What is the real advantage of using unsigned variables in C++? [duplicate]

Question

So I understand that unsigned variables can only hold positive values and signed variables can hold negative and positive. However, it is unclear to me why would someone use unsigned variables? Isn't that risky? I mean I would personally just stick with signed variables just in case. Is there a memory/performance advantage in using unsigned variables?

Answer 1

Selecting the right kind of primitive data type for your particular problem is all about being correct about showing your intent. While, for example, a size of an array might as well be stored in a signed type (like it is the case in Java or C#), but why should you? An array cannot be of negative size. Doing so anyway is only confusing to readers of your program and will not give you any benefit.

There is no measurable performance gain by not using unsigned values; actually it can even be dangerous to do so since unsigned values can hold bigger positive numbers than signed values of the same memory size, thus risking a narrowing conversion when assigning, for example, array sizes that are naturally unsigned to a signed value of same memory size:

// While unlikely, truncation can happen
int64_t x = sizeof(...);
//          ~~~~^~~~~~~ uint64_t on my system

Those bugs are generally hard to track, but compilers have gotten better at warning you about committing them.

Naturally, you should be aware that using unsigned integers can indeed be dangerous in some cases. As an example I wrote a simple for loop. We do not expect the value i to be negative, so we do the seemingly correct decision to use a value of unsigned type:

for(unsigned i = 5; i >= 0; --i)
{
}

But in this case, the loop will never terminate since the unsigned value of 0 - 1 (happens in fifth iteration here) will be a big positive value (this is called wrap around), thus defeating the loop termination check.

This can, for example, be solved like this:

for(unsigned i = 5; (i+1) > 0; --i)
{
}

But this should not deter you from using the right data type. Just exercise caution about things like value ranges and wrap around and you will be fine.

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In conclusion, use the type that is most appropriate and seems to show your intent the best.