boost smart_ptr库包含两种类型,它们允许类为自己提供shared_ptrs,enable_shared_from_this(也可用于stl的智能指针)和enable_shared_from_raw.对我来说,后者似乎更优越,因为它允许在构造函数中创建共享指针.但是,文档中根本没有提到这个类,不是root boost / headers的一部分,而且谷歌搜索它主要在实际的.hpp文件中提供命中.
boost :: enable_shared_from_raw是否已被弃用或以其他方式不适合使用？我错过了什么吗？
解决方法
enable_shared_from_raw的问题在于它很危险;你可能会无意中最终泄漏物体.
如果在不授予对shared_ptr实例的原始指针的所有权的情况下调用shared_from_raw,则enable_shared_from_raw基类将保留对自身的强引用,直到您这样做为止.
只要它拥有对自身的强引用,在手动删除对象之前,引用计数将无法达到0,这完全消除了使用shared_ptr的所有好处.一旦某些东西取得了原始指针的所有权,它就会降低它对弱引用的强引用,一切都很好.
class object : boost::enable_shared_from_raw { }
// Leak! When shared_from_raw is called,raw takes ownership of itself.
object* raw = new object;
boost::shared_ptr<object> bar = boost::shared_from_raw(raw);
// This is fine; bar already owns the object when shared_from_raw is invoked.
object* raw = new object;
boost::shared_ptr<object> bar(raw);
boost::shared_ptr<object> foo = boost::shared_from_raw(raw);
// Also fine; ownership is transferred to bar.
object* raw = new object;
boost::shared_ptr<object> foo = boost::shared_from_raw(raw);
boost::shared_ptr<object> bar(raw);
// Still a leak! Nothing external has taken ownership of the raw pointer.
object* raw = new object;
boost::shared_ptr<object> bar = boost::shared_from_raw(raw);
boost::shared_ptr<object> foo = bar;
我认为你想在构造函数中调用shared_from_this()的最常见情况是你想用某种管理器注册对象;这很可能导致这里描述的泄漏.
// This use case results in a leak.
// Nothing external takes ownership of the raw pointer.
struct Object;
struct ObjectManager
{
void RegisterObject(boost::shared_ptr<Object> obj)
{
m_objects.push_back(obj);
}
std::list<boost::shared_ptr<Object> > m_objects;
};
static ObjectManager gObjectManager;
struct Object : boost::enable_shared_from_raw
{
Object()
{
gObjectManager.RegisterObject(boost::shared_from_raw(this));
}
}
enable_shared_from_this设置了额外的限制来防止这种类型的泄漏.
