我在我编写的模板稀疏矩阵类YaleStorage<D>上有一个STL样式的迭代器。我为矩阵类编写了operator==，它允许比较两个矩阵D可能不同。
我的迭代器也是模板 - 因此我可以将const_iterator和iterator作为iterator_T<RefType>的特化，其中RefType可以是const D&或{ {1}}。
即使D&不同，我也需要这些迭代器具有可比性。我并不特别需要constness来区分（D不需要与const_iterator进行比较）。以下是我编写iterator等内容的方法：
operator!=请注意，我编写了自己的template <typename D>
class YaleStorage {
template <typename RefType>
class iterator_T {
template <typename E, typename ERefType = typename enable_if_else<std::is_const<RefType>::value, const E&, E&>::type>
bool operator!=(const typename YaleStorage<E>::template stored_diagonal_iterator_T<ERefType>& rhs) const { /* some comparison code */ }
};
typedef iterator_T<D&> iterator;
typedef iterator_T<const D&> const_iterator;
const_iterator cbegin() const { /* returns a const_iterator */ }
const_iterator cend() const { /* returns the end const_iterator */ }
};
，因为我在STL中找不到类似的功能。这是：
enable_if_else不幸的是，当我尝试进行常规迭代器比较（template <bool B, typename TA, typename TB>
struct enable_if_else {};
template <typename TA, typename TB>
struct enable_if_else<true, TA, TB> { typedef TA type; };
template <typename TA, typename TB>
struct enable_if_else<false, TA, TB> { typedef TB type; };
）时，我遇到了大量错误，例如：
D=E我已经尝试明确地投射compiling ../../../../ext/nmatrix/storage/yale.cpp
../../../../ext/nmatrix/storage/yale.cpp: In instantiation of ‘VALUE nm::yale_storage::each_stored_with_indices(VALUE) [with DType = unsigned char; VALUE = long unsigned int]’:
../../../../ext/nmatrix/storage/yale.cpp:1686:3: required from here
../../../../ext/nmatrix/storage/yale.cpp:1567:83: error: no match for ‘operator!=’ in ‘d != nm::YaleStorage<D>::cend() const [with D = unsigned char; nm::YaleStorage<D>::const_iterator = nm::YaleStorage<unsigned char>::iterator_T<const unsigned char&>]()’
../../../../ext/nmatrix/storage/yale.cpp:1567:83: note: candidates are:
In file included from ../../../../ext/nmatrix/storage/yale.cpp:64:0:
../../../../ext/nmatrix/storage/yale.h:438:10: note: template<class E, class ERefType> bool nm::YaleStorage<D>::iterator_T::operator!=(const typename nm::YaleStorage<E>::iterator_T<ERefType>&) const [with E = E; ERefType = ERefType; RefType = const unsigned char&; D = unsigned char]
../../../../ext/nmatrix/storage/yale.h:438:10: note: template argument deduction/substitution failed:
../../../../ext/nmatrix/storage/yale.cpp:1567:83: note: couldn't deduce template parameter ‘E’
的结果，所以它知道比较的两边都有什么，但我不明白为什么它会扼杀这个。
cend()我知道如何为非template <typename DType>
static VALUE each_with_indices(VALUE nm) {
YaleStorage<DType> y(s);
// The next line is 1567:
for (typename YaleStorage<DType>::const_iterator d = y.cbegin(); d != y.cend()); ++d) {
/* Do some stuff with d */
}
}
函数明确指定模板参数，但我该如何为运算符执行此操作？还有另一个更好的方法吗？在我看来，编译器应该明白operator在这里是什么。