c++ - 为什么我不能返回对压缩字段的引用？

Question

我使用 g++ 来编译带有打包字段的代码。但是，我在尝试返回对打包字段的引用时收到错误消息。

示例:

struct __attribute__((packed)) Foo {
   int* ptr;
   uint16_t foo;
   int*& getPtr(){
      return ptr;
   }
};

产生错误:

test.cpp:22:14: error: cannot bind packed field ‘((Foo*)this)->Foo::ptr’ to ‘int*&’
   return ptr;

为什么我不能返回对压缩字段的引用？

Answer 1

有一个 gcc 错误报告 Cannot bind packed field涵盖了这一点，它说:

The C++ spec (C++03, Sects. 3.9, 3.9.1, 3.9.2) are very clear that T and "pointer to T" have implementation-specific alignment requirements. If you have a "pointer to T" then you may assume that it meets the alignment requirements. I'm sure the C spec has similar language.

In the OP's case, the following code could violate the alignment requirements

他们建议使用 alignment 属性来定义您自己的对齐类型，但它看起来并不奏效。