Header-only runtime reflection system in C++

The reflection system was born within 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.

You can help me make the difference.
Many thanks to those who supported me
and still support me today.

Introduction
Build Instructions
Crash course
Contributors
License
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.

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:

#include <meta/factory.hpp>
#include <meta/meta.hpp>

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.
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 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<std::string_view> 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:

std::hash<std::string_view> hash{};
auto factory = meta::reflect<my_type>(hash("reflected_type"));

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

auto factory = meta::reflect<my_type>(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:

meta::factory factory = meta::reflect<my_type>(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:
```
meta::reflect<my_type>(hash("reflected")).ctor<int, char>().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:
```
meta::reflect<my_type>(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:
```
meta::reflect<my_type>(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:
```
meta::reflect<my_type>(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:
```
meta::reflect<derived_type>(hash("derived")).base<base_type>();
```
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:
```
meta::reflect<double>().conv<int>();
```

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:

// 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:

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:

// search for a reflected type by type
meta::type by_type = meta::resolve<my_type>();

// 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:

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:
```
meta::ctor ctor = meta::resolve<my_type>().ctor<int, char>();
```
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:
```
meta::dtor dtor = meta::resolve<my_type>().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:
```
meta::data data = meta::resolve<my_type>().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:
```
meta::func func = meta::resolve<my_type>().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:
```
meta::base base = meta::resolve<derived_type>().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:
```
meta::conv conv = meta::resolve<double>().conv<int>();
```
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:

meta::func func = meta::resolve<my_type>().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:

meta::resolve<my_type>().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:
```
meta::reflect<my_type>(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:
```
meta::reflect<my_type>(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:

meta::reflect<my_enum>()
        .data<my_enum::a_value>(hash("a_value"))
        .data<my_enum::another_value>(hash("another_value"));

meta::reflect<int>().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:

my_enum value = meta::resolve<my_enum>().data(hash("a_value")).get({}).cast<my_enum>();
int max = meta::resolve<int>().data(hash("max_int")).get({}).cast<int>();

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:

meta::reflect<my_type>(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:

// iterate all the properties of a meta type
meta::resolve<my_type>().prop([](meta::prop prop) {
    // ...
});

// search for a given property by name
meta::prop prop = meta::resolve<my_type>().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:

meta::unregister<my_type>();

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).

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 before to create issues or pull
requests.

Take also a look at the
contributors list to
know who has participated so far.

License

Code released under
the MIT license.
Documentation released under
CC BY 4.0.

Support

If you want to support this project, you can
offer me an espresso.

If you find that it’s not enough, feel free to
help me the way you prefer.

meta