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https://github.com/skypjack/meta

Header-only, non-intrusive and macro-free runtime reflection system in C++
https://github.com/skypjack/meta

cpp cpp-17 entt header-only macro-free meta modern-cpp non-instrusive reflection runtime runtime-reflection

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Header-only, non-intrusive and macro-free runtime reflection system in C++

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# Header-only runtime reflection system in C++



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> The reflection system was born within [EnTT](https://github.com/skypjack/entt)

> and is developed and enriched there. This project is designed for those who

> are interested only in a header-only, full-featured, non-intrusive and macro

> free reflection system which certainly deserves to be treated also separately

> due to its quality and its rather peculiar features.



If you use `meta` and you want to say thanks or support the project, please

**consider becoming a

[sponsor](https://github.com/users/skypjack/sponsorship)**.


You can help me make the difference.

[Many thanks](https://skypjack.github.io/sponsorship/) to those who supported me

and still support me today.



# Table of Contents



* [Introduction](#introduction)

* [Build Instructions](#build-instructions)

  * [Requirements](#requirements)

  * [Library](#library)

  * [Documentation](#documentation)

  * [Tests](#tests)

* [Crash course](#crash-course)

  * [Names and identifiers](#names-and-identifiers)

  * [Reflection in a nutshell](#reflection-in-a-nutshell)

  * [Any as in any type](#any-as-in-any-type)

  * [Enjoy the runtime](#enjoy-the-runtime)

  * [Policies: the more, the less](#policies-the-more-the-less)

  * [Named constants and enums](#named-constants-and-enums)

  * [Properties and meta objects](#properties-and-meta-objects)

  * [Unregister types](#unregister-types)

* [Contributors](#contributors)

* [License](#license)

* [Support](#support)



# Introduction



Reflection (or rather, its lack) is a trending topic in the C++ world. I looked

for a third-party library that met my needs on the subject, but I always came

across some details that I didn't like: macros, being intrusive, too many

allocations.


In one word: **unsatisfactory**.



I finally decided to write a built-in, non-intrusive and macro-free runtime

reflection system for my own.


Maybe I didn't do better than others or maybe yes. Time will tell me.



# Build Instructions



## Requirements



To be able to use `meta`, users must provide a full-featured compiler that

supports at least C++17.


The requirements below are mandatory to compile the tests and to extract the

documentation:



* CMake version 3.2 or later.

* Doxygen version 1.8 or later.



If you are looking for a C++14 version of `meta`, feel free to

[contact me](https://github.com/skypjack).



## Library



`meta` is a header-only library. This means that including the `factory.hpp` and

`meta.hpp` headers is enough to include the library as a whole and use it.


It's a matter of adding the following lines to the top of a file:



```cpp

#include 

#include 

```



Then pass the proper `-I` argument to the compiler to add the `src` directory to

the include paths.



## Documentation



The documentation is based on [doxygen](http://www.stack.nl/~dimitri/doxygen/).

To build it:



    $ cd build

    $ cmake .. -DBUILD_DOCS=ON

    $ make



The API reference will be created in HTML format within the directory

`build/docs/html`. To navigate it with your favorite browser:



    $ cd build

    $ your_favorite_browser docs/html/index.html



It's also available [online](https://skypjack.github.io/meta/) for the latest

version.



## Tests



To compile and run the tests, `meta` requires *googletest*.


`cmake` will download and compile the library before compiling anything else.

In order to build without tests set CMake option `BUILD_TESTING=OFF`.



To build the most basic set of tests:



* `$ cd build`

* `$ cmake ..`

* `$ make`

* `$ make test`



# Crash course



## Names and identifiers



The meta system doesn't force the user to use a specific tool when it comes to

working with names and identifiers. It does this by offering an API that works

with opaque identifiers that for example 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 doesn't matter if they are generated at runtime, at

compile-time or with custom functions.



However, the examples in the following sections are all based on

`std::hash` as provided by the standard library. Therefore,

where an identifier is required, it's likely that an instance of this class is

used as follows:



```cpp

std::hash hash{};

auto factory = meta::reflect(hash("reflected_type"));

```



For what it's worth, this is likely completely equivalent to:



```cpp

auto factory = meta::reflect(42);

```



Obviously, human-readable identifiers are more convenient to use and highly

recommended.



## Reflection in a nutshell



Reflection always starts from real types (users cannot reflect imaginary types

and it would not make much sense, we wouldn't be talking about reflection

anymore).


To _reflect_ a type, the library provides the `reflect` function:



```cpp

meta::factory factory = meta::reflect(hash("reflected_type"));

```



It accepts the type to reflect as a template parameter and an optional

identifier as an argument. Identifiers are important because users can retrieve

meta types at runtime by searching for them by _name_. However, there are cases

in which users can be interested in adding features to a reflected type so that

the reflection system can use it correctly under the hood, but they don't want

to allow searching the type by _name_.


