为什么 boost/etc 在模板成员函数的参数类型上做这么多 SFINAE？

Question

当使用enable_if 等测试的参数不满足各种标准时，有选择地从类接口中删除模板成员函数的基本原理是什么？如果保留成员函数模板，尝试使用它们会失败，在我看来，在更复杂的情况下，编译器错误比“替换失败”更有用？

如果编译失败，支持这些极其严格的基于 SFINAE 的模板成员函数要求的论据是什么？

Answer 1

已发现模板代码中 2-5 深的编译器错误几乎是不可理解的。你会得到一堆模板噪音。

SFINAE 替换失败通常会列出一个模板并说它不起作用，因为无法推断出某些参数，通常会显示失败的特征。不完美，但比模板喷射更好。

更重要的是，这样的模板可以阻止其他有效的模板。

此外，如果您使用类似 SFINAE 的技术，您可以测试给定方法是否存在并且在编译时是否有效；如果正文无法编译，则无法编译。

Answer 2

您觉得哪个错误消息更容易理解。这个是 SFINAE 的吗？

template <class C, class = decltype(begin(std::declval<C&>())[0])>
void sort_sfinae(C& c) {
    std::sort(c.begin(), c.end());
}

main.cpp: In function 'int main()':
main.cpp:11:18: error: no matching function for call to 'sort_sfinae(std::__cxx11::list<int>&)'
     sort_sfinae(l);
                  ^
main.cpp:5:6: note: candidate: template<class C, class> void sort_sfinae(C&)
 void sort_sfinae(C& c) {
      ^
main.cpp:5:6: note:   template argument deduction/substitution failed:
main.cpp:4:62: error: no match for 'operator[]' (operand types are 'std::__cxx11::list<int>::iterator {aka std::_List_iterator<int>}' and 'int')
 template <class C, class = decltype(begin(std::declval<C&>())[0])>
                                                              ^

---

或者这个没有？

template <class C>
void sort_no_sfinae(C& c) {
    std::sort(c.begin(), c.end());
}

