模板对象作为模板类的静态成员
想象一下下面的模板类（省略了成员_t的setter和getter）：
template<class T>
class chain
{
public:
static chain<T> NONE;
chain()
: _next(&NONE)
{}
~chain() {}
chain<T>& getNext() const
{
return *_next;
}
void setNext(chain<T>* next)
{
if(next && next != this)
_next = next;
}
chain<T>& getLast() const
{
if (_next == &NONE)
return *this;
else
return _next->getLast();
}
private:
T _t;
chain<T>* _next;
};
这个概念的基本思想是，我不使用空指针，而是有一个静态默认元素来承担这个角色，而仍然是技术上有效的对象；这可以防止空指针的一些问题，同时使代码更加冗长...
我可以很好地实例化此模板，但链接器在静态上给出了 unresolved-external 错误成员对象NONE。
我假设在实例化模板时，行 static chain；实际上也将是一个定义，因为它实际上发生在实例化模板的实现中。然而，事实证明并非如此......
我的问题是：是否有可能，如果可以，如何在每个模板实例化之前不显式定义 NONE 元素？
Imagine the following template class (setter and getter for the member _t omitted):
template<class T>
class chain
{
public:
static chain<T> NONE;
chain()
: _next(&NONE)
{}
~chain() {}
chain<T>& getNext() const
{
return *_next;
}
void setNext(chain<T>* next)
{
if(next && next != this)
_next = next;
}
chain<T>& getLast() const
{
if (_next == &NONE)
return *this;
else
return _next->getLast();
}
private:
T _t;
chain<T>* _next;
};
The basic idea of this concept is, instead of using null-pointers, I have a static default element that takes in this role while still being a technically valid object; this could prevent some of the issues with null pointers while making the code more verbose at the same time...
I can instantiate this template just fine, but the linker gives an unresolved-external error on the static member object NONE.
I would have assumed that when instantiating the template, the line static chain<T> NONE; would effectively be a definition, too, as it actually happens within the implementation instantiating the template. However, it turns out not to be...
My question is: is something like possible at all, and if so, how, without explicitly defining the NONE element before each and every template instantiation?
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评论（2）
您仍然需要像非模板类一样在类外部定义它。就像在非模板类中一样，您只在类定义中声明了
NONE并且仍然需要定义它：You still need to define that outside the class just like a non-template class. Just like in a non-template class, you have only declared
NONEinside the class definition and still need to define it:应该有效
should work