create read-only properties from a non-const data member: ```cpp entt::meta_factory{}.data("member"_hs); ``` * _Member functions_. Meta member functions are actual member functions of the underlying type but also plain free functions. From the point of view of the client, all the functions associated with the reflected type appear as if they were part of the type itself: ```cpp entt::meta_factory{} .func<&my_type::static_function>("static"_hs) .func<&my_type::member_function>("member"_hs) .func<&free_function>("free"_hs); ``` The `func` function requires the identifier to use for the meta data function. This is then used for runtime access.
Overloading of meta functions is supported. Overloaded functions are resolved at runtime by the reflection system according to the types of the arguments. * _Base classes_. A base class is such that the underlying type is actually derived from it: ```cpp entt::meta_factory{}.base(); ``` The reflection system tracks the relationship and allows for implicit casts at runtime when required. In other terms, wherever a `base_type` is required, an instance of `derived_type` is also accepted. * _Conversion functions_. Conversion functions allow users to define conversions that are implicitly performed by the reflection system when required: ```cpp entt::meta_factory{}.conv(); ``` This is everything users need to create meta types. Refer to the inline documentation for further details. ## Any to the rescue The reflection system offers a kind of _extended version_ of the `entt::any` class (see the core module for more details).
The purpose is to add some feature on top of those already present, so as to integrate it with the meta type system without having to duplicate the code. The API is very similar to that of the `any` type. The class `meta_any` _wraps_ many of the feature to infer a meta node, before forwarding some or all of the arguments to the underlying storage.
Among the few relevant differences, `meta_any` adds support for containers and pointer-like types, while `any` does not.
Similar to `any`, this class is also used to create _aliases_ for unmanaged objects either with `forward_as_meta` or using the `std::in_place_type` disambiguation tag, as well as from an existing object by means of the `as_ref` member function. Additionally, it can take ownership of pointers passed as arguments along with `std::in_place`.
Unlike `any` instead, `meta_any` treats an empty instance and one initialized with `void` differently: ```cpp entt::meta_any empty{}; entt::meta_any other{std::in_place_type}; ``` While `any` considers both as empty, `meta_any` treats objects initialized with `void` as if they were _valid_ ones. This allows differentiating between failed function calls and function calls that are successful but return nothing. Finally, the member functions `try_cast`, `cast` and `allow_cast` are used to cast the underlying object to a given type (either a reference or a value type) or to _convert_ a `meta_any` in such a way that a cast becomes viable for the resulting object.
There is in fact no `any_cast` equivalent for `meta_any`. ## Enjoy the runtime Once the web of reflected types is constructed, it is a matter of using it at runtime where required.
There are a few options to search for a reflected type: ```cpp // direct access to a reflected type auto by_type = entt::resolve(); // look up a reflected type by identifier auto by_id = entt::resolve("reflected_type"_hs); // look up a reflected type by type info auto by_type_id = entt::resolve(entt::type_id()); ``` There exists also an overload of the `resolve` function to use to iterate all reflected types at once. It returns an iterable object to be used in a range-for loop: ```cpp for(auto &&[id, type]: entt::resolve()) { // ... } ``` In all cases, the returned value is an instance of `meta_type` (possibly with its id). These objects offer an API to know their _runtime identifiers_, to iterate all the meta objects associated with them and even to build instances of the underlying type.
Meta data members and functions are accessed by means of their identifiers: * Meta data members: ```cpp auto data = entt::resolve().data("member"_hs); ``` The returned type is `meta_data` and may be invalid if there is no meta data object associated with the given identifier.
A meta data object offers an API to query the underlying type (for example, to know if it is a const or a static one), to get the meta type of the variable and to set or get the contained value. * Meta function members: ```cpp auto func = entt::resolve().func("member"_hs); ``` The returned type is `meta_func` and may be invalid if there is no meta function object associated with the given identifier.
A meta function object offers an API to query the underlying type (for example, to know if it is a const or a static function), to know the number of arguments, the meta return type and the meta types of the parameters. In addition, a meta function object is used to invoke the underlying function and then get the return value in the form of a `meta_any` object. Both functions search for the elements throughout the meta type hierarchy. However, they offer the option of passing a second boolean argument to stop the search at the top-level meta type.
