如何引用参数参数的嵌套模板中的类型？
假设我有以下定义
template <class T>
class Sequence
{
}
例如 Sequence 可能是一个字符串数组，类似于 vector
// Now define iterator template
template <class T>
class SequenceIterator
{
}
这里的想法当然是能够在某个序列上创建迭代器 例如，
SequenceIterator< Sequence<string> > iter1;
SequenceIterator< Sequence<int> > iter2;
我现在的问题是如何定义 SequenceIterator 中存在的成员函数，其目的是返回序列中的下一个值。通常，我希望将其写为然而
bool Next(T1 & value); // If the iterator has not finished, set value to the next item
，SequenceIterator 已以模板化名称传递，即 Sequence 或 Sequence
所以问题是，我如何一般地引用该基础类型（string 或 int），以便我可以定义 Next 成员函数。
谢谢， 大卫
Suppose I have the following definitions
template <class T>
class Sequence
{
}
E.gSequence<string> might be an array of strings, similar to vector<string>
// Now define iterator template
template <class T>
class SequenceIterator
{
}
The idea here of course is to be able to create an iterator over some sequence
E.g.
SequenceIterator< Sequence<string> > iter1;
SequenceIterator< Sequence<int> > iter2;
The question I now have is how to define the member function that would exist inside SequenceIterator and whose purpose is to return the next value in the sequence. Typically I would expect to write that as
bool Next(T1 & value); // If the iterator has not finished, set value to the next item
However, the SequenceIterator has been passed in a templated name already, i.e, Sequence<string> or Sequence<int>
So the question is, how do I generically refer to that underlying type (string or int) so that I can define the Next member function.
Thanks,
David
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有三种方法：更改
Sequence的定义以包含，或者更改
SequenceIterator的模板参数以明确识别Sequence本身是模板虽然
SequenceIterator的实例化没有改变，但您现在可以直接访问T。第三，您可以使用容器特征类来处理为您类型扣除。第三个选项提供了Sequence和SequenceIterator之间的最小耦合，但是，就像 马克说，标准容器倾向于使用第一种方法。There are three ways: change the definition of
Sequenceto includeor, change the template parameters for
SequenceIteratorto explicitly recognize thatSequenceis a template itselfand while the instantiation of
SequenceIteratordoes not change, you can now accessTdirectly. Thirdly, you can use a container traits class that handles the type deduction for you. The third option provides the least coupling betweenSequenceandSequenceIterator, but, like Mark said, the standard containers tend to use the first method.标准库通过在每个容器中包含 typedef 来解决这个问题。在这种情况下，
Sequence将具有typedef T value_type;，因此您可以使用Sequence::value_type 来引用那种类型。另外，我会高度考虑像标准库一样使用
operator++和operator*，这样就不会让人对非标准迭代器接口感到困惑。The standard library solves this by having typedefs within every container. In this case
Sequence<T>would have atypedef T value_type;so you can then useSequence<T>::value_typeto refer to that type.Also, I would highly consider using
operator++andoperator*like the standard library so you don't confuse people with a non-standard-like iterator interface.