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Header-only runtime reflection system in C++

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

Table of Contents

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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.<br/> In one word: unsatisfactory.

I finally decided to write a built-in, non-intrusive and macro-free runtime reflection system for my own.<br/> 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.<br/> The requirements below are mandatory to compile the tests and to extract the documentation:

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.<br/> 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
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It's also available online for the latest version.

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Tests

To compile and run the tests, meta requires googletest.<br/> 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:

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.<br/> 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).<br/> 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.<br/> 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:

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.<br/> 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.<br/> 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.<br/> 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.<br/> 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.<br/> Refer to the inline documentation for all the details.

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

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.<br/> 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.<br/> 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.<br/> 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.<br/> 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:

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

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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).<br/> 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.<br/> Take also a look at the contributors list to know who has participated so far.

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License

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

Code released under the MIT license. Documentation released under CC BY 4.0.<br/>

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Support

If you want to support this project, you can offer me an espresso.<br/> If you find that it's not enough, feel free to help me the way you prefer.

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