[英]How to use forward declarations for a template class in a shared pointer (in C++)
I have a base class Base
defined in Base.h : 我在Base.h中定义了一个基类
Base
:
class Base
{ /* ... */ };
And a class template Child
that derives from Base
, defined in Child.h : 还有一个从
Base
派生的类模板Child
,它在Child.h中定义:
#include "Base.h"
template <class T>
class Child : public Base
{ /* ... */ };
Now I want to create some factory methods within the Base
class, which should return a std::shared_ptr
to the Child
class. 现在,我想在
Base
类中创建一些工厂方法,这些方法应将std::shared_ptr
返回到Child
类。 To avoid circular dependencies I tried to use a forward declaration instead. 为了避免循环依赖,我尝试使用前向声明。 So Base.h now looks like this:
因此, Base.h现在看起来像这样:
class Child; // new forward declaration
class Base
{
/* ... */
// new factory method
static std::shared_ptr<Base> CreateChildInt(int i = 0)
{
return std::make_shared<Child<int>>(i);
}
};
However, the definition of CreateChildInt()
leads to the following compiler error: 但是,
CreateChildInt()
的定义导致以下编译器错误:
"error C2947: expecting '>' to terminate template-argument-list, found '<'" “错误C2947:期望'>'终止模板参数列表,找到'<'”
So is this even possible what I am trying to achieve? 那么,这甚至有可能是我想要达到的目标吗?
If not, are there any workarounds / best practices for this approach? 如果没有,这种方法是否有任何解决方法/最佳做法?
EDIT : The reason for why I want to put the factory method into the Base
class instead of Child
is the following. 编辑 :为什么我想将工厂方法放入
Base
类而不是Child
的原因如下。 When I put the factory into Child
I would need to call the factory method like this: 当我将工厂放入
Child
我需要像下面这样调用factory方法:
std::shared_ptr<Base> p = Child<int>::CreateChildInt(3);
However, I would like to omit the template type <int>
in this call, thus: 但是,我想在此调用中省略模板类型
<int>
,因此:
std::shared_ptr<Base> p = Base::CreateChildInt(3);
Firstly, you declared a class, but Child
that you define is actually a template. 首先,您声明了一个类,但是您定义的
Child
实际上是一个模板。 The correct way to declare a class template is: 声明类模板的正确方法是:
template <class T>
class Child;
However, a correct forward declaration alone won't help you. 但是,仅靠正确的前向声明并不能帮助您。 The implementation of
CreateChildInt::CreateChildInt
must know the full definition of Child
because it creates an instance of it. CreateChildInt::CreateChildInt
的实现必须知道Child
的完整定义,因为它创建了它的实例。 You can't define Child
before Base
either, because inheritance also depends on the full definition, so you'd end up with a cicrular dependency. 您也不能在
Base
之前定义Child
,因为继承也取决于完整的定义,因此最终会产生关键的依赖性。
Solution: Forward declare Child
, then define Base
but don't define Base::CreateChildInt
inline, then define Child
and finally define Base::CreateChildInt
. 解决方案:转发声明
Child
,然后定义Base
但不要内联定义Base::CreateChildInt
,然后定义Child
,最后定义Base::CreateChildInt
。
PS. PS。 I find it dubious from OOP perspective that the implementation of the base class member function depends on the child class.
从OOP角度看,我怀疑基类成员函数的实现取决于子类。 I recommend that you consider re-designing your approach.
我建议您考虑重新设计方法。
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