为什么boost :: enable_shared_from_raw没有记录？

Question

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？

Answer 1

问题enable_shared_from_raw是它很危险; 你可能会无意中最终泄漏物体.

如果在shared_from_raw不授予对实例的原始指针的所有权的情况下调用shared_ptr,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.

只要它具有对自身的强引用,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a的所有好处shared_ptr.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;

我认为你想要shared_from_this()在构造函数中调用的最常见的情况是你想要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 设置额外的限制以防止此类泄漏.