All the meta objects thus obtained as well as the meta types explicitly convert to a boolean value to check for validity: ```cpp if(auto func = entt::resolve().func("member"_hs); func) { // ... } ``` Furthermore, all of them (and a few more, like meta basis) are returned by a bunch of overloads that provide the caller with iterable ranges of top-level elements. As an example: ```cpp for(auto &&[id, type]: entt::resolve().base()) { // ... } ``` Meta types are also used to `construct` actual instances of the underlying type.
In particular, the `construct` member function accepts a variable number of arguments and searches for a match. It then returns a `meta_any` object that may or may not be initialized, depending on whether a suitable constructor was found or not.
Conversion functions are not accessible instead. They are used internally by `meta_any` and the meta objects when needed. Meta types and meta objects in general contain much more than what was said. Refer to the inline documentation for further details. ### Tell me your name For meta types, data and functions, users can also provide custom _names_: ```cpp entt::meta_factory{} .type("type"_hs, "my_type") .data<&variable>("data"_hs, "variable") .func<&function>("func"_hs, "function"); ``` The _label_ provided **should** be a string literal. The library does not make copies. It is up to the user to guarantee the lifetime of the name itself.
String identifiers are returned from the meta objects via the `name` function: ```cpp const char *name = entt::resolve().name(); ``` Since most of the time there is no need to differentiate the _name_ from the numeric identifier associated with a meta object, `EnTT` also offers a more compact version of these functions: ```cpp entt::meta_factory{} .type("my_type") .data<&variable>("variable") .func<&function>("function"); ``` Again, the name provided **should** be a string literal. The string is then used to generate a numeric identifier with the `hashed_string` class.
Despite support for names, there are no string-based lookup functions available. That is, types (`resolve`) as well as data members (`data`) and function members (`func`) are only _searchable_ by numeric identifiers. ## Container support The runtime reflection system also supports containers of all types.
Moreover, _containers_ does not necessarily mean those offered by the C++ standard library. In fact, user defined data structures can also work with the meta system in many cases. To make a container be recognized as such by the meta system, users are required to provide specializations for either the `meta_sequence_container_traits` class or the `meta_associative_container_traits` class, according to the actual _type_ of the container.
`EnTT` already exports the specializations for some common classes. In particular: * `std::vector`, `std::array`, `std::deque` and `std::list` (but not `std::forward_list`) are supported as _sequence containers_. * `std::map`, `std::set` and their unordered counterparts are supported as _associative containers_. It is important to include the header file `container.hpp` to make these specializations available to the compiler when needed.
The same file also contains many examples for the users that are interested in making their own containers available to the meta system. When a specialization of the `meta_sequence_container_traits` class exists, the meta system treats the wrapped type as a sequence container. In a similar way, a type is treated as an associative container if a specialization of the `meta_associative_container_traits` class is found for it.
Proxy objects are returned by dedicated members of the `meta_any` class. The following is a deliberately verbose example of how users can access a proxy object for a sequence container: ```cpp std::vector vec{1, 2, 3}; entt::meta_any any = entt::forward_as_meta(vec); if(any.type().is_sequence_container()) { if(auto view = any.as_sequence_container(); view) { // ... } } ``` The method to use to get a proxy object for associative containers is `as_associative_container` instead.
It is not necessary to perform a double check actually. Instead, it is enough to query the meta type or verify that the proxy object is valid. In fact, proxies are contextually convertible to bool to check for validity. For example, invalid proxies are returned when the wrapped object is not a container.
In all cases, users are not expected to _reflect_ containers explicitly. It is sufficient to assign a container for which a specialization of the _traits_ classes exists to a `meta_any` object to be able to get its proxy object. The interface of the `meta_sequence_container` proxy object is the same for all types of sequence containers, although the available features differ from case to case. In particular: * The `value_type` member function returns the meta type of the elements. * The `size` member function returns the number of elements in the container as an unsigned integer value. * The `resize` member function allows resizing the wrapped container and returns true in case of success.
For example, it is not possible to resize fixed size containers. * The `clear` member function allows clearing the wrapped container and returns true in case of success.
For example, it is not possible to clear fixed size containers. * The `reserve` member function allows increasing the capacity of the wrapped container and returns true in case of success.
For example, it is not possible to increase capacity of fixed size containers. * The `begin` and `end` member functions return opaque iterators that is used to iterate the container directly: ```cpp for(entt::meta_any element: view) { // ... } ``` In all cases, given an underlying container of type `C`, the returned element contains an object of type `C::value_type` which therefore depends on the actual container.
