Implementing Tribool with int8_t

Question

For digital electronic circuit simulation, i wanted to implement Three states. i.e High, Low and Undefined in C++.

I saw Boost::tribool, and they implement it using enum

Now for conserving some memory i have implemented it using 1 Byte Int

#ifdef _WINDOWS
typedef __int8 State;
#elif
typedef int8_t State;
#endif
const State kLow = 0;
const State kHigh = 1;
const State kUndefined = 2;

1. Do you see any potential problem with it?
2. And why would one waste 4 bytes instead of a single byte for 3 values only
3. Do you have a still better way?

• user36: That's an 8-bit int, not a 1-byte int.
• Vinayak Garg: Of course, but I was more concerned about memory usage, that's why byte. Ha, I would have loved to do this in 2 bits instead!

Answer 1

I would definitely recommend using enumerations. As this is December 2011, I would imagine your compiler supports an explicit underlying type on enumerations (GCC supported this since 4.4 and Visual C++ now supports it in 11.0). As a side note, <cstdint> is supported by visual c++ now (as of 10.0).

As an addenum to traditional c-style enumerations:

enum state : std::int8_t {
 S_UNKNWON,
 S_HIGH,
 S_LOW
};

Or adding scope to the enumeration's memebers (avoid polluting global scope):

enum class state : std::int8_t {
 unknown,
 high,
 low
};
state status = state::unknown;

Of course, after looking at boost::tribool, I would recommend that over anything -- unless you really need to ensure that state is exactly 1 byte.

• Vinayak Garg: +1 for informing about cstdint. no more #ifdef required. And yeah i wanted some savings, so enum state : int8_t solved the problem, with type safety!

Answer 2

The main reason there is no accidental conversion (type safety is one of the keys to using C++ correctly).

enum Tri { Yes, No, Maybe };
int main()
{
 Tri y = Yes;
 y= 1; // Fails to compile.
}

Secondly you are using three bytes to hold the different states here

const State kLow = 0;
const State kHigh = 1;
const State kUndefined = 2;

With an enum there is no space taken up (though potentially the above may be optimized out).

C++11 also allows you to specify the size of an enum:

enum class Tri : char { Yes, No, Maybe };
// ^^^^ Uses a char sized object

Do you see any potential problem with it?

Yes. Not type safe

And why would one waste 4 bytes instead of a single byte for 3 values only

Why not. Does it really matter in any modern PC.
OK for embedded systems maybe (but you obviously are using WINDOWS)

Do you have a still better way?

Yes. Use enum in C++11

• Adam: I got a kick out of pushing you over the 5K border ^^
• Vinayak Garg: every time one reminds me of good habit (here not ditching enum) i +1 :)