Optimize IfcFile::entitiesByReference() by reserving capacity for IfcEntityList in advance.

Around 18.4 % speed-up (avg. of first 2000 calls) when converting a somewhat large (176 MB) file to XML.

src/ifcparse/IfcEntityList.h

@@ -40,6 +40,7 @@ public:
 	it end();
 	IfcUtil::IfcBaseClass* operator[] (int i);
 	unsigned int size() const;
+    void reserve(unsigned capacity);
 	bool contains(IfcUtil::IfcBaseClass*) const;
 	template <class U>
 	typename U::list::ptr as() {

src/ifcparse/IfcParse.cpp

@@ -1793,17 +1793,16 @@ IfcEntityList::ptr IfcFile::entitiesByType(const std::string& t) {
 
 IfcEntityList::ptr IfcFile::entitiesByReference(int t) {
 	entities_by_ref_t::const_iterator it = byref.find(t);
-	IfcEntityList::ptr return_value;
+	IfcEntityList::ptr ret;
 	if (it != byref.end()) {
+        ret.reset(new IfcEntityList);
+        ret->reserve((unsigned)it->second.size());
 		const std::vector<unsigned>& ids = it->second;
 		for (std::vector<unsigned>::const_iterator jt = ids.begin(); jt != ids.end(); ++jt) {
-			if (!return_value) {
-				return_value.reset(new IfcEntityList);
-			}
-			return_value->push(entityById(*jt));
+			ret->push(entityById(*jt));
 		}
 	}
-	return return_value;
+	return ret;
 }
 
 IfcUtil::IfcBaseClass* IfcFile::entityById(int id) {

src/ifcparse/IfcUtil.cpp

@@ -48,6 +48,7 @@ void IfcEntityList::push(const IfcEntityList::ptr& l) {
 	}
 }
 unsigned int IfcEntityList::size() const { return (unsigned int) ls.size(); }
+void IfcEntityList::reserve(unsigned capacity) { ls.reserve((size_t)capacity); }
 IfcEntityList::it IfcEntityList::begin() { return ls.begin(); }
 IfcEntityList::it IfcEntityList::end() { return ls.end(); }
 IfcUtil::IfcBaseClass* IfcEntityList::operator[] (int i) {