Implementing Component system from Unity in c++
Apologies if this is not what you are looking for, but I had an idea to use the unordered map with a type index and, with the help of some metaprogramming and TR2, place multiple pointers to the component into the map, including its direct base classes as additional keys. So getComponent<SphereCollider>() and getComponent<Collider>() along with a down-cast will have the same pointee.
#include <tr2/type_traits>
#include <tuple>
#include <typeindex>
#include <unordered_map>
#include <iostream>
class Component {
public:
virtual ~Component() {}
};
class GameObject {
public:
template <typename T>
void addComponent(T *component);
template <typename T>
T *getComponent();
std::unordered_map<std::typeindex, Component *> components;
};
template <typename>
struct direct_bases_as_tuple {};
template <typename... Types>
struct direct_bases_as_tuple<std::tr2::__reflection_typelist<Types...>> {
typedef std::tuple<Types...> type;
};
template <std::size_t N, typename ComponentBases, typename ComponentType>
struct AddComponent {
GameObject *owner;
explicit AddComponent(GameObject *owner) : owner(owner) {}
void operator()(ComponentType *component) {
AddComponent<N-1, ComponentBases, ComponentType>{owner}(component);
using BaseType = std::tuple_element<N-1, ComponentBases>::type;
owner->components[typeid(BaseType)] = component;
}
};
template <typename ComponentBases, typename ComponentType>
struct AddComponent<0u, ComponentBases, ComponentType> {
GameObject *owner;
explicit AddComponent(GameObject *owner) : owner(owner) {}
void operator()(ComponentType *component) {
return;
}
};
template <typename T>
void GameObject::addComponent(T *component) {
using ComponentBases = direct_bases_as_tuple<std::tr2::direct_bases<ComponentType>::type>::type;
constexpr classCount = std::tuple_size<ComponentBases>::value;
AddComponent<classCount, ComponentBases, T>{this}(component);
components[typeid(T)] = component;
}
template <typename T>
T * GameObject::getComponent() {
auto iter = components.find(typeid(T));
if (iter != std::end(components)) {
return dynamic_cast<T *>(iter->second);
}
return nullptr;
}
class Collider : public Component {};
class SphereCollider : public Collider {};
int main() {
GameObject gameObject;
gameObject.addComponent(new SphereCollider);
//get by derived class
SphereCollider *sphereColliderA = gameObject.getComponent<SphereCollider>();
//get by subclass
SphereCollider *sphereColliderB = dynamic_cast<SphereCollider *>(
gameObject.getComponent<Collider>()
);
if (sphereColliderA == sphereColliderB) {
std::cout << "good" << std::endl;
}
}
I created the AddComponent struct to recurse through the component base classes at compile-time and insert the pointer (value) with the corresponding class (key) each iteration. The helper struct direct_bases_as_tuple was inspired by Andy Prowl's answer to change the direct bases into a tuple. I compiled this using GCC 4.9.2 using C++11 features.
Since I'm writing my own game engine and incorporating the same design, I thought I'd share my results.
Overview
I wrote my own RTTI for the classes I cared to use as Components of my GameObject instances. The amount of typing is reduced by #defineing the two macros: CLASS_DECLARATION and CLASS_DEFINITION
CLASS_DECLARATION declares the unique static const std::size_t that will be used to identify the class type (Type), and a virtual function that allows objects to traverse their class hierarchy by calling their parent-class function of the same name (IsClassType).
CLASS_DEFINITION defines those two things. Namely the Type is initialized to a hash of a stringified version of the class name (using TO_STRING(x) #x), so that Type comparisons are just an int compare and not a string compare.
std::hash<std::string> is the hash function used, which guarantees equal inputs yield equal outputs, and the number of collisions is near-zero.
Aside from the low risk of hash collisions, this implementation has the added benefit of allowing users to create their own Component classes using those macros without ever having to refer to|extend some master include file of enum classs, or use typeid (which only provides the run-time type, not the parent-classes).
AddComponent
This custom RTTI simplifies the call syntax for Add|Get|RemoveComponent to just specifying the template type, just like Unity.
The AddComponent method perfect-forwards a universal reference variadic parameter pack to the user's constructor. So, for example, a user-defined Component-derived class CollisionModel could have the constructor:
CollisionModel( GameObject * owner, const Vec3 & size, const Vec3 & offset, bool active );
then later on the user simply calls:
myGameObject.AddComponent<CollisionModel>(this, Vec3( 10, 10, 10 ), Vec3( 0, 0, 0 ), true );
Note the explicit construction of the Vec3s because perfect-forwarding can fail to link if using deduced initializer-list syntax like { 10, 10, 10 } regardless of Vec3's constructor declarations.
This custom RTTI also resolves 3 issues with the std::unordered_map<std::typeindex,...> solution:
- Even with the hierarchy traversal using
std::tr2::direct_basesthe end result is still duplicates of the same pointer in the map. - A user can't add multiple Components of equivalent type, unless a map is used that allows/solves collisions without overwriting, which further slows down the code.
- No uncertain and slow
dynamic_castis needed, just a straightstatic_cast.
