我来自Java背景。 (Hash-)地图和集合是我在工作过程中每天处理的内容。谈到C ++(或一般的手动管理内存),我想知道如何正确使用它们。
根据定义,在将对象添加到集合/映射时,只有在它/标识符不存在的情况下才会添加它。所以调用Add()
/ Put()
的函数在添加之后不得销毁该对象(或者一个破坏的指针将保留在集合中),如果没有添加它,它必须销毁它,因为相同的对象是在集合中找到(或者存在内存泄漏),如果在集合中发现了一个恰好是试图添加的对象的相等对象,它就不能销毁它。创建深层副本实际上是不可能的(或者我不清楚如何),因为在我的情况下,数据对象可能以递归方式相互依赖。
当从集合中获取子集(假设包含foos和bar的集合中的所有条形图),并且原始集合被释放时,所有foos必须被释放,而子集中包含的条带不能被释放。如果集合被释放,它们必须被释放,但前提是它们不再用于任何其他集合。如果从原始集合中导出了一组长条,并且这组条形图不知道,甚至可能在不同的线程中,该怎么办?
为了说明问题,这里是Java中用于管理内存中链接数据的小记录的最小示例。下面的main()
方法显示了一些用例可能性。
interface IObject { }
interface INode extends IObject { }
interface IIdentifiable extends IObject {
String m_csSubject();
}
class CLiteral implements IObject {
private String m_csValue;
CLiteral(String csValue) { m_csValue = csValue; }
String m_csValue() { return m_csValue; }
public int hashCode() {
return 31 * 1 + ((m_csValue == null) ? 0 : m_csValue.hashCode());
}
public boolean equals(Object cObject) {
if (this == cObject) return true;
if (cObject == null) return false;
if (getClass() != cObject.getClass()) return false;
CLiteral cOther = (CLiteral) cObject;
if (m_csValue == null) {
if (cOther.m_csValue != null) return false;
} else if (!m_csValue.equals(cOther.m_csValue)) return false;
return true;
}
}
class CLangString extends CLiteral {
private final String m_csLanguage;
CLangString(String csValue, String csLanguage) {
super(csValue);
m_csLanguage = csLanguage;
}
public int hashCode() {
return 31 * super.hashCode() + ((m_csLanguage == null) ? 0 : m_csLanguage.hashCode());
}
public boolean equals(Object cObject) {
if (this == cObject) return true;
if (!super.equals(cObject)) return false;
if (getClass() != cObject.getClass()) return false;
CLangString cOther = (CLangString) cObject;
if (m_csLanguage == null) {
if (cOther.m_csLanguage != null) return false;
} else if (!m_csLanguage.equals(cOther.m_csLanguage)) return false;
return true;
}
}
class CLink implements IIdentifiable, INode {
private String m_csSubject;
CLink(String csSubject) { m_csSubject = csSubject; }
public String m_csSubject() { return m_csSubject; }
public int hashCode() {
return 31 * 1 + ((m_csSubject == null) ? 0 : m_csSubject.hashCode());
}
public boolean equals(Object cObject) {
if (this == cObject) return true;
if (cObject == null) return false;
if (getClass() != cObject.getClass()) return false;
CLink cOther = (CLink) cObject;
if (m_csSubject == null) {
if (cOther.m_csSubject != null) return false;
} else if (!m_csSubject.equals(cOther.m_csSubject)) return false;
return true;
}
}
import java.util.*;
import java.util.Map.Entry;
class CNode implements INode {
static final String m_csTYPE = "http://www.w3.org/1999/02/22-rdf-syntax-ns#type";
final Map<String, Collection<IObject>> m_cEdges = new HashMap<>();
CNode() { }
CNode(CLink cType) { put(m_csTYPE, cType); }
CNode(String csType) { this(new CLink(csType)); }
CResult get(String csPredicate) {
CResult cResult = new CResult();
cResult.addAll(m_cEdges.get(csPredicate));
return cResult;
}
CResult getById(String csPredicate, String csIdPredicate, String csIdValue) {
CResult cResult = new CResult();
for (Entry<String, Collection<IObject>> cEntry : m_cEdges.entrySet())
for (IObject cValue : cEntry.getValue())
if (cValue instanceof CNode) {
CNode cNode = (CNode) cValue;
if (cNode.hasType(csPredicate) && cNode.get(csIdPredicate).Strings().contains(csIdValue))
cResult.add(cNode);
}
return cResult;
}
boolean hasType(String csType) {
for (IIdentifiable cIdentifiable : get(m_csTYPE).iIdentifiables())
if (csType.equals(cIdentifiable.m_csSubject())) return true;
return false;
}
<T extends CNode> T put(String csPredicate, IObject cObject) {
