CODEWITHSUNDEEP
c
Venkata
Venkata

Reputation: 513

About C struct members

I am looking into the code written by other members and I came across the code shown below:

struct myType
{
 int   myInt  : 1;
 int   reserved : 31;
};

What is 1 and 31 stands above and when above notation is used?

Upvotes: 1

Views: 159

Answers (2)

Kiril Kirov
Kiril Kirov

Reputation: 38173

These are bit fields. This code means, that myInt is just one bit and reserved is 31 bits

For example, on my machine

struct a
{
    int asd : 1;
    int b : 2;
};

std::cout << sizeof( a );

prints 4 (it's platform dependent). In you're example, the exact size of the struct is 32bits, but it's possible the actual size to be different - depends on the alignment

Upvotes: 1

unwind
unwind

Reputation: 399833

Those are bit fields, the number after the colon specifies the width, in bits, reserved for that field. They are often used when trying to conserve space, or when trying to map an external (think hardware-owned) register that has bitfields. Note that packing and endianness affect how the bits are layed out in memory, so they're not to be used if portability is important.

Note that it's a very bad idea to use signed bit fields with very small sizes, such as 1. Since one bit is needed for the sign, no bits are left for the actual value which is generally not a very good situation. Using unsigned int myUnsigned : 1 fixes this.

Upvotes: 4

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