[英]How can I use std::reference_wrapper as a class member?
I use a library that only returns references to created objects Entity& Create(int id)
.我使用一个只返回对创建的对象
Entity& Create(int id)
的引用的库。 In my class, I need to create one of these and store it.在我的 class 中,我需要创建其中一个并存储它。
I had thought to use class member std::reference_wrapper<Entity> MyClass::m_Entity
but the problem is, I would like to create this object in a call to a method like MyClass::InitEntity()
– so I run into a compile error "no default constructor available" because m_Entity
is not initialised in my constructor.我曾想过使用 class 成员
std::reference_wrapper<Entity> MyClass::m_Entity
但问题是,我想在调用像MyClass::InitEntity()
这样的方法时创建这个 object - 所以我遇到了编译错误“没有可用的默认构造函数”,因为m_Entity
未在我的构造函数中初始化。
Is there any way around this, other than to change my class design?除了改变我的 class 设计之外,有什么办法可以解决这个问题吗? Or is this a case where using pointers would make more sense?
或者这是使用指针更有意义的情况?
Is MyClass
in a valid state if it doesn't have a valid reference to an Entity
?如果
MyClass
没有对Entity
的有效引用,它是否在有效的 state 中? If it is, then you should use a pointer.如果是,那么您应该使用指针。 The constructor initializes the pointer to
nullptr
, and the InitEntity
function assigns it to the address of a valid Entity
object.构造函数初始化指向
nullptr
的指针, InitEntity
function 将其分配给有效Entity
object 的地址。
class MyClass
{
public:
MyClass(): _entity(nullptr) {}
void InitEntity() { _entity = &Create(123); }
void doSomethingWithEntity()
{
if (_entity) ...
}
private:
Entity *_entity;
};
If MyClass
isn't in a valid state without a valid reference to an Entity
, then you can use a std::reference_wrapper<Entity>
and initialize it in the constructor.如果
MyClass
不在有效的 state 中而没有对Entity
的有效引用,那么您可以使用std::reference_wrapper<Entity>
并在构造函数中对其进行初始化。
class MyClass
{
public:
MyClass(): _entity(Create(123)) {}
void doSomethingWithEntity()
{
...
}
private:
std::reference_wrapper<Entity> _entity;
};
Of course which one you go with depends on how MyClass
is supposed to be used.当然,您使用哪一个 go 取决于
MyClass
应该如何使用。 Personally, the interface for std::reference_wrapper
is a little awkward for me, so I'd use a pointer in the second case as well (while still ensuring that it's always not null).就个人而言,
std::reference_wrapper
的接口对我来说有点尴尬,所以我也会在第二种情况下使用指针(同时仍然确保它始终不为空)。
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