使用（无效*）时，作为一个通用的数据容器L值的麻烦 [英] L-value trouble when using a (void *) as a generic data container

问题描述

下面是在程序中使用的结构：

Here is a structure used in a program:

struct basic_block
{
  void * aux;

  /* Many other fields,
  which are irrelevant.  */
};

现在

1. basic_block 的几个实例程序的执行过程中存在。


2. 该项目工程分阶段/通行证，这是一个接一个执行。


3. 的辅助字段是为存储级和 basic_block A阶段的执行过程中的具体数据，并释放通过舞台本身（所以下一个阶段可以重复使用它）。这就是为什么它是一个无效* 。

1. Several instances of basic_block exist during the execution of the program.
2. The program works in stages/passes, which are executed one after another.
3. The aux field is meant for storing stage and basic_block specific data during the execution of a stage, and freed by the stage itself (so the next stage can reuse it). This is why it is a void *.

我的舞台使用辅助来存储结构，所以我想访问的东西每一次，我一定要做投：

My stage uses aux to store a struct, so each time I want to access something, I have to do a cast:

( (struct a_long_struct_name *) (bb->aux))->foo_field = foo;

现在，我的问题：这样每次铸造这是一个痛苦，难以阅读时，它是更为复杂的前pressions的一部分。我提出的解决办法是：使用宏做演员对我来说：

Now, my problem: Casting it each time like this is a pain, and difficult to read when it is part of more complicated expressions. My proposed solution was: Use a macro to do the cast for me:

#define MY_DATA(bb) \
  ( (struct a_long_struct_name *) (bb)->aux)

然后我可以访问我的数据：

Then I can access my data with:

MY_DATA(bb)->foo_field = foo;

但我不能用 MY_DATA（BB）的本身的作为L值（一个的malloc ），所以我使用的宏是不是一个好主意毕竟：

But: I cannot use MY_DATA(bb) itself as an L-value (for a malloc), so I using that macro isn't such a good idea after all:

/* WRONG! I cannot assign to the result of a cast:  */
MY_DATA(bb) = malloc (sizeof (struct a_long_struct_name));

我的问题：

的我可以以参考做辅助在一个干净的方式，仍然可以使用它作为一个L值。的

What can I do in order to refer to aux in a clean way and still be able to use it as an L-value.

推荐答案

您可以施放的地址的领域，以结构a_long_struct_name ** 然后取消对它的引用：

You can cast the address of the field to struct a_long_struct_name ** and then dereference it:

#define MY_DATA(bb) \
   (* ((struct a_long_struct_name **) &(bb)->aux) )

这将在这两个你都呈现构建工作。

This will work in both the constructs you have shown.

如果设定的可能性的具体类型的辅助可在该点被称为其中，结构basic_block 是宣布，不过，这将是更清洁的使用工会：

If the set of possibilities for the concrete type of aux can be known at the point where struct basic_block is declared, though, it would be cleaner to use a union:

struct basic_block
{
  union {
      struct a_long_struct_name *alsn;
      /* etc */
  } aux;

  /* Many other fields,
     which are irrelevant.  */
};

然后

#define MY_DATA(bb) ((bb)->aux.alsn)

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