All meta iterators are input iterators and do not offer an indirection operator on purpose. * The `insert` member function is used to add elements to the container. It accepts a meta iterator and the element to insert: ```cpp auto last = view.end(); // appends an integer to the container view.insert(last, 42); ``` This function returns a meta iterator pointing to the inserted element and a boolean value to indicate whether the operation was successful or not. A call to `insert` may silently fail in case of fixed size containers or whether the arguments are not at least convertible to the required types.
Since meta iterators are contextually convertible to bool, users can rely on them to know if the operation failed on the actual container or upstream, for example due to an argument conversion problem. * The `erase` member function is used to remove elements from the container. It accepts a meta iterator to the element to remove: ```cpp auto first = view.begin(); // removes the first element from the container view.erase(first); ``` This function returns a meta iterator following the last removed element and a boolean value to indicate whether the operation was successful or not. A call to `erase` may silently fail in case of fixed size containers. * The `operator[]` is used to access container elements. It accepts a single argument, the position of the element to return: ```cpp for(std::size_t pos{}, last = view.size(); pos < last; ++pos) { entt::meta_any value = view[pos]; // ... } ``` The function returns instances of `meta_any` that directly refer to the actual elements. Modifying the returned object directly modifies the element inside the container.
Depending on the underlying sequence container, this operation may not be as efficient. For example, in the case of an `std::list`, a positional access translates to a linear visit of the list itself (probably not what the user expects). Similarly, also the interface of the `meta_associative_container` proxy object is the same for all types of associative containers. However, there are some differences in behavior in the case of key-only containers. In particular: * The `key_only` member function returns true if the wrapped container is a key-only one. * The `key_type` member function returns the meta type of the keys. * The `mapped_type` member function returns an invalid meta type for key-only containers and the meta type of the mapped values for all other types of containers. * The `value_type` member function returns the meta type of the elements.
For example, it returns the meta type of `int` for `std::set` while it returns the meta type of `std::pair` for `std::map`. * The `size` member function returns the number of elements in the container as an unsigned integer value. * The `clear` member function allows clearing the wrapped container and returns true in case of success. * The `reserve` member function allows increasing the capacity of the wrapped container and returns true in case of success.
For example, it is not possible to increase capacity of standard maps. * The `begin` and `end` member functions return opaque iterators that are used to iterate the container directly: ```cpp for(std::pair element: view) { // ... } ``` In all cases, given an underlying container of type `C`, the returned element is a key-value pair where the key has type `C::key_type` and the value has type `C::mapped_type`. Since key-only containers do not have a mapped type, their _value_ is nothing more than an invalid `meta_any` object.
All meta iterators are input iterators and do not offer an indirection operator on purpose. While the accessed key is usually constant in the associative containers and is therefore returned by copy, the value (if any) is wrapped by an instance of `meta_any` that directly refers to the actual element. Modifying it directly modifies the element inside the container. * The `insert` member function is used to add elements to a container. It gets two arguments, the key and the value to insert: ```cpp auto last = view.end(); // appends an integer to the container view.insert(last.handle(), 42, 'c'); ``` This function returns a boolean value to indicate whether the operation was successful or not. A call to `insert` may fail when the arguments are not at least convertible to the required types. * The `erase` member function is used to remove elements from a container. It gets a single argument, the key to remove: ```cpp view.erase(42); ``` This function returns a boolean value to indicate whether the operation was successful or not. A call to `erase` may fail when the argument is not at least convertible to the required type. * The `operator[]` is used to access elements in a container. It gets a single argument, the key of the element to return: ```cpp entt::meta_any value = view[42]; ``` The function returns instances of `meta_any` that directly refer to the actual elements. Modifying the returned object directly modifies the element inside the container. Container support is minimal but likely sufficient to satisfy all needs. ## Pointer-like types As with containers, it is also possible to _tell_ to the meta system which types are _pointers_. This makes it possible to dereference instances of `meta_any`, thus obtaining light _references_ to pointed objects that are also correctly associated with their meta types.
To make the meta system recognize a type as _pointer-like_, users can specialize the `is_meta_pointer_like` class. `EnTT` already exports the specializations for some common classes. In particular: * All types of raw pointers. * `std::unique_ptr` and `std::shared_ptr`. * All classes that _export_ a type member called `is_meta_pointer_like`: ```cpp struct smart_pointer { using is_meta_pointer_like = void; // ... }; ``` The actual type is irrelevant and will not be used in any way. It is important to include the header file `pointer.hpp` to make these specializations available to the compiler when needed.
