类（或结构）通过模板自引用 [英] class (or struct) self-reference by template

问题描述

以下是否合法？

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

对这篇文章似乎暗示它不是。

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.

在我的测试这似乎工作正常（我使用了 GCC 和 Clang - 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

N4371 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):

不完整的类型 T 在实例化向量时使用，如果分配器满足分配器的完整性
要求17.6.3.5.1。 T 必须在引用
向量的所有专业化成员之前完成。

An incomplete type T may be used when instantiating vector if the allocator satisfies the allocator completeness requirements 17.6.3.5.1. T shall be complete before any member of the resulting specialization of vector is referenced.

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.

这篇关于类（或结构）通过模板自引用的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！