Collection<IObject> cObjects = m_cEdges.get(csPredicate);
if (cObjects == null) m_cEdges.put(csPredicate, cObjects = new HashSet<>());
cObjects.add(cObject);
return (T) this;
}
}
class CNamedNode extends CNode implements IIdentifiable {
private String m_csSubject;
CNamedNode(String csSubject, CLink cType) {
super(cType);
m_csSubject = csSubject;
}
public String m_csSubject() { return m_csSubject; }
public int hashCode() {
return 31 * 1 + ((m_csSubject == null) ? 0 : m_csSubject.hashCode());
}
public boolean equals(Object cObject) {
if (this == cObject) return true;
if (cObject == null) return false;
if (getClass() != cObject.getClass()) return false;
CNamedNode cOther = (CNamedNode) cObject;
if (m_csSubject == null) {
if (cOther.m_csSubject != null) return false;
} else if (!m_csSubject.equals(cOther.m_csSubject)) return false;
return true;
}
}
import java.util.*;
class CResult extends HashSet<IObject> {
Set<CNamedNode> CNamedNodes() {
Set<CNamedNode> cResult = new HashSet<>();
for (IObject cMember : this)
if (cMember instanceof CNamedNode) cResult.add((CNamedNode) cMember);
return cResult;
}
Set<CNode> CNodes() {
Set<CNode> cResult = new HashSet<>();
for (IObject cMember : this)
if (cMember instanceof CNode) cResult.add((CNode) cMember);
return cResult;
}
Set<IIdentifiable> iIdentifiables() {
Set<IIdentifiable> cResult = new HashSet<>();
for (IObject cMember : this)
if (cMember instanceof IIdentifiable) cResult.add((IIdentifiable) cMember);
return cResult;
}
boolean IsAnyIdentifiable() {
boolean bResult = false;
for (IObject cMember : this) if (cMember instanceof IIdentifiable) return true;
return bResult;
}
String String() {
StringBuilder cResult = new StringBuilder();
boolean bFirst = true;
for (String csString : Strings()) {
if(bFirst) bFirst = false; else cResult.append(", ");
cResult.append(csString);
}
return cResult.toString();
}
Set<String> Strings() {
Set<String> cResult = new HashSet<>();
for (IObject cMember : this)
if (cMember instanceof CLiteral) cResult.add(((CLiteral) cMember).m_csValue());
return cResult;
}
}
public static void main(String[] args) {
final String m_csNS = "http://animalShelter.example/";
CResult cDogs;
CResult cFidos;
{
CNode cShelter = new CNode(m_csNS + "AnimalShelter");
cShelter.put(
m_csNS + "hasDog",
new CNode(new CLink(m_csNS + "Dog"))
.put(m_csNS + "hasName", new CLangString("Fido", "en"))
.put(m_csNS + "residence", cShelter)
);
cShelter.put(
m_csNS + "hasDog",
new CNamedNode(
"http://en.wikipedia.org/wiki/Scooby-Doo_(character)", new CLink(m_csNS + "Dog")
)
.put(m_csNS + "hasName", new CLangString("Scoobert \"Scooby\" Doo", "en"))
.put(m_csNS + "residence", cShelter)
);
cDogs = cShelter.get(m_csNS + "hasDog");
cFidos = cShelter.getById(m_csNS + "Dog", m_csNS + "hasName", "Fido");
}
if(!cDogs.isEmpty()){
System.out.println("No. of dogs: " + cDogs.size());
}
if (cDogs.IsAnyIdentifiable()) {
System.out.println("Famous dogs:");
for (CNamedNode cWellKnownDog : cDogs.CNamedNodes()) {
System.out.println("- " + cWellKnownDog.get(m_csNS + "hasName").String() + " ("
+ cWellKnownDog.m_csSubject() + ')');
}
}
if(!cFidos.isEmpty()){
System.out.println(cFidos.CNodes().size() + " dog(s) called Fido.");
}
}
我如何对C ++这样做?如何在我的数据模型类中实现对象销毁,使用它的函数(在这种情况下为main()
)没有(或者最不可能)关心模型的内部?
答案 0 :(得分:1)
插入C ++哈希映射/集(std::unordered_map
和std::unordered_set
)是通过复制完成的。那应该是你的默认选择。不要像编写Java那样尝试编写C ++。
如果你想避免深度复制,你的map / set可以包含对象的原始指针,但是对象的生命周期是单独维护的(例如在std::vector
中)。
但是,可能有理由想要更多类似Java的语义。或许,例如,您希望map / set拥有多态类型。在这种情况下,您可能需要智能指针。最好拥有唯一的所有权(每个对象只有一个智能指针),在这种情况下,您可以使用std::unique_ptr
,或者如果您必须拥有共享所有权,请使用std::shared_ptr
。两种智能指针都可以存储在std::unordered_map
或std::unordered_set
中,并且应该大致具有您期望的语义。