GetComponent
GetComponent just uses the static const std::size_t Type of the template type as an argument to the virtual bool IsClassType method and iterates over std::vector< std::unique_ptr< Component > > looking for the first match.
I've also implemented a GetComponents method that can get all components of the requested type, again including getting from the parent-class.
Note that the static member Type can be accessed both with and without an instance of the class.
Also note that Type is public, declared for each Component-derived class, ...and capitalized to emphasize its flexible use, despite being a POD member.
RemoveComponent
Lastly, RemoveComponent uses C++14's init-capture to pass that same static const std::size_t Type of the template type into a lambda so it can basically do the same vector traversal, this time getting an iterator to the first matching element.
There are a few comments in the code about ideas for a more flexible implementation, not to mention const versions of all these could also easily be implemented.
The Code
Classes.h
#ifndef TEST_CLASSES_H
#define TEST_CLASSES_H
#include <string>
#include <functional>
#include <vector>
#include <memory>
#include <algorithm>
#define TO_STRING( x ) #x
//****************
// CLASS_DECLARATION
//
// This macro must be included in the declaration of any subclass of Component.
// It declares variables used in type checking.
//****************
#define CLASS_DECLARATION( classname ) \
public: \
static const std::size_t Type; \
virtual bool IsClassType( const std::size_t classType ) const override; \
//****************
// CLASS_DEFINITION
//
// This macro must be included in the class definition to properly initialize
// variables used in type checking. Take special care to ensure that the
// proper parentclass is indicated or the run-time type information will be
// incorrect. Only works on single-inheritance RTTI.
//****************
#define CLASS_DEFINITION( parentclass, childclass ) \
const std::size_t childclass::Type = std::hash< std::string >()( TO_STRING( childclass ) ); \
bool childclass::IsClassType( const std::size_t classType ) const { \
if ( classType == childclass::Type ) \
return true; \
return parentclass::IsClassType( classType ); \
} \
namespace rtti {
//***************
// Component
// base class
//***************
class Component {
public:
static const std::size_t Type;
virtual bool IsClassType( const std::size_t classType ) const {
return classType == Type;
}
public:
virtual ~Component() = default;
Component( std::string && initialValue )
: value( initialValue ) {
}
public:
std::string value = "uninitialized";
};
//***************
// Collider
//***************
class Collider : public Component {
CLASS_DECLARATION( Collider )
public:
Collider( std::string && initialValue )
: Component( std::move( initialValue ) ) {
}
};
//***************
// BoxCollider
//***************
class BoxCollider : public Collider {
CLASS_DECLARATION( BoxCollider )
public:
BoxCollider( std::string && initialValue )
: Collider( std::move( initialValue ) ) {
}
};
//***************
// RenderImage
//***************
class RenderImage : public Component {
CLASS_DECLARATION( RenderImage )
public:
RenderImage( std::string && initialValue )
: Component( std::move( initialValue ) ) {
}
};
//***************
// GameObject
//***************
class GameObject {
public:
std::vector< std::unique_ptr< Component > > components;
public:
template< class ComponentType, typename... Args >
void AddComponent( Args&&... params );
template< class ComponentType >
ComponentType & GetComponent();
template< class ComponentType >
bool RemoveComponent();
template< class ComponentType >
std::vector< ComponentType * > GetComponents();
template< class ComponentType >
int RemoveComponents();
};
//***************
// GameObject::AddComponent
// perfect-forwards all params to the ComponentType constructor with the matching parameter list
// DEBUG: be sure to compare the arguments of this fn to the desired constructor to avoid perfect-forwarding failure cases
// EG: deduced initializer lists, decl-only static const int members, 0|NULL instead of nullptr, overloaded fn names, and bitfields
//***************
template< class ComponentType, typename... Args >
void GameObject::AddComponent( Args&&... params ) {
components.emplace_back( std::make_unique< ComponentType >( std::forward< Args >( params )... ) );
}
//***************
// GameObject::GetComponent
// returns the first component that matches the template type
// or that is derived from the template type
// EG: if the template type is Component, and components[0] type is BoxCollider
// then components[0] will be returned because it derives from Component
//***************
template< class ComponentType >
ComponentType & GameObject::GetComponent() {
for ( auto && component : components ) {
if ( component->IsClassType( ComponentType::Type ) )
return *static_cast< ComponentType * >( component.get() );
}
return *std::unique_ptr< ComponentType >( nullptr );
}
//***************
// GameObject::RemoveComponent
// returns true on successful removal
// returns false if components is empty, or no such component exists
//***************
template< class ComponentType >
bool GameObject::RemoveComponent() {
if ( components.empty() )
return false;
auto & index = std::find_if( components.begin(),
