1/17/2024 0 Comments Integer overflow wrap aroundX = -1 // -1 is out of our range, so we get modulo wrap-around Unsigned short x // smallest 2-byte unsigned value possible Let’s take a look at this using 2-byte shorts: #include 256, however, is outside the range, so it wraps around to the value 0. 255 is in range of a 1-byte integer, so 255 is fine. Any number bigger than the largest number representable by the type simply “wraps around” (sometimes called “modulo wrapping”). Here’s another way to think about the same thing. Therefore, we divide 280 by 256, getting 1 remainder 24. 1 greater than the largest number of the type is 256. The number 280 is too big to fit in our 1-byte range of 0 to 255. If an unsigned value is out of range, it is divided by one greater than the largest number of the type, and only the remainder kept. Given that most programmers would consider this overflow, we’ll call this overflow despite C++’s statements to the contrary. This is contrary to general programming consensus that integer overflow encompasses both signed and unsigned use cases ( cite). Oddly, the C++ standard explicitly says “a computation involving unsigned operands can never overflow”. What happens if we try to store the number 280 (which requires 9 bits to represent) in a 1-byte (8-bit) unsigned integer? The answer is overflow. An integer without a sign (an unsigned integer) assumes all values are positive. Consequently, an integer with a sign (a signed integer) can tell the difference between positive and negative. If a sign is not provided, we assume a number is positive. The following is a simple way to remember the difference: in order to differentiate negative numbers from positive ones, we use a negative sign. New programmers sometimes get signed and unsigned mixed up. Remembering the terms signed and unsigned When no negative numbers are required, unsigned integers are well-suited for networking and systems with little memory, because unsigned integers can store more positive numbers without taking up extra memory. Hence it should be something like wrapping_into, analogous to wrapping_add, wrapping_sub, etc.Size/Type Range 8 bit unsigned 0 to 255 16 bit unsigned 0 to 65,535 32 bit unsigned 0 to 4,294,967,295 64 bit unsigned 0 to 18,446,744,073,709,551,615Īn n-bit unsigned variable has a range of 0 to (2 n)-1. I'd still argue that most of the time when you want to convert between types, you don't expect just the low bits - that's rather specialized and low-level. So I want to use u64, and then if the number is actually too big to fit in memory, I want this to fail when I try to allocate memory by calling Vec::new(y_into().unwrap()) or something like that. For instance, if I'm calculating the size of a data structure I need, and I know my calculation will fit in 40 bits, I don't want to use usize for that calculation because usize might not have 40 bits, which would make my calculation overflow. So you have to convert sometimes.Īlso usize is not portable, so actually I don't think this is great advice. But even std functions give you other types. In practice that would mean all your integers are usize because most everything can be used for an index calculation. If one needs indices, one should always use usize for storing the index in the first place. There might be a lot of words in the documentation, but that's not what the actual Kolmogorov complexity of its semantics is. The Rust documentation tries to be friendly, elaborate, and eloquent as a work of prose, but if you look at the content that really matters, it all just boils down to the high-level idea of "preserve bits if converting between ints, preserve values when converting to/from float, and both kinds of cast are on a best-effort basis". See, this can be written down in 1 or 2 sentences. Everything else aims to produce the closest value in the target type (when converting between integers and floats-used-as-reals) or does what's the most hardware-efficient while still yielding a reasonable value (truncate or zero/sign-extend). The only thing I couldn't guess without reading the documentation if I were new to Rust is whatever NaN as integer does. That long-winded explanation is basically what anyone would sensibly expect (and that's no coincidence). Nonetheless Rust reference includes a table which includes more than dozen possibilities and few pages of explanations
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