EnTT: Crash Course: poly

EnTT 3.15.0

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Crash Course: poly

# Table of Contents

- Introduction
  - Other libraries
- Concept and implementation
  - Deduced interface
  - Defined interface
  - Fulfill a concept
- Inheritance
- Static polymorphism in the wild
- Storage size and alignment requirement

# Introduction

Static polymorphism is a very powerful tool in C++, albeit sometimes cumbersome to obtain.
This module aims to make it simple and easy to use.

The library allows to define concepts as interfaces to fulfill with concrete classes without having to inherit from a common base.
Among others, this is one of the advantages of static polymorphism in general and of a generic wrapper like that offered by the `poly` class template in particular.
The result is an object to pass around as such and not through a reference or a pointer, as it happens when it comes to working with dynamic polymorphism.

Since the `poly` class template makes use of `entt::any` internally, it also supports most of its feature. For example, the possibility to create aliases to existing and thus unmanaged objects. This allows users to exploit the static polymorphism while maintaining ownership of objects.
Likewise, the `poly` class template also benefits from the small buffer optimization offered by the `entt::any` class and therefore minimizes the number of allocations, avoiding them altogether where possible.

## Other libraries

There are some very interesting libraries regarding static polymorphism.
The ones that I like more are:

- `dyno`: runtime polymorphism done right.
- `Poly`: a class template that makes it easy to define a type-erasing polymorphic object wrapper.

The former is admittedly an experimental library, with many interesting ideas. I have some doubts about the usefulness of some features in real world projects, but perhaps my lack of experience comes into play here. In my opinion, its only flaw is the API that I find slightly more cumbersome than other solutions.
The latter was undoubtedly a source of inspiration for this module. Although I opted for different choices in the implementation of both the final API and some features.

Either way, the authors are gurus of the C++ community, people I only have to learn from.

# Concept and implementation

The first thing to do to create a type-erasing polymorphic object wrapper (to use the terminology introduced by Eric Niebler) is to define a concept that types will have to adhere to.
For this purpose, the library offers a single class that supports both deduced and fully defined interfaces. Although having interfaces deduced automatically is convenient and allows users to write less code in most cases, it has some limitations. It is therefore useful to be able to get around the deduction by providing a custom definition for the static virtual table.

Once the interface is defined, a generic implementation is needed to fulfill the concept itself.
Also in this case, the library allows customizations based on types or families of types, so as to be able to go beyond the generic case where necessary.

## Deduced interface

This is how a concept with a deduced interface is defined:

struct Drawable: entt::type_list<> {
template<typename Base>
struct type: Base {
void draw() { this->template invoke<0>(*this); }
};
// ...
};
entt::type_list
A class to use to push around lists of types, nothing more.
Definition type_traits.hpp:111

It is recognizable by the fact that it inherits from an empty type list.
Functions can also be const, accept any number of parameters and return a type other than `void`:

struct Drawable: entt::type_list<> {
template<typename Base>
struct type: Base {
bool draw(int pt) const { return this->template invoke<0>(*this, pt); }
};
// ...
};

In this case, all parameters are passed to `invoke` after the reference to `this` and the return value is whatever the internal call returns.
As for `invoke`, this is a name that is injected into the concept through `Base`, from which one must necessarily inherit. Since it is also a dependent name, the `this-> template` form is unfortunately necessary due to the rules of the language. However, there also exists an alternative that goes through an external call:

struct Drawable: entt::type_list<> {
template<typename Base>
struct type: Base {
void draw() const { entt::poly_call<0>(*this); }
};
// ...
};
entt::poly_call
decltype(auto) poly_call(Poly &&self, Args &&...args)
Shortcut for calling poly_base<Type>::invoke.
Definition poly.hpp:173

Once the concept is defined, users must provide a generic implementation of it in order to tell the system how any type can satisfy its requirements. This is done via an alias template within the concept itself.
The index passed as a template parameter to either `invoke` or `poly_call` refers to how this alias is defined.

## Defined interface

A fully defined concept is no different to one for which the interface is deduced, with the only difference that the list of types is not empty this time:

struct Drawable: entt::type_list<void()> {
template<typename Base>
struct type: Base {
void draw() { entt::poly_call<0>(*this); }
};
// ...
};

Again, parameters and return values other than `void` are allowed. Also, the function type must be const when the method to bind to it is const:

struct Drawable: entt::type_list<bool(int) const> {
template<typename Base>
struct type: Base {
bool draw(int pt) const { return entt::poly_call<0>(*this, pt); }
};
// ...
};

Why should a user fully define a concept if the function types are the same as the deduced ones?
In fact, this is the limitation that can be worked around by manually defining the static virtual table.

When things are deduced, there is an implicit constraint.
If the concept exposes a member function called `draw` with function type `void()`, a concept is satisfied:

- Either by a class that exposes a member function with the same name and the same signature.
- Or through a lambda that makes use of existing member functions from the interface itself.

In other words, it is not possible to make use of functions not belonging to the interface, even if they are part of the types that fulfill the concept.
Similarly, it is not possible to deduce a function in the static virtual table with a function type different from that of the associated member function in the interface itself.

