C++ 打包 typedef 枚举

Question

typedef enum BeNeLux
{
   BELGIUM,
   NETHERLANDS,
   LUXEMBURG
} _ASSOCIATIONS_ BeNeLux;

When I try to compile this with C++ Compiler, I am getting errors, but it seems to work fine with a C compiler. So here's the question. Is it possible to pack an enum in C++, or can someone see why I would get the error?

The error is:

"semicolon missing after declaration of BeNeLux".

I know, after checking and rechecking, that there definitely is a semicolon there, and in any places required in the rest of the code.

Addendum:

_PACKAGE_ was just an example. I am renaming it.

_ASSOCIATIONS_ is not a type of BeNeLux:

#define _ASSOCIATIONS_ __attribute__((packed))

The code is iffed, but only to make sure it is GNU C/C++.

#if defined (__GNUC__) 
#define _ASSOCIATIONS_ __attribute__((packed))
#else
#define _ASSOCIATIONS_

Would this cause problems? I thought ( GNUC ) worked for both C and C++

Addendum 2:

I even tried

#ifdef __cplusplus
extern "C" {
#endif

    typedef enum BeNeLux
    {
       BELGIUM,
       NETHERLANDS,
       LUXEMBURG
    } _ASSOCIATIONS_ BeNeLux;

#ifdef __cplusplus
}
#endif

No joy. Anyone?

Note: -fshort-enums is not a possibility; looking for a programmatic solution.

Answer 1

UPDATE:

For C++11 and later, you can specify the underlying type of enum s. For example:

enum BeNeLux : uint8_t {
   BELGIUM,
   NETHERLANDS,
   LUXEMBURG
};

But this only applies if the code will be C++ only. If the code needs to be compatible with both C and C++, I believe my original answer still applies.

---

I don't think that there is something that does exactly what you want here. I assume you are trying to create a type that is the smallest type for the enum's range.

If you need this type of control, I would recommend something like this:

typedef unsigned char BeNeLux;
static const BeNeLux BELGIUM = 0;
static const BeNeLux NETHERLANDS = 1;
static const BeNeLux LUXEMBURG = 2;

not quite as pretty and possibly a little less type safe. But has the effect that you want. sizeof(BeNeLux) == 1 and you have a named constant for all values in the range. A good compiler won't even allocate a variable for static const integer values so long as you never attempt to use the address of it.

Answer 2

#if defined (__GNUC__)
#  if defined (__cplusplus)
#     define _ASSOCIATIONS_(X) __attribute__((packed))
#  else
#     define _ASSOCIATIONS_(X) __attribute__((packed)) X
#  endif
#else
#  if defined (__cplusplus)
#     define _ASSOCIATIONS_(X)
#  else
#     define _ASSOCIATIONS_(X) X
#  endif
#endif

typdef enum BeNeLux {
  BELGIUM,
  NETHERLANDS,
  LUXEMBURG
} _ASSOCIATIONS_ (BeNeLux);

This seems to compile in my g++ (GCC) 4.2.4 (Ubuntu 4.2.4-1ubuntu4)

Answer 3

There are no real breakthroughs here. I just reordered things in the hopes that your compiler would like it better. I did not have your version of gcc or g++ so I could not test with those. I did run it through version 3.4.5 gcc and g++ (mingw special) which both warned 'packed' attribute ignored when ordered like your code, but did not complain about mine.

#ifdef __GNUC__
#define attribute(x) __attribute__((x));
#else
#define attribute(x)
#endif


#ifdef __cplusplus
extern "C" {
#endif

enum BeNeLux
{
    BELGIUM,
    NETHERLANDS,
    LUXEMBURG
} attribute(packed);
// I declared attribute to look like a f() so that it would not look like I was
// declaring a variable here.

#ifndef __cplusplus
typedef enum BeNeLux BeNeLux; // the typedef is separated into a separate stmt
#else
}
#endif

Answer 4

I suppose you're using GCC and G++. The code compiles fine with either, for me. It would be rather suprising to see such a feature disappear when enabling C++ on any one particular compiler.

Make sure that your #define is not #if 'ed out for C++ code, eg #ifndef __cplusplus .

Try g++ -E to see the preprocessor output and verify that the #define appears.

Also, even for preprocessor macros, names beginning with an underscore and capital letter or two underscores are reserved for the compiler and library. If you must have a macro, PACKED might be the best name for it.

Answer 5

In C++, you don't need the typedef . Just start with enum BeNeLux . It's also possible (I can never remember) that declaring a type and a variable with the same identifier may not be legal in one of the languages.

Answer 6

enum BeNeLux
{
   BELGIUM,
   NETHERLANDS,
   LUXEMBURG
};

This is what is expected of C++ code.

Answer 7

#if defined (__GNUC__)

#  if defined (__cplusplus)
#     define ENUM enum
#  else
#     define ENUM typedef enum
#  endif

#  if defined (__cplusplus)
#     define _ASSOCIATIONS_(X) __attribute__((packed))
#  else
#     define _ASSOCIATIONS_(X) __attribute__((packed)) X
#  endif
#else
#  if defined (__cplusplus)
#     define _ASSOCIATIONS_(X)
#  else
#     define _ASSOCIATIONS_(X) X
#  endif
#endif

ENUM BeNeLux {
  BELGIUM,
  NETHERLANDS,
  LUXEMBURG
} _ASSOCIATIONS_ (BeNeLux);

Another snippet. see the new #define ENUM