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/* Copyright (C) 2012 Wildfire Games.
* This file is part of 0 A.D.
*
* 0 A.D. is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 2 of the License, or
* (at your option) any later version.
*
* 0 A.D. is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with 0 A.D. If not, see <http://www.gnu.org/licenses/>.
*/
#ifndef INCLUDED_CMPPTR
#define INCLUDED_CMPPTR
#include "simulation2/system/Entity.h"
class CSimContext;
class CSimulation2;
class IComponent;
// Helper functions for CmpPtr
IComponent* QueryInterface(const CSimContext& context, entity_id_t ent, int iid);
IComponent* QueryInterface(const CSimulation2& simulation, entity_id_t ent, int iid);
/**
* A simplified syntax for accessing entity components.
* E.g. to get the @c Position component, write:
*
* @code
* CmpPtr<ICmpPosition> cmpPosition(context, ent);
* if (!cmpPosition)
* // do something (maybe just silently abort; you should never crash if the
* // component is missing, even if you're sure it should never be missing)
* @endcode
*
* where @c context is (if you're writing component code) a CSimContext object, or
* (if you're writing external engine code that makes use of the simulation system)
* a CSimulation2 object; and @c ent is the entity ID.
*
* @c ent can be CComponentManager::SYSTEM_ENTITY (if you're writing a component), or
* CSimulation2::SYSTEM_ENTITY (for external code), if you want to access the global
* singleton system components.
*
* You should never hold onto a component pointer outside of the method in which you acquire
* it, because it might get deleted and invalidate your pointer. (Components will never be
* deleted while inside a simulation method.)
*/
template<typename T>
class CmpPtr
{
private:
T* m;
// "Safe Bool Idiom" based on http://www.artima.com/cppsource/safebool.html
// to allow "if (cmp)" and "if (!cmp)" etc, without also allowing "int i = cmp"
// or "if (cmp1 == cmp2)" etc.
typedef void (CmpPtr::*bool_type)() const;
void this_type_does_not_support_comparisons() const {}
public:
CmpPtr(const CSimContext& context, entity_id_t ent)
{
m = static_cast<T*>(QueryInterface(context, ent, T::GetInterfaceId()));
}
CmpPtr(const CSimulation2& simulation, entity_id_t ent)
{
m = static_cast<T*>(QueryInterface(simulation, ent, T::GetInterfaceId()));
}
CmpPtr(CEntityHandle ent)
{
SEntityComponentCache* cache = ent.GetComponentCache();
if (cache != NULL && T::GetInterfaceId() < (int)cache->numInterfaces)
m = static_cast<T*>(cache->interfaces[T::GetInterfaceId()]);
else
m = NULL;
}
T* operator->() { return m; }
operator bool_type() const
{
return (m != NULL) ? &CmpPtr::this_type_does_not_support_comparisons : 0;
}
};
template<typename T, typename U>
bool operator!=(const CmpPtr<T>& lhs, const U& UNUSED(rhs))
{
lhs.this_type_does_not_support_comparisons();
return false;
}
template<typename T, typename U>
bool operator==(const CmpPtr<T>& lhs, const U& UNUSED(rhs))
{
lhs.this_type_does_not_support_comparisons();
return false;
}
#endif // INCLUDED_CMPPTR
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