In file included from /usr/local/include/c++/5.3.0/algorithm:62:0,
                 from main.cpp:2:
/usr/local/include/c++/5.3.0/bits/stl_algo.h: In instantiation of 'void std::__sort(_RandomAccessIterator, _RandomAccessIterator, _Compare) [with _RandomAccessIterator = std::_List_iterator<int>; _Compare = __gnu_cxx::__ops::_Iter_less_iter]':
/usr/local/include/c++/5.3.0/bits/stl_algo.h:4698:18:   required from 'void std::sort(_RAIter, _RAIter) [with _RAIter = std::_List_iterator<int>]'
main.cpp:6:14:   required from 'void sort_no_sfinae(C&) [with C = std::__cxx11::list<int>]'
main.cpp:11:21:   required from here
/usr/local/include/c++/5.3.0/bits/stl_algo.h:1964:22: error: no match for 'operator-' (operand types are 'std::_List_iterator<int>' and 'std::_List_iterator<int>')
     std::__lg(__last - __first) * 2,
                      ^
In file included from /usr/local/include/c++/5.3.0/bits/stl_algobase.h:67:0,
                 from /usr/local/include/c++/5.3.0/list:60,
                 from main.cpp:1:
/usr/local/include/c++/5.3.0/bits/stl_iterator.h:328:5: note: candidate: template<class _Iterator> typename std::reverse_iterator<_Iterator>::difference_type std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_Iterator>&)
     operator-(const reverse_iterator<_Iterator>& __x,
     ^
/usr/local/include/c++/5.3.0/bits/stl_iterator.h:328:5: note:   template argument deduction/substitution failed:
In file included from /usr/local/include/c++/5.3.0/algorithm:62:0,
                 from main.cpp:2:
/usr/local/include/c++/5.3.0/bits/stl_algo.h:1964:22: note:   'std::_List_iterator<int>' is not derived from 'const std::reverse_iterator<_Iterator>'
     std::__lg(__last - __first) * 2,
                      ^
In file included from /usr/local/include/c++/5.3.0/bits/stl_algobase.h:67:0,
                 from /usr/local/include/c++/5.3.0/list:60,
                 from main.cpp:1:
/usr/local/include/c++/5.3.0/bits/stl_iterator.h:380:5: note: candidate: template<class _IteratorL, class _IteratorR> decltype ((__y.base() - __x.base())) std::operator-(const std::reverse_iterator<_Iterator>&, const std::reverse_iterator<_IteratorR>&)
     operator-(const reverse_iterator<_IteratorL>& __x,
     ^
/usr/local/include/c++/5.3.0/bits/stl_iterator.h:380:5: note:   template argument deduction/substitution failed:
In file included from /usr/local/include/c++/5.3.0/algorithm:62:0,
                 from main.cpp:2:
/usr/local/include/c++/5.3.0/bits/stl_algo.h:1964:22: note:   'std::_List_iterator<int>' is not derived from 'const std::reverse_iterator<_Iterator>'
     std::__lg(__last - __first) * 2,
                      ^
In file included from /usr/local/include/c++/5.3.0/bits/stl_algobase.h:67:0,
                 from /usr/local/include/c++/5.3.0/list:60,
                 from main.cpp:1:
/usr/local/include/c++/5.3.0/bits/stl_iterator.h:1138:5: note: candidate: template<class _IteratorL, class _IteratorR> decltype ((__x.base() - __y.base())) std::operator-(const std::move_iterator<_Iterator>&, const std::move_iterator<_IteratorR>&)
     operator-(const move_iterator<_IteratorL>& __x,
     ^
/usr/local/include/c++/5.3.0/bits/stl_iterator.h:1138:5: note:   template argument deduction/substitution failed:
In file included from /usr/local/include/c++/5.3.0/algorithm:62:0,
                 from main.cpp:2:
/usr/local/include/c++/5.3.0/bits/stl_algo.h:1964:22: note:   'std::_List_iterator<int>' is not derived from 'const std::move_iterator<_Iterator>'
     std::__lg(__last - __first) * 2,
                      ^
In file included from /usr/local/include/c++/5.3.0/bits/stl_algobase.h:67:0,
                 from /usr/local/include/c++/5.3.0/list:60,
                 from main.cpp:1:
/usr/local/include/c++/5.3.0/bits/stl_iterator.h:1145:5: note: candidate: template<class _Iterator> decltype ((__x.base() - __y.base())) std::operator-(const std::move_iterator<_Iterator>&, const std::move_iterator<_Iterator>&)
     operator-(const move_iterator<_Iterator>& __x,
     ^
/usr/local/include/c++/5.3.0/bits/stl_iterator.h:1145:5: note:   template argument deduction/substitution failed:
In file included from /usr/local/include/c++/5.3.0/algorithm:62:0,
                 from main.cpp:2:
/usr/local/include/c++/5.3.0/bits/stl_algo.h:1964:22: note:   'std::_List_iterator<int>' is not derived from 'const std::move_iterator<_Iterator>'
     std::__lg(__last - __first) * 2,
                      ^
In file included from /usr/local/include/c++/5.3.0/vector:65:0,
                 from /usr/local/include/c++/5.3.0/bits/random.h:34,
                 from /usr/local/include/c++/5.3.0/random:49,
                 from /usr/local/include/c++/5.3.0/bits/stl_algo.h:66,
                 from /usr/local/include/c++/5.3.0/algorithm:62,
                 from main.cpp:2:
/usr/local/include/c++/5.3.0/bits/stl_bvector.h:208:3: note: candidate: std::ptrdiff_t std::operator-(const std::_Bit_iterator_base&, const std::_Bit_iterator_base&)
   operator-(const _Bit_iterator_base& __x, const _Bit_iterator_base& __y)
   ^
/usr/local/include/c++/5.3.0/bits/stl_bvector.h:208:3: note:   no known conversion for argument 1 from 'std::_List_iterator<int>' to 'const std::_Bit_iterator_base&'