The same file also contains many examples for the users that are interested in making their own pointer-like types available to the meta system. When a type is recognized as a pointer-like one by the meta system, it is possible to dereference the instances of `meta_any` that contain these objects. The following is a deliberately verbose example to show how to use this feature: ```cpp int value = 42; // meta type equivalent to that of int * entt::meta_any any{&value}; if(any.type().is_pointer_like()) { // meta type equivalent to that of int if(entt::meta_any ref = *any; ref) { // ... } } ``` It is not necessary to perform a double check. Instead, it is enough to query the meta type or verify that the returned object is valid. For example, invalid instances are returned when the wrapped object is not a pointer-like type.
Dereferencing a pointer-like object returns an instance of `meta_any` which _refers_ to the pointed object. Modifying it means modifying the pointed object directly (unless the returned element is const). In general, _dereferencing_ a pointer-like type boils down to a `*ptr`. However, `EnTT` also supports classes that do not offer an `operator*`. In particular: * It is possible to exploit a solution based on ADL lookup by implementing a function (also a template one) named `dereference_meta_pointer_like`: ```cpp template Type & dereference_meta_pointer_like(const custom_pointer_type &ptr) { return ptr.deref(); } ``` * When not in control of the type's namespace, it is possible to inject into the `entt` namespace a specialization of the `adl_meta_pointer_like` class template to bypass the adl lookup as a whole: ```cpp template struct entt::adl_meta_pointer_like> { static decltype(auto) dereference(const custom_pointer_type &ptr) { return ptr.deref(); } }; ``` In all other cases and when dereferencing a pointer works as expected regardless of the pointed type, no user intervention is required. ## Template information Meta types also provide a minimal set of information about the _nature_ of the original type in case it is a class template.
By default, this works out of the box and requires no user action. However, it is important to include the header file `template.hpp` to make this information available to the compiler when needed. Meta template information is easily found: ```cpp // this method returns true if the type is recognized as a class template specialization if(auto type = entt::resolve>(); type.is_template_specialization()) { // meta type of the class template conveniently wrapped by entt::meta_class_template_tag auto class_type = type.template_type(); // number of template arguments std::size_t arity = type.template_arity(); // meta type of the i-th argument auto arg_type = type.template_arg(0u); } ``` Typically, when template information for a type is required, what the library provides is sufficient. However, there are some cases where a user may want more details or a different set of information.
Consider the case of a class template that is meant to wrap function types: ```cpp template struct function_type; template struct function_type {}; ``` In this case, rather than the function type, it might be useful to provide the return type and unpacked arguments as if they were different template parameters for the original class template.
To achieve this, users must enter the library internals and provide their own specialization for the class template `entt::meta_template_traits`, such as: ```cpp template struct entt::meta_template_traits> { using class_type = meta_class_template_tag; using args_type = type_list; }; ``` The reflection system does not verify the accuracy of the information nor infer a correspondence between real types and meta types.
Therefore, the specialization is used as is and the information it contains is associated with the appropriate type when required. ## Automatic conversions In C++, there are a number of conversions allowed between arithmetic types that make it convenient to work with this kind of data.
If this were to be translated into explicit registrations with the reflection system, it would result in a long series of instructions such as the following: ```cpp entt::meta_factory{} .conv() .conv() // ... .conv(); ``` Repeated for each type eligible to undergo this type of conversions. This is both error-prone and repetitive.
Similarly, the language allows users to silently convert unscoped enums to their underlying types and offers what it takes to do the same for scoped enums. It would result in the following if it were to be done explicitly: ```cpp entt::meta_factory{} .conv>(); ``` Fortunately, all of this can also be avoided. `EnTT` offers implicit support for these types of conversions: ```cpp entt::meta_any any{42}; any.allow_cast(); double value = any.cast(); ``` With no need for registration, the conversion takes place automatically under the hood. The same goes for a call to `allow_cast` involving a meta type: ```cpp entt::meta_type type = entt::resolve(); entt::meta_any any{my_enum::a_value}; any.allow_cast(type); int value = any.cast(); ``` This makes working with arithmetic types and scoped or unscoped enums as easy as it is in C++.
