C / C ++枚举：检测多个项目映射到相同值时 [英] C/C++ enums: Detect when multiple items map to same value

问题描述

是否有编译时检测/防止C / C ++枚举中的重复值？

捕获是有多个项目被初始化为显式值。

背景

我继承了一些C代码，如下所示：

MsgFoo1A = BASE1_VAL（5）
$ b ，// 5
MsgFoo1B，// 6
MsgFoo1C，// 7
MsgFoo1D，// 8
MsgFoo1E，// 9
MsgFoo2A = BASE2_VAL，//呃哦7再次...
MsgFoo2B //呃哦！ 8再次...
} FOO;

问题是随着代码的增长作为开发人员向 MsgFoo1x 组添加更多消息，最终超过 BASE2_VAL 。

这段代码最终将被迁移到C ++，所以如果有一个C ++的解决方案（模板魔术？），那没关系 - 但是一个适用于C和C ++的解决方案更好。

解决方案

有几种方法来检查这个编译时间，但是它们可能并不总是适合你。首先在MsgFoo2A之前插入一个标记枚举值。

  typedef枚举
 {
 MsgFoo1A = BASE1_VAL，
 MsgFoo1B，
 MsgFoo1C，
 MsgFoo1D，
 MsgFoo1E，
 MARKER_1_DONT_USE，/ *不要使用此值，但将其留在此处。 * / 
 MsgFoo2A = BASE2_VAL，
 MsgFoo2B 
} FOO; 
  

现在我们需要一种方法来确保 MARKER_1_DONT_USE< BASE2_VAL 在编译时。有两种常见的技术。

负数大小数组

声明一个负数大小的数组是一个错误。这看起来有点丑陋，但它有效。

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

如果MARKER_1_DONT_USE大于BASE_2_VAL，几乎所有编写的编译器都会生成错误。 GCC吐出来：

  test.c：16：错误：数组IGNORE_ENUM_CHECK的大小为负
  





静态断言





如果您的编译器支持C11， code> _Static_assert 。对C11的支持并不普及，但您的编译器可能支持 _Static_assert ，特别是因为C ++中的相应功能得到广泛支持。

  _Static_assert（MARKER_1_DONT_USE< BASE2_VAL，枚举值重叠）; 
  

GCC发出以下消息：

  test.c：16：1：错误：静态断言失败：枚举值重叠。 
 _Static_assert（MARKER_1_DONT_USE< BASE2_VAL，枚举值重叠）; 
 ^ 
  

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

The catch is that there are multiple items which are initialized to explicit values.

Background:

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;

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

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.

解决方案

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;

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

Negative size arrays

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];

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

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 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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