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