In both cases, the returned value is a factory object to use to continue

building the meta type.



A factory is such that all its member functions returns the factory itself.

It can be used to extend the reflected type and add the following:



* _Constructors_. Actual constructors can be assigned to a reflected type by

  specifying their list of arguments. Free functions (namely, factories) can be

  used as well, as long as the return type is the expected one. From a client's

  point of view, nothing changes if a constructor is a free function or an

  actual constructor.


  Use the `ctor` member function for this purpose:



  ```cpp

  meta::reflect(hash("reflected")).ctor().ctor<&factory>();

  ```



* _Destructors_. Free functions can be set as destructors of reflected types.

  The purpose is to give users the ability to free up resources that require

  special treatment before an object is actually destroyed.


  Use the `dtor` member function for this purpose:



  ```cpp

  meta::reflect(hash("reflected")).dtor<&destroy>();

  ```



  A function should neither delete nor explicitly invoke the destructor of a

  given instance.



* _Data members_. Both real data members of the underlying type and static and

  global variables, as well as constants of any kind, can be attached to a meta

  type. From a client's point of view, all the variables associated with the

  reflected type will appear as if they were part of the type itself.


  Use the `data` member function for this purpose:



  ```cpp

  meta::reflect(hash("reflected"))

      .data<&my_type::static_variable>(hash("static"))

      .data<&my_type::data_member>(hash("member"))

      .data<&global_variable>(hash("global"));

  ```



  This function requires as an argument the identifier to give to the meta data

  once created. Users can then access meta data at runtime by searching for them

  by _name_.


  Data members can be set also by means of a couple of functions, namely a

  setter and a getter. Setters and getters can be either free functions, member

  functions or mixed ones, as long as they respect the required signatures.


  Refer to the inline documentation for all the details.



* _Member functions_. Both real member functions of the underlying type and free

  functions can be attached to a meta type. From a client's point of view, all

  the functions associated with the reflected type will appear as if they were

  part of the type itself.


  Use the `func` member function for this purpose:



  ```cpp

  meta::reflect(hash("reflected"))

      .func<&my_type::static_function>(hash("static"))

      .func<&my_type::member_function>(hash("member"))

      .func<&free_function>(hash("free"));

  ```



  This function requires as an argument the identifier to give to the meta

  function once created. Users can then access meta functions at runtime by

  searching for them by _name_.



* _Base classes_. A base class is such that the underlying type is actually

  derived from it. In this case, the reflection system tracks the relationship

  and allows for implicit casts at runtime when required.


  Use the `base` member function for this purpose:



  ```cpp

  meta::reflect(hash("derived")).base();

  ```



  From now on, wherever a `base_type` is required, an instance of `derived_type`

  will also be accepted.



* _Conversion functions_. Actual types can be converted, this is a fact. Just

  think of the relationship between a `double` and an `int` to see it. Similar

  to bases, conversion functions allow users to define conversions that will be

  implicitly performed by the reflection system when required.


  Use the `conv` member function for this purpose:



  ```cpp

  meta::reflect().conv();

  ```



That's all, everything users need to create meta types and enjoy the reflection

system. At first glance it may not seem that much, but users usually learn to

appreciate it over time.


Also, do not forget what these few lines hide under the hood: a built-in,

non-intrusive and macro-free system for reflection in C++. Features that are

definitely worth the price, at least for me.



## Any as in any type



The reflection system comes with its own meta any type. It may seem redundant

since C++17 introduced `std::any`, but it is not.


In fact, the _type_ returned by an `std::any` is a const reference to an

`std::type_info`, an implementation defined class that's not something everyone

wants to see in a software. Furthermore, the class `std::type_info` suffers from

some design flaws and there is even no way to _convert_ an `std::type_info` into

a meta type, thus linking the two worlds.



A meta any object provides an API similar to that of its most famous counterpart

and serves the same purpose of being an opaque container for any type of

value.


It minimizes the allocations required, which are almost absent thanks to _SBO_

techniques. In fact, unless users deal with _fat types_ and create instances of

them though the reflection system, allocations are at zero.



A meta any object can be created by any other object or as an empty container

to initialize later:



```cpp

// a meta any object that contains an int

meta::any any{0};



// an empty meta any object

meta::any empty{};

```



It takes the burden of destroying the contained instance when required.


Moreover, it can be used as an opaque container for unmanaged objects if needed:



```cpp

int value;

meta::any any{std::ref(value)};

```



In other words, whenever `any` intercepts a `reference_wrapper`, it acts as a

reference to the original instance rather than making a copy of it. The

contained object is never destroyed and users must ensure that its lifetime

exceeds that of the container.



