In 本次问答 https://stackoverflow.com/a/42221591/819272我编写了一个小包装类,它提供对范围的反向迭代器访问,依赖于类模板的 c++1z 语言功能模板参数推导(p0091r3 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0091r3.html, p0512r0 http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0512r0.pdf)
#include <iostream>
#include <iterator>
#include <vector>
template<class Rng>
class Reverse
{
Rng const& rng;
public:
Reverse(Rng const& r) noexcept
:
rng(r)
{}
auto begin() const noexcept { using std::end; return std::make_reverse_iterator(end(rng)); }
auto end() const noexcept { using std::begin; return std::make_reverse_iterator(begin(rng)); }
};
int main()
{
std::vector<int> my_stack;
my_stack.push_back(1);
my_stack.push_back(2);
my_stack.puhs_back(3);
// prints 3,2,1
for (auto const& elem : Reverse(my_stack)) {
std::cout << elem << ',';
}
}
然而,做一个嵌套应用程序Reverse
不产生原始迭代顺序
// still prints 3,2,1 instead of 1,2,3
for (auto const& elem : Reverse(Reverse(my_stack))) {
std::cout << elem << ',';
}
实例 http://melpon.org/wandbox/permlink/xk0yEumtsG1hp6VT(g++ 7.0 SVN 和 clang 5.0 SVN 的输出相同)
罪魁祸首似乎是类模板的模板参数推导,因为通常的包装函数确实允许正确的嵌套
template<class Rng>
auto MakeReverse(Rng const& rng) { return Reverse<Rng>(rng); }
// prints 1,2,3
for (auto const& elem : MakeReverse(MakeReverse(my_stack))) {
std::cout << elem << ',';
}
实例 http://melpon.org/wandbox/permlink/uS9dUBYhWH9Ql4fK(g++ 和 clang 的输出相同)
Question:类模板的嵌套模板参数推导是否应该仅在“一层”深度工作,或者这是 g++ 和 clang 当前实现中的错误?