类（或结构）通过模板自引用

问题描述

以下是否合法？

template< typename T >
struct tree_node
   {
   T t;
   std::vector<tree_node> children;
   };

对似乎暗示它不是。

作为未定义行为类型的场景。目标语义是明确的。如果它是不完整类型的无效使用，那么它应该是编译时错误。

This doesn't strike me as an "undefined behavior" type of scenario. The intended semantics are unambiguous. If it is an invalid usage of an incomplete type then it should be a compile-time error.

这似乎工作正常（我使用了和 - 都使用 -Wall -Werror -std = c ++ 11 ）。

In my tests this seems to work fine (I have used both GCC and Clang -- both with -Wall -Werror -std=c++11).

在语言定义中（在C ++ 17之前），有直接或间接指定为未定义行为的东西，或者只是 / em>

Is there something in the language definition (prior to C++17) that directly or indirectly specifies this as undefined behavior, or is it just under-specified?

请记住，这在结构上非常相似，如下所示：

Keep in mind that this is very similar, structurally, to something like the following:

typedef int T;
struct tree_node;

struct tree_node
   {
   T t;
   tree_node * children;
   }

推荐答案

我们有（从N4527，[vector.overview] ，将在C ++ 17中）：

Actually, as a result of N4371 we have (from N4527, [vector.overview], will be in C++17):

在此之前，向量无法使用不完整类型构造（ tree_node 那个点），这将是未定义的行为。

Prior to this, vector could not be constructed with an incomplete type (which tree_node is at that point), and that would be undefined behavior.

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