C++ anonymous structs

c++ struct unions anonymous-struct
Dejwi picture Dejwi · Apr 24, 2013 · Viewed 8.5k times · Source

I use the following union to simplify byte, nibble and bit operations:

union Byte
{
  struct {
    unsigned int bit_0: 1;
    unsigned int bit_1: 1;
    unsigned int bit_2: 1;
    unsigned int bit_3: 1;
    unsigned int bit_4: 1;
    unsigned int bit_5: 1;
    unsigned int bit_6: 1;
    unsigned int bit_7: 1;
  };

  struct {
    unsigned int nibble_0: 4;
    unsigned int nibble_1: 4;
  };

  unsigned char byte;
};

It works nice, but it also generates this warning:

warning: ISO C++ prohibits anonymous structs [-pedantic]

Ok, nice to know. But... how to get this warning out of my g++ output? Is there a possibility to write something like this union without this issue?

Answer

Drew Dormann picture Drew Dormann · Apr 24, 2013

The gcc compiler option -fms-extensions will allow non-standard anonymous structs without warning.

(That option enables what it considers "Microsoft extensions")

You can also achieve the same effect in valid C++ using this convention.

union Byte
{
  struct bits_type {
    unsigned int _0: 1;
    unsigned int _1: 1;
    unsigned int _2: 1;
    unsigned int _3: 1;
    unsigned int _4: 1;
    unsigned int _5: 1;
    unsigned int _6: 1;
    unsigned int _7: 1;
  } bit;
  struct nibbles_type {
    unsigned int _0: 4;
    unsigned int _1: 4;
  } nibble;
  unsigned char byte;
};

With this, your non-standard byte.nibble_0 becomes the legal byte.nibble._0

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