Thread: How to use only 4 bytes of an int?

You are misunderstanding things. All types have certain sizes. An integer on a 32-bit system is normally 4 bytes.

You can, however, tell the compiler you only want to use certain BITS of a type, which is what you are doing. Single bits cannot actually be stored, only single bytes, so you will always be allocating at least a byte, and the bits are actually being retrieved with bitwise arithmetic.

The only valid use of bitfields is within structures (and classes in C++), where you can store multiple smaller integers in one larger unit:

Code:

struct a 
{
    int x:4;
    int y:8;
    int z:4;
};

This struct will still take up 4 bytes, most likely, but you can store 3 different integer values there.

If you want to have an array of bits, then you have to do it yourself. You can make an array of smaller integers, such as char or short, which will reduce the amount of space used.

Using less than 8 bits at a time will most likely lead to extra code, and for most intents and purpose, will only be valuable if you have a HUGE number of these things. And, as I said earlier, it will have to involve writing your own code for it.

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Mats

A struct or a regular variable isn't any different in the instructions generated as a general rule.
However, a bitfield (with number of bits different from the number of bits of char, short or int) WILL take more time to access because the compiler will have to produce the relevant and/or/shift instructions to modify the bits of the whole item.

How to learn about these things? Basically, understand how the compiler works and study what it ACTUALLY generates compared to what you think it will do. I can almost always predict with good accuracy what instructions come out of a few lines of C or C++ code.

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Mats

Originally Posted by slingerland3g

matsp, wouldn't your struct be only 2 bytes in size and not 32 bits?

Depends on the size of int itself. If you want to make it smaller, you probably need short instead of int. But I have a vague memory it is not clearly defined by the standard.

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Mats