It is still possible to set up conversion functions manually, and these are always preferred over the automatic ones. ## Implicitly generated default constructor Creating objects of default constructible types through the reflection system while not having to explicitly register the meta type or its default constructor is also possible.
For example, in the case of primitive types like `int` or `char`, but not just them. For default constructible types only, default constructors are automatically defined and associated with their meta types, whether they are explicitly or implicitly generated.
Therefore, this is all needed to construct an integer from its meta type: ```cpp entt::resolve().construct(); ``` Where the meta type is, for example, the one returned from a meta container, useful for building keys without knowing or having to register the actual types. In all cases, when users register default constructors, they are preferred both during searches and when the `construct` member function is invoked. ## From void to any Sometimes all a user has is an opaque pointer to an object of a known meta type. It would be handy in this case to be able to construct a `meta_any` element from it.
For this purpose, the `meta_type` class offers a `from_void` member function designed to convert an opaque pointer into a `meta_any`: ```cpp entt::meta_any any = entt::resolve(id).from_void(element); ``` Unfortunately, it is not possible to do a check on the actual type. Therefore, this call can be considered as a _static cast_ with all its _problems_.
On the other hand, the ability to construct a `meta_any` from an opaque pointer opens the door to some pretty interesting uses that are worth exploring. ## Policies: the more, the less Policies are a kind of compile-time directives that can be used when registering reflection information.
Their purpose is to require slightly different behavior than the default in some specific cases. For example, when reading a given data member, its value is returned wrapped in a `meta_any` object which, by default, makes a copy of it. For large objects or if the caller wants to access the original instance, this behavior is not desirable. Policies are there to offer a solution to this and other problems. There are a few alternatives available at the moment: * The _as-value_ policy, associated with the type `entt::as_value_t`.
This is the default policy. In general, it should never be used explicitly, since it is implicitly selected if no other policy is specified.
In this case, the return values of the functions as well as the properties exposed as data members are always returned by copy in a dedicated wrapper and therefore associated with their original meta types. * The _as-void_ policy, associated with the type `entt::as_void_t`.
Its purpose is to discard the return value of a meta object, whatever it is, thus making it appear as if its type were `void`: ```cpp entt::meta_factory{}.func<&my_type::member_function, entt::as_void_t>("member"_hs); ``` If the use with functions is obvious, perhaps less so is use with constructors and data members. In the first case, the returned wrapper is always empty even though the constructor is still invoked. In the second case, the property is not accessible for reading instead. * The _as-ref_ and _as-cref_ policies, associated with the types `entt::as_ref_t` and `entt::as_cref_t`.
They allow to build wrappers that act as references to unmanaged objects. Accessing the object contained in the wrapper for which the _reference_ was requested makes it possible to directly access the instance used to initialize the wrapper itself: ```cpp entt::meta_factory{}.data<&my_type::data_member, entt::as_ref_t>("member"_hs); ``` These policies work with constructors (for example, when objects are taken from an external container rather than created on demand), data members and functions in general.
If on the one hand `as_cref_t` always forces the return type to be const, `as_ref_t` _adapts_ to the constness of the passed object and to that of the return type if any. * The _as-is_ policy, associated with the type `entt::as_is_t`.
Useful for decoupling meta type creation code from calling code while still preserving the behavior of data members and member functions as defined: ```cpp entt::meta_factory{}.func<&my_type::any_member, entt::as_is_t>("member"_hs); ``` For data members or member functions that return a reference type, the value is returned by reference with the same constness. In all other cases, the value is returned by copy. Some uses are rather trivial, but it is useful to note that there are some less obvious corner cases that can in turn be solved with the use of policies. ## Named constants and enums As mentioned, the `data` member function is used to reflect constants of any type.
This allows users to create meta types for enums that work exactly like any other meta type built from a class. Similarly, arithmetic types are _enriched_ with constants of special meaning where required.