Explicitly defining a static virtual table suppresses the deduction step and allows maximum flexibility when providing the implementation for a concept.

## Fulfill a concept

The `impl` alias template of a concept is used to define how it is fulfilled:

struct Drawable: entt::type_list<> {
// ...
template<typename Type>
using impl = entt::value_list<&Type::draw>;
};
entt::value_list
A class to use to push around lists of constant values, nothing more.
Definition type_traits.hpp:366

In this case, it is stated that the `draw` method of a generic type is enough to satisfy the requirements of the `Drawable` concept.
Both member functions and free functions are supported to fulfill concepts:

template<typename Type>
void print(Type &self) { self.print(); }
struct Drawable: entt::type_list<void()> {
// ...
template<typename Type>
using impl = entt::value_list<&print<Type>>;
};

Likewise, as long as the parameter types and return type support conversions to and from those of the function type referenced in the static virtual table, the actual implementation may differ in its function type since it is erased internally.
Moreover, the `self` parameter is not strictly required by the system and can be left out for free functions if not required.

Refer to the inline documentation for more details.

# Inheritance

Concept inheritance is straightforward due to how poly looks like in `EnTT`. Therefore, it is quite easy to build hierarchies of concepts if necessary.
The only constraint is that all concepts in a hierarchy must belong to the same family, that is, they must be either all deduced or all defined.

For a deduced concept, inheritance is achieved in a few steps:

struct DrawableAndErasable: entt::type_list<> {
template<typename Base>
struct type: typename Drawable::type<Base> {
static constexpr auto base = Drawable::impl<Drawable::type<entt::poly_inspector>>::size;
void erase() { entt::poly_call<base + 0>(*this); }
};
template<typename Type>
using impl = entt::value_list_cat_t<
typename Drawable::impl<Type>,
entt::value_list<&Type::erase>
>;
};
entt::value_list_cat_t
typename value_list_cat< List... >::type value_list_cat_t
Helper type.
Definition type_traits.hpp:517

The static virtual table is empty and must remain so.
On the other hand, `type` no longer inherits from `Base`. Instead, it forwards its template parameter to the type exposed by the base class. Internally, the size of the static virtual table of the base class is used as an offset for the local indexes.
Finally, by means of the `value_list_cat_t` utility, the implementation consists in appending the new functions to the previous list.

As for a defined concept instead, the list of types is extended in a similar way to what is shown for the implementation of the above concept.
To do this, it is useful to declare a function that allows to convert a concept into its underlying `type_list` object:

template<typename... Type>
entt::type_list<Type...> as_type_list(const entt::type_list<Type...> &);

The definition is not strictly required, since the function is only used through a `decltype` as it follows:

struct DrawableAndErasable: entt::type_list_cat_t<
decltype(as_type_list(std::declval<Drawable>())),
entt::type_list<void()>> {
// ...
};
entt::type_list_cat_t
typename type_list_cat< List... >::type type_list_cat_t
Helper type.
Definition type_traits.hpp:252

Similar to above, `type_list_cat_t` is used to concatenate the underlying static virtual table with the new function types.
Everything else is the same as already shown instead.

# Static polymorphism in the wild

Once the concept and implementation are defined, it is possible to use the `poly` class template to wrap instances that meet the requirements:

using drawable = entt::poly<Drawable>;
struct circle {
void draw() { /* ... */ }
};
struct square {
void draw() { /* ... */ }
};
// ...
drawable instance{circle{}};
instance->draw();
instance = square{};
instance->draw();
entt::poly
basic_poly< Concept > poly
Alias declaration for the most common use case.
Definition fwd.hpp:17

This class offers a wide range of constructors, from the default one (which returns an uninitialized `poly` object) to the copy and move constructors, as well as the ability to create objects in-place.
Among others, there is also a constructor that allows users to wrap unmanaged objects in a `poly` instance (either const or non-const ones):

circle shape;
drawable instance{std::in_place_type<circle &>, shape};

Similarly, it is possible to create non-owning copies of `poly` from an existing object:

drawable other = instance.as_ref();

In both cases, although the interface of the `poly` object does not change, it does not construct any element or take care of destroying the referenced objects.

Note also how the underlying concept is accessed via a call to `operator->` and not directly as `instance.draw()`.
This allows users to decouple the API of the wrapper from that of the concept. Therefore, where `instance.data()` invokes the `data` member function of the poly object, `instance->data()` maps directly to the functionality exposed by the underlying concept.

# Storage size and alignment requirement

Under the hood, the `poly` class template makes use of `entt::any`. Therefore, it can take advantage of the possibility of defining at compile-time the size of the storage suitable for the small buffer optimization as well as the alignment requirements:

entt::basic_poly<Drawable, sizeof(double[4]), alignof(double[4])>
entt::basic_poly
Static polymorphism made simple and within everyone's reach.
Definition poly.hpp:193

The default size is `sizeof(double[2])`, which seems like a good compromise between a buffer that is too large and one unable to hold anything larger than an integer. The alignment requirement is optional, and by default such that it is the most stringent (the largest) for any object whose size is at most equal to the one provided.
It is worth noting that providing a size of 0 (which is an accepted value in all respects) will force the system to dynamically allocate the contained objects in all cases.