A meta any object has a `type` member function that returns the meta type of the

contained value, if any. The member functions `try_cast`, `cast` and `convert`

are used to know if the underlying object has a given type as a base or if it

can be converted implicitly to it.



## Enjoy the runtime



Once the web of reflected types has been constructed, it's a matter of using it

at runtime where required.



To search for a reflected type there are two options: by type or by _name_. In

both cases, the search can be done by means of the `resolve` function:



```cpp

// search for a reflected type by type

meta::type by_type = meta::resolve();



// search for a reflected type by name

meta::type by_name = meta::resolve(hash("reflected_type"));

```



There exits also a third overload of the `resolve` function to use to iterate

all the reflected types at once:



```cpp

resolve([](meta::type type) {

    // ...

});

```



In all cases, the returned value is an instance of `type`. This type of objects

offer an API to know the _runtime identifier_ of the type, to iterate all the

meta objects associated with them and even to build or destroy instances of the

underlying type.


Refer to the inline documentation for all the details.



The meta objects that compose a meta type are accessed in the following ways:



* _Meta constructors_. They are accessed by types of arguments:



  ```cpp

  meta::ctor ctor = meta::resolve().ctor();

  ```



  The returned type is `ctor` and may be invalid if there is no constructor that

  accepts the supplied arguments or at least some types from which they are

  derived or to which they can be converted.


  A meta constructor offers an API to know the number of arguments, the expected

  meta types and to invoke it, therefore to construct a new instance of the

  underlying type.



* _Meta destructor_. It's returned by a dedicated function:



  ```cpp

  meta::dtor dtor = meta::resolve().dtor();

  ```



  The returned type is `dtor` and may be invalid if there is no custom

  destructor set for the given meta type.


  All what a meta destructor has to offer is a way to invoke it on a given

  instance. Be aware that the result may not be what is expected.



* _Meta data_. They are accessed by _name_:



  ```cpp

  meta::data data = meta::resolve().data(hash("member"));

  ```



  The returned type is `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 (ie to know if

  it's a const or a static one), to get the meta type of the variable and to set

  or get the contained value.



* _Meta functions_. They are accessed by _name_:



  ```cpp

  meta::func func = meta::resolve().func(hash("member"));

  ```



  The returned type is `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 (ie to know

  if it's 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 can be used to invoke the underlying function and then get the

  return value in the form of meta any object.



* _Meta bases_. They are accessed through the _name_ of the base types:



  ```cpp

  meta::base base = meta::resolve().base(hash("base"));

  ```



  The returned type is `base` and may be invalid if there is no meta base object

  associated with the given identifier.


  Meta bases aren't meant to be used directly, even though they are freely

  accessible. They expose only a few methods to use to know the meta type of the

  base class and to convert a raw pointer between types.



* _Meta conversion functions_. They are accessed by type:



  ```cpp

  meta::conv conv = meta::resolve().conv();

  ```



  The returned type is `conv` and may be invalid if there is no meta conversion

  function associated with the given type.


  The meta conversion functions are as thin as the meta bases and with a very

  similar interface. The sole difference is that they return a newly created

  instance wrapped in a meta any object when they convert between different

  types.



All the objects thus obtained as well as the meta types can be explicitly

converted to a boolean value to check if they are valid:



```cpp

meta::func func = meta::resolve().func(hash("member"));



if(func) {

    // ...

}

```



Furthermore, all meta objects with the exception of meta destructors can be

iterated through an overload that accepts a callback through which to return

them. As an example:



```cpp

meta::resolve().data([](meta::data data) {

    // ...

});

```



A meta type can also be used to `construct` or `destroy` actual instances of the

underlying type.


In particular, the `construct` member function accepts a variable number of

arguments and searches for a match. It returns a `any` object that may or may

not be initialized, depending on whether a suitable constructor has been found

or not. On the other side, the `destroy` member function accepts instances of

`any` as well as actual objects by reference and invokes the registered

destructor if any.


Be aware that the result of a call to `destroy` may not be what is expected. The

purpose is to give users the ability to free up resources that require special

treatment and **not** to actually destroy instances.



Meta types and meta objects in general contain much more than what is said: a

plethora of functions in addition to those listed whose purposes and uses go

unfortunately beyond the scope of this document.


I invite anyone interested in the subject to look at the code, experiment and

read the official documentation to get the best out of this powerful tool.



## Policies: the more, the less



Policies are a kind of compile-time directives that can be used when recording

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 `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 isn't desirable. Policies are there to offer a solution to this and

other problems.



There are a few alternatives available at the moment:



* The _as-is_ policy, associated with the type `meta::as_is_t`.


  This is the default policy. In general, it should never be used explicitly,

  since it's 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 `meta::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`.


