检测多个枚举项何时映射到相同的值 [英] Detect when multiple enum items map to same value

问题描述

是否有一种编译时方法来检测/防止 C/C++ 枚举中的重复值?

Is there a compile-time way to detect / prevent duplicate values within a C/C++ enumeration?

问题在于有多个项目被初始化为显式值.

背景:

我继承了一些 C 代码，例如:

I've inherited some C code such as the following:

#define BASE1_VAL    (5)
#define BASE2_VAL    (7)

typedef enum
{
  MsgFoo1A = BASE1_VAL,       // 5
  MsgFoo1B,                   // 6
  MsgFoo1C,                   // 7
  MsgFoo1D,                   // 8
  MsgFoo1E,                   // 9
  MsgFoo2A = BASE2_VAL,       // Uh oh!  7 again...
  MsgFoo2B                    // Uh oh!  8 again...
} FOO;

问题是随着代码的增长&随着开发人员向 MsgFoo1x 组添加更多消息，最终它会超出 BASE2_VAL.

The problem is that as the code grows & as developers add more messages to the MsgFoo1x group, eventually it overruns BASE2_VAL.

这段代码最终会迁移到 C++，所以如果有一个仅限 C++ 的解决方案(模板魔术?)，那没问题——但是一个适用于 C 和 C++ 的解决方案更好.

This code will eventually be migrated to C++, so if there is a C++-only solution (template magic?), that's OK -- but a solution that works with C and C++ is better.

推荐答案

有几种方法可以检查此编译时间，但它们可能并不总是适合您.首先在 MsgFoo2A 之前插入一个标记"枚举值.

There are a couple ways to check this compile time, but they might not always work for you. Start by inserting a "marker" enum value right before MsgFoo2A.

typedef enum
{
    MsgFoo1A = BASE1_VAL,
    MsgFoo1B,
    MsgFoo1C,
    MsgFoo1D,
    MsgFoo1E,
    MARKER_1_DONT_USE, /* Don't use this value, but leave it here.  */
    MsgFoo2A = BASE2_VAL,
    MsgFoo2B
} FOO;

现在我们需要一种方法来确保 MARKER_1_DONT_USE 在编译时.有两种常见的技术.

Now we need a way to ensure that MARKER_1_DONT_USE < BASE2_VAL at compile-time. There are two common techiques.

声明大小为负的数组是错误的.这看起来有点难看，但确实有效.

It is an error to declare an array with negative size. This looks a little ugly, but it works.

extern int IGNORE_ENUM_CHECK[MARKER_1_DONT_USE > BASE2_VAL ? -1 : 1];

如果 MARKER_1_DONT_USE 大于 BASE_2_VAL，几乎所有编写过的编译器都会产生错误.GCC 吐槽:

Almost every compiler ever written will generate an error if MARKER_1_DONT_USE is greater than BASE_2_VAL. GCC spits out:

test.c:16: error: size of array ‘IGNORE_ENUM_CHECK’ is negative

静态断言

如果您的编译器支持 C11，您可以使用 _Static_assert.对 C11 的支持并不普遍，但您的编译器无论如何都可能支持 _Static_assert，尤其是因为 C++ 中的相应特性得到广泛支持.

Static assertions

If your compiler supports C11, you can use _Static_assert. Support for C11 is not ubiquitous, but your compiler may support _Static_assert anyway, especially since the corresponding feature in C++ is widely supported.

_Static_assert(MARKER_1_DONT_USE < BASE2_VAL, "Enum values overlap.");

GCC 发出以下消息:

GCC spits out the following message:

test.c:16:1: error: static assertion failed: "Enum values overlap."
 _Static_assert(MARKER_1_DONT_USE < BASE2_VAL, "Enum values overlap.");
 ^

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