What does this short C++ code mean?

Question

This was a code snippet from a test question. The question was what the size of S would be.

struct S
{
    char a : 4;
    unsigned char b : 3;
    signed char : 2;
    char c : 1;
    char d : 5;
};

What does the ":" do? Is there any difference when it is applied to a signed or unsigned char (or any other data type)? When is this usually used?

Also worth reading https://stackoverflow.com/questions/44169863/what-does-char-3-in-a-struct-mean-and-how-large-is-the-member as a warning of how ugly asking this question can get. — user4581301, May 26 '17 at 19:34
Note that "Multiple adjacent bit fields are usually packed together (although this behavior is implementation-defined)". The answer is implementation defined. — François Andrieux, May 26 '17 at 19:35
It is also possible that changing types will break the bit packing. — Weather Vane, May 26 '17 at 19:53

score 1 · Answer 1 · answered May 26 '17 at 19:32

It's a bit field, it says that for example char a will only have 4 bits of memory instead of normal 8. Unsigned char b will have only 3 bits of memory. Number of bits limits the range of values it can hold.

Bit Field declares a class data member with explicit size, in bits. Adjacent bit field members may be packed to share and straddle the individual bytes. http://en.cppreference.com/w/cpp/language/bit_field

What does this short C++ code mean?

1 Answers1