Is it possible to specify the bit width of an enum in C++11?

Question

I'm exchanging packets with an embedded device and I'd really like to be able to use enums in the sub-byte parts of the packet definitions too. But I can't guess a syntax that might work and I suspect it's not possible, since I can't work out how to declare a partial-byte subtype in C++:

enum class communication_path_t : uint8_t  {      Ethernet = 0, Wifi = 1 };  typedef struct {     communication_path_t pathByte;  // works, uses one byte     // ...      // single byte split three ways     uint8_t retryCount : 3;     communication_path_t path : 3;  // compile error     uint8_t deviceType : 2; } packet_t; 

That doesn't compile, because you can't fit an 8 bit enum into a 3 bit field. Edited in the exact error:

<anonymous struct>::path’ is too small to hold all values    of ‘enum class MyNamespace::communication_path_t’ [-Werror] 

What I'd like to do is something like this:

enum class communication_path_t : uint8_t : 3 { ... 

or

typedef uint8_t:3 three_bit_int_t; enum class communication_path_t : three_bit_int_t { ... 

Neither of those compile, and I'm having trouble finding documentation that refers to both bit fields and enums, making me suspect there is none. Before I spend hours looking, is what I'm trying to do even possible?

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Edit: upgrading to g++-4.9 does not fix the problem. It's remarkably painless, just:

sudo apt-get install g++-4.9 g++-4.9 --version  g++-4.9 (Ubuntu 4.9.2-0ubuntu1~14.04) 4.9.2 GCC 4.9.2 released [2014-10-30] 

Then change my build chain to use "g++-4.9" instead of "g++". Unfortunately I get the same error:

g++-4.9 -Dlinux -std=c++11 -pthread (...) ../common/LogPacketBreakdown.cpp In file included from ../common/LogPacketBreakdown.cpp:12:0: ../common/PacketInfo.h:104:50: error: ‘Digiflex::<anonymous     struct>::communicationPath’ is too small to hold all values of      ‘enum class Digiflex::communication_path_t’ [-Werror]     communication_path_t communicationPath : 3; 

Looks as though I need 5.0 and that's not in the Ubuntu experimental tools list so I'd need to build from source. I think I'll just live with the workaround for now. Thanks all for your help.

Answer 1

The code you posted should be accepted by the most recent compilers. You can see this bug report where the fix should have occurred: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=51242

In today's gcc, a warning should still be emitted. In clang, you should see nothing.