Check incomplete type

1、由于delete incomplete type是undefined的behavior，因此我们需要考虑如何判断是否是incomplete type。

2、与本节内容相关的是Detect-whether-a-type-is-defined章节

stackoverflow Delete objects of incomplete type # A

It is undefined behavior.

However, you can make the compiler check for incomplete types, like boost:

// verify that types are complete for increased safety
template<class T>
inline void checked_delete(T *x)
{
    // intentionally complex - simplification causes regressions
    typedef char type_must_be_complete[sizeof(T) ? 1 : -1];
    (void) sizeof(type_must_be_complete);
    delete x;
}

Applying sizeof to an incomplete type should trigger an error, and I suppose if that passes with some compiler, then an array of negative size would trigger an error.

std::unique_ptr

本地文件位置: /usr/include/c++/4.8.2/bits/unique_ptr.h

URL: https://gcc.gnu.org/onlinedocs/libstdc++/libstdc++-html-USERS-4.4/a01404.html

  /// Primary template, default_delete.
  template<typename _Tp>
    struct default_delete
    {
      constexpr default_delete() noexcept = default;

      template<typename _Up, typename = typename
               enable_if<is_convertible<_Up*, _Tp*>::value>::type>
        default_delete(const default_delete<_Up>&) noexcept { }

      void
      operator()(_Tp* __ptr) const
      {
        static_assert(sizeof(_Tp)>0,
                      "can't delete pointer to incomplete type");
        delete __ptr;
      }
    };

stackoverflow std::unique_ptr with an incomplete type won't compile

NOTE: 从提问者贴出的编译报错信息来看，也是采用的上面描述的那种方式。