All values thus exported appear to users as if they were constant data members of the reflected types. This avoids the need to _export_ what is the difference between enums and classes in C++ directly in the space of the reflected types. Exposing constant values or elements from an enum is quite simple: ```cpp entt::meta_factory{} .data("a_value"_hs) .data("another_value"_hs); entt::meta_factory{}.data<2048>("max_int"_hs); ``` Accessing them is trivial as well. It is a matter of doing the following, as with any other data member of a meta type: ```cpp auto value = entt::resolve().data("a_value"_hs).get({}).cast(); auto max = entt::resolve().data("max_int"_hs).get({}).cast(); ``` All this happens behind the scenes without any allocation because of the small 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 _traits_ or arbitrary _custom data_ to the meta objects created. The main difference between them is that: * 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. * Custom data are stored in a generic quick access area reserved for the user and which the library will never use under any circumstances. In all cases, this support is currently available only for meta types, meta data and meta functions. ### Traits User-defined traits are set via a meta factory: ```cpp entt::meta_factory{}.traits(my_traits::required | my_traits::hidden); ``` In the example above, `EnTT` bitmask enum support is used, but any integral value is fine, as long as it does not exceed 16 bits.
Traits can be assigned at different times. Subsequent calls to the `traits` function do not reset previously set values. However, users must reset the factory to the meta object of interest: ```cpp entt::meta_factory{} .data<&my_type::data_member, entt::as_ref_t>("member"_hs) .traits(my_traits::internal); ``` Once created, all meta objects offer a member function named `traits` to get the currently set value: ```cpp auto value = entt::resolve().traits(); ``` Note that the type is erased upon registration and must therefore be repeated when traits are _extracted_, so as to allow the library to _reconstruct_ them correctly. ### Custom data Custom arbitrary data are set via a meta factory: ```cpp entt::meta_factory{}.custom("name"); ``` The way to do this is by specifying the data type to the `custom` function and passing the necessary arguments to construct it correctly.
It is not possible to assign custom data at different times. Therefore, multiple calls to the `custom` function overwrite previous values. However, this value can be read from meta objects and used to update existing data with a factory, effectively updating them as needed.
Likewise, users can also set custom data on meta objects later if needed, as long as the factory is reset to the meta object of interest: ```cpp entt::meta_factory{} .func<&my_type::member_function>("member"_hs) .custom("tooltip"); ``` Once created, all meta objects offer a member function named `custom` to get the currently set value as a reference or as a pointer to an element: ```cpp const type_data &value = entt::resolve().custom(); ``` Note that the returned object performs an extra check in debug before converting to the requested type, so as to avoid subtle bugs.
Only in the case of conversion to a pointer is this check safe and such that a null pointer is returned to inform the user of the failed attempt. ## Unregister types A type registered with the reflection system can also be _unregistered_. This means unregistering all its data members, member functions, conversion functions and so on. However, base classes are not unregistered as well, since they do not necessarily depend on it.
Roughly speaking, unregistering a type means disconnecting all associated meta objects from it and making its identifier no longer available: ```cpp entt::meta_reset(); ``` It is also possible to reset types by their unique identifiers: ```cpp entt::meta_reset("my_type"_hs); ``` Finally, there exists a non-template overload of the `meta_reset` function that does not accept arguments and resets all meta types at once: ```cpp entt::meta_reset(); ``` A type can be re-registered later with a completely different name and form. ## Meta context All meta types and their parts are created at runtime and stored in a default _context_. This is obtained via a service locator as: ```cpp auto &&context = entt::locator::value_or(); ``` By itself, a context is an opaque object that the user can do little with. However, users can replace an existing context with another at any time: ```cpp entt::meta_context other{}; auto &&context = entt::locator::value_or(); std::swap(context, other); ``` This is useful for testing purposes or to define multiple context objects with different meta types to use as appropriate. If _replacing_ the default context is not enough, `EnTT` also offers the ability to use multiple and externally managed contexts with the runtime reflection system.
For example, to create new meta types within a context other than the default one, simply pass it as an argument to the `meta_factory` constructor: ```cpp entt::meta_ctx context{}; entt::meta_factory{context}.type("reflected_type"_hs); ``` By doing so, the new meta type is not available in the default context but is usable by passing around the new context when needed, such as when creating a new `meta_any` object: ```cpp entt::meta_any any{context, std::in_place_type}; ``` Similarly, to search for meta types in a context other than the default one, it is necessary to pass it to the `resolve` function: ```cpp entt::meta_type type = entt::resolve(context, "reflected_type"_hs) ``` More generally, when using externally managed contexts, it is always required to provide the system with the context to use, at least at the _entry point_.
For example, once the `meta_type` instant is obtained, it is no longer necessary to pass the context around as the meta type takes the information with it and eventually propagates it to all its parts.
On the other hand, it is necessary to instruct the library on where meta types are to be fetched when `meta_any`s and `meta_handle`s are constructed, a factory created or a meta type resolved.