Colon : used in variable initialization?

Question

I found this line here:

uint32 bIsHungry : 1;

... and I've never seen this syntax used to initialize a variable.

I'm used to seeing this:

uint32 bIsHungry = 1;

It looks kind of like an initialization list, but for a single field?

What is it, what does it do, why should I care?

To initialize a scalar variable you can use either the equality sign or parentheses or braces. — Vlad from Moscow, Aug 06 '15 at 08:21

score 15 · Accepted Answer · edited May 23 '17 at 11:53

15

That line is a bit field declaration and it declares a data member with an explicit bit-level size

Example from cppreference:

#include <iostream>
struct S {
 // three-bit unsigned field,
 // allowed values are 0...7
 unsigned int b : 3;
};
int main()
{
    S s = {7};
    ++s.b; // unsigned overflow
    std::cout << s.b << '\n'; // output: 0
}

Notice that in the example above the unsigned overflow is defined behavior (same doesn't apply if b were declared as a signed type)

The documentation you links also states that

Boolean types can be represented either with the C++ bool keyword or as a bitfield

Regarding why should I care I recommend reading this other question

edited May 23 '17 at 11:53

Community

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answered Aug 06 '15 at 08:15

Marco A.

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Interesting! Thanks, I've literally never seen that before. – Cody Smith Aug 06 '15 at 08:20
It might come in handy under particular circumstances where e.g. the hardware will map at bit level parts of a memory area. – Marco A. Aug 06 '15 at 08:22

Nishant · Answer 2 · 2015-08-06T08:25:45.800

2

This is bitfield declaration, Declares a class data member with explicit size, in bits.

edited Aug 06 '15 at 08:25

answered Aug 06 '15 at 08:15

Nishant

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1

There is neither initialization. – Vlad from Moscow Aug 06 '15 at 08:23

score 2 · Answer 3 · answered Aug 06 '15 at 08:20

It is not initialization, it is a fragment of a declaration.

struct {
    // ...
    uint32 bIsHungry : 1;
    // ...
};

declares a bIsHungry as a bitfield member of the struct. It says that bIsHungry is an unsigned int whose length is 1 bit. Its possible values are 0 and 1.

Read more about bitfields: http://en.cppreference.com/w/c/language/bit_field

Colon : used in variable initialization?

3 Answers3