C special syntax (adding ':' ) while defining a struct [duplicate]

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Possible Duplicate:

What does the : do in a struct declaration after a member

I would like to ask why the : character was added i开发者_JAVA百科n this struct:

typedef union A
{
struct 
    {
        ubyte B:4;
         }
} struct_a;

Thanks in advance;

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The :4 is putting a 4-bit limit on the variable. See Section 6.9 of Kernighan & Ritchie.

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It's called a bit field. In this case it is saying the B is 4 bits in size.

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It declares a bit field with 4 bits.