  If the use with functions is obvious, it must be said that it's also possible

  to use this policy with constructors and data members. In the first case, the

  constructor will be invoked but the returned wrapper will actually be empty.

  In the second case, instead, the property will not be accessible for

  reading.



  As an example of use:



  ```cpp

  meta::reflect(hash("reflected"))

      .func<&my_type::member_function, meta::as_void_t>(hash("member"));

  ```



* The _as-alias_ policy, associated with the type `meta::as_alias_t`


  It allows to build wrappers that act as aliases for the objects used to

  initialize them. Modifying the object contained in the wrapper for which the

  _aliasing_ was requested will make it possible to directly modify the instance

  used to initialize the wrapper itself.


  This policy works with constructors (for example, when objects are taken from

  an external container rather than created on demand), data members and

  functions in general (as long as their return types are lvalue references).



  As an example of use:



  ```cpp

  meta::reflect(hash("reflected"))

      .data<&my_type::data_member, meta::as_alias_t>(hash("member"));

  ```



Some uses are rather trivial, but it's 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



A special mention should be made for constant values and enums. It wouldn't be

necessary, but it will help distracted readers.



As mentioned, the `data` member function can be used to reflect constants of any

type among the other things.


This allows users to create meta types for enums that will work exactly like any

other meta type built from a class. Similarly, arithmetic types can be enriched

with constants of special meaning where required.


Personally, I find it very useful not to export what is the difference between

enums and classes in C++ directly in the space of the reflected types.



All the values thus exported will appear to users as if they were constant data

members of the reflected types.



Exporting constant values or elements from an enum is as simple as ever:



```cpp

meta::reflect()

        .data(hash("a_value"))

        .data(hash("another_value"));



meta::reflect().data<2048>(hash("max_int"));

```



It goes without saying that accessing them is trivial as well. It's a matter of

doing the following, as with any other data member of a meta type:



```cpp

my_enum value = meta::resolve().data(hash("a_value")).get({}).cast();

int max = meta::resolve().data(hash("max_int")).get({}).cast();

```



As a side note, remember that all this happens behind the scenes without any

allocation because of the small object optimization performed by the meta any

class.



## Properties and meta objects



Sometimes (for example, when it comes to creating an editor) it might be useful

to be able to attach properties to the meta objects created. Fortunately, this

is possible for most of them.


To attach a property to a meta object, no matter what as long as it supports

properties, it is sufficient to provide an object at the time of construction

such that `std::get<0>` and `std::get<1>` are valid for it. In other terms, the

properties are nothing more than key/value pairs users can put in an

`std::pair`. As an example:



```cpp

meta::reflect(hash("reflected"), std::make_pair(hash("tooltip"), "message"));

```



The meta objects that support properties offer then a couple of member functions

named `prop` to iterate them at once and to search a specific property by key:



```cpp

// iterate all the properties of a meta type

meta::resolve().prop([](meta::prop prop) {

    // ...

});



// search for a given property by name

meta::prop prop = meta::resolve().prop(hash("tooltip"));

```



Meta properties are objects having a fairly poor interface, all in all. They

only provide the `key` and the `value` member functions to be used to retrieve

the key and the value contained in the form of meta any objects, respectively.



## 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, the base classes won't be unregistered, since they don't

necessarily depend on it. Similarly, implicitly generated types (as an example,

the meta types implicitly generated for function parameters when needed) won't

be unregistered.



To unregister a type, users can use the `unregister` function from the global

namespace:



```cpp

meta::unregister();

```



This function returns a boolean value that is true if the type is actually

registered with the reflection system, false otherwise.


The type can be re-registered later with a completely different name and form.



# Contributors



Requests for features, PR, suggestions ad feedback are highly appreciated.



If you find you can help me and want to contribute to the project with your

experience or you do want to get part of the project for some other reasons,

feel free to contact me directly (you can find the mail in the

[profile](https://github.com/skypjack)).


I can't promise that each and every contribution will be accepted, but I can

assure that I'll do my best to take them all seriously.



If you decide to participate, please see the guidelines for

[contributing](docs/CONTRIBUTING.md) before to create issues or pull

requests.


Take also a look at the

[contributors list](https://github.com/skypjack/meta/blob/master/AUTHORS) to

know who has participated so far.



# License



Code and documentation Copyright (c) 2018-2019 Michele Caini.



Code released under

[the MIT license](https://github.com/skypjack/meta/blob/master/LICENSE).

Documentation released under

[CC BY 4.0](https://creativecommons.org/licenses/by/4.0/).



# Support



If you want to support this project, you can

[offer me](https://github.com/users/skypjack/sponsorship) an espresso.


If you find that it's not enough, feel free to

[help me](https://www.paypal.me/skypjack) the way you prefer.