components.end(),
[ classType = ComponentType::Type ]( auto & component ) {
return component->IsClassType( classType );
} );
bool success = index != components.end();
if ( success )
components.erase( index );
return success;
}
//***************
// GameObject::GetComponents
// returns a vector of pointers to the the requested component template type following the same match criteria as GetComponent
// NOTE: the compiler has the option to copy-elide or move-construct componentsOfType into the return value here
// TODO: pass in the number of elements desired (eg: up to 7, or only the first 2) which would allow a std::array return value,
// except there'd need to be a separate fn for getting them *all* if the user doesn't know how many such Components the GameObject has
// TODO: define a GetComponentAt<ComponentType, int>() that can directly grab up to the the n-th component of the requested type
//***************
template< class ComponentType >
std::vector< ComponentType * > GameObject::GetComponents() {
std::vector< ComponentType * > componentsOfType;
for ( auto && component : components ) {
if ( component->IsClassType( ComponentType::Type ) )
componentsOfType.emplace_back( static_cast< ComponentType * >( component.get() ) );
}
return componentsOfType;
}
//***************
// GameObject::RemoveComponents
// returns the number of successful removals, or 0 if none are removed
//***************
template< class ComponentType >
int GameObject::RemoveComponents() {
if ( components.empty() )
return 0;
int numRemoved = 0;
bool success = false;
do {
auto & index = std::find_if( components.begin(),
components.end(),
[ classType = ComponentType::Type ]( auto & component ) {
return component->IsClassType( classType );
} );
success = index != components.end();
if ( success ) {
components.erase( index );
++numRemoved;
}
} while ( success );
return numRemoved;
}
} /* rtti */
#endif /* TEST_CLASSES_H */
Classes.cpp
#include "Classes.h"
using namespace rtti;
const std::size_t Component::Type = std::hash<std::string>()(TO_STRING(Component));
CLASS_DEFINITION(Component, Collider)
CLASS_DEFINITION(Collider, BoxCollider)
CLASS_DEFINITION(Component, RenderImage)
main.cpp
#include <iostream>
#include "Classes.h"
#define MORE_CODE 0
int main( int argc, const char * argv ) {
using namespace rtti;
GameObject test;
// AddComponent test
test.AddComponent< Component >( "Component" );
test.AddComponent< Collider >( "Collider" );
test.AddComponent< BoxCollider >( "BoxCollider_A" );
test.AddComponent< BoxCollider >( "BoxCollider_B" );
#if MORE_CODE
test.AddComponent< RenderImage >( "RenderImage" );
#endif
std::cout << "Added:\n------\nComponent\t(1)\nCollider\t(1)\nBoxCollider\t(2)\nRenderImage\t(0)\n\n";
// GetComponent test
auto & componentRef = test.GetComponent< Component >();
auto & colliderRef = test.GetComponent< Collider >();
auto & boxColliderRef1 = test.GetComponent< BoxCollider >();
auto & boxColliderRef2 = test.GetComponent< BoxCollider >(); // boxColliderB == boxColliderA here because GetComponent only gets the first match in the class hierarchy
auto & renderImageRef = test.GetComponent< RenderImage >(); // gets &nullptr with MORE_CODE 0
std::cout << "Values:\n-------\ncomponentRef:\t\t" << componentRef.value
<< "\ncolliderRef:\t\t" << colliderRef.value
<< "\nboxColliderRef1:\t" << boxColliderRef1.value
<< "\nboxColliderRef2:\t" << boxColliderRef2.value
<< "\nrenderImageRef:\t\t" << ( &renderImageRef != nullptr ? renderImageRef.value : "nullptr" );
// GetComponents test
auto allColliders = test.GetComponents< Collider >();
std::cout << "\n\nThere are (" << allColliders.size() << ") collider components attached to the test GameObject:\n";
for ( auto && c : allColliders ) {
std::cout << c->value << '\n';
}
// RemoveComponent test
test.RemoveComponent< BoxCollider >(); // removes boxColliderA
auto & boxColliderRef3 = test.GetComponent< BoxCollider >(); // now this is the second BoxCollider "BoxCollider_B"
std::cout << "\n\nFirst BoxCollider instance removed\nboxColliderRef3:\t" << boxColliderRef3.value << '\n';
#if MORE_CODE
// RemoveComponent return test
int removed = 0;
while ( test.RemoveComponent< Component >() ) {
++removed;
}
#else
// RemoveComponents test
int removed = test.RemoveComponents< Component >();
#endif
std::cout << "\nSuccessfully removed (" << removed << ") components from the test GameObject\n";
system( "PAUSE" );
return 0;
}
Output
Added:
------
Component (1)
Collider (1)
BoxCollider (2)
RenderImage (0)
Values:
-------
componentRef: Component
colliderRef: Collider
boxColliderRef1: BoxCollider_A
boxColliderRef2: BoxCollider_A
renderImageRef: nullptr
There are (3) collider components attached to the test GameObject:
Collider
BoxCollider_A
BoxCollider_B
First BoxCollider instance removed
boxColliderRef3: BoxCollider_B
Successfully removed (3) components from the test GameObject
Side-note: granted Unity uses Destroy(object) and not RemoveComponent, but my version suits my needs for now.
I know this post is already answered, but if you look into Game Programming Patterns, in this book he has a design pattern called Service Locator, and at the end, it says Unity uses this pattern together with the Component Pattern. I wish I could answer into more specifics, but this could be another way to approach this.