C++ 如何在给定其 shared_ptr 的情况下正确复制对象
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How to properly duplicate an object given its shared_ptr
提问by Marc
I'm trying to make a duplicate of an object of a custom class Event. I have a shared pointer to the object that I've obtained from its allocation:
我正在尝试复制自定义类的对象Event。我有一个指向从分配中获得的对象的共享指针：
std::shared_ptr<Event> e = std::make_shared<Event>();
In order to get a true duplicate of e(not just a copy of the pointer) I've tried:
为了获得真正的副本e（不仅仅是指针的副本），我尝试过：
std::shared_ptr<Event> o = std::make_shared<Event>(*e);
But I'm not sure if this is the correct way as it seems that if I delete eit also deletes o...
但我不确定这是否是正确的方法，因为似乎如果我删除e它也会删除o......
Btw, I haven't defined a copy constructor Event::Event(const Event &orig)but in my understanding this is not necessary as the compiler provides a default copy constructor. The event class only contains variables and no further pointers.
顺便说一句，我还没有定义复制构造函数，Event::Event(const Event &orig)但据我所知，这不是必需的，因为编译器提供了默认的复制构造函数。事件类只包含变量，没有进一步的指针。
采纳答案by B?ови?
std::make_sharedis just a simple template function that creates the objects, passing all arguments to the constructor :
std::make_shared只是一个创建对象的简单模板函数，将所有参数传递给构造函数：
template<class T, class... Args>
shared_ptr<T> make_shared(Args&&... args)
{
return shared_ptr<T>( new T( std::forward<Args>( args )... ) );
}
In your particular case :
在您的特定情况下：
std::shared_ptr<Event> o = std::make_shared<Event>(*e);
the object is copied.
对象被复制。
If your code is such :
如果您的代码是这样的：
void foo() {
// create new object using default constructor
std::shared_ptr<Event> e = std::make_shared<Event>();
// create new object using copy constructor constructor
std::shared_ptr<Event> o = std::make_shared<Event>(*e);
}
then of course both objects are destroyed, when they go out of scope.
当然，当它们超出范围时，两个对象都会被销毁。
回答by Jonathan Wakely
What you tried should work correctly, ifthe dynamic type of *eis Event, and not some class derived from Event. (If *eis actually an object derived from Eventthen you will create a new Event(not the derived type) as a copy of the base class part of *ei.e. you will "slice" *e).
你试过应该可以正常工作，如果动态类型*e的Event，而不是一些类衍生Event。（如果*e实际上是一个派生的对象，Event那么您将创建一个新的Event（不是派生类型）作为*eie的基类部分的副本，您将“切片” *e）。
Since you create eusing make_shared<Event>()you know that in this case it really is an Event, so std::make_shared<Event>(*e)should make a new shared_ptrthat owns a copy of *e.
由于您e使用创建，make_shared<Event>()您知道在这种情况下它确实是一个Event，因此std::make_shared<Event>(*e)应该创建一个shared_ptr拥有*e.
