(我知道这不是常规实现,但我想尝试一下这个想法。)
struct TrieNode {
std::unordered_map<char, TrieNode> next;
};
这个类在 visual studio 2017 下编译得很好,可以按预期工作。但是,它不能用 gcc (c++14) 编译(如我所料)。
In file included from /usr/include/c++/8/bits/stl_algobase.h:64, from /usr/include/c++/8/bits/char_traits.h:39, from /usr/include/c++/8/ios:40, from /usr/include/c++/8/ostream:38, from /usr/include/c++/8/iostream:39, from prog.cpp:1: /usr/include/c++/8/bits/stl_pair.h: In instantiation of ‘struct std::pair<const char, TrieNode>’: /usr/include/c++/8/bits/stl_vector.h:1610:27: required from ‘struct __gnu_cxx::__aligned_buffer<std::pair<const char, TrieNode> >’ /usr/include/c++/8/bits/hashtable_policy.h:234:43: required from ‘struct std::__detail::_Hash_node_value_base<std::pair<const char, TrieNode> >’ /usr/include/c++/8/bits/hashtable_policy.h:280:12: required from ‘struct std::__detail::_Hash_node<std::pair<const char, TrieNode>, false>’ /usr/include/c++/8/bits/hashtable_policy.h:2027:49: required from ‘struct std::__detail::_Hashtable_alloc<std::allocator<std::__detail::_Hash_node<std::pair<const char, TrieNode>, false> > >’ /usr/include/c++/8/bits/hashtable.h:173:11: required from ‘class std::_Hashtable<char, std::pair<const char, TrieNode>, std::allocator<std::pair<const char, TrieNode> >, std::__detail::_Select1st, std::equal_to<char>, std::hash<char>, std::__detail::_Mod_range_hashing, std::__detail::_Default_ranged_hash, std::__detail::_Prime_rehash_policy, std::__detail::_Hashtable_traits<false, false, true> >’ /usr/include/c++/8/bits/unordered_map.h:105:18: required from ‘class std::unordered_map<char, TrieNode>’ prog.cpp:9:37: required from here /usr/include/c++/8/bits/stl_pair.h:215:11: error: ‘std::pair<_T1, _T2>::second’ has incomplete type _T2 second; /// @c second is a copy of the second object ^~~~~~ prog.cpp:7:8: note: forward declaration of ‘struct TrieNode’ struct TrieNode { ^~~~~~~~
我想知道 visual c++ 实现怎么没有任何问题?标准对此类情况有何规定?
最佳答案
我无法告诉您为什么它在 MSVS2017 中有效,但根据标准,将不完整类型与 std::unordered_map
一起使用是未定义的行为。类的完整类上下文由 [class.mem]/6 定义作为
A complete-class context of a class is a
function body ([dcl.fct.def.general]),
default argument,
noexcept-specifier ([except.spec]), or
default member initializer
只有在那些地方,类名才表示一个完整的类型。由于我们不在任何这些地方,因此名称命名为不完整的类型。如果我们检查我们是否可以将它与 std::unordered_map
一起使用,我们检查 [res.on.functions]/2我们有
In particular, the effects are undefined in the following cases: [...]
- If an incomplete type ([basic.types]) is used as a template argument when instantiating a template component or evaluating a concept, unless specifically allowed for that component.
所以一般来说这是不允许的。我们需要检查 [container.requirements]和 [unord.map]但那里没有说允许不完整的类型。这意味着我们退回到一般规则,这是不允许的。
关于c++ - 在自己的声明中使用类作为模板参数,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/57910461/