Isize

Isize is a primitive signed integer type in the Rust programming language. Its width is determined by the target platform’s pointer size, commonly 32 bits on 32‑bit targets and 64 bits on 64‑bit targets. Consequently, isize uses the same number of bits as a pointer on the target architecture. The range is the standard signed range for that width: isize::MIN to isize::MAX, corresponding to -(2^(N-1)) to 2^(N-1) - 1.

Isize is the signed counterpart to usize, the unsigned pointer-sized integer type. It is typically used for calculations that involve offsets, pointer arithmetic, or interfaces with C code that provide or expect signed offsets. It is not the preferred choice for indexing collections, where usize is generally used, since indexing requires nonnegative indices.

In practice, isize can be used for arithmetic that may produce negative results, for example when computing differences between pointers or relative offsets in certain algorithms. It can also appear in FFI boundaries where a signed offset is meaningful. Developers should be mindful when converting between isize and unsigned types; casting a negative isize to a usize with as will wrap around to a large positive value. For fallible conversions, explicit checks or TryFrom can be used to prevent unintended wraparound.

Related types include usize (the unsigned, pointer-sized counterpart) and, in C interop, the similarly named ssize_t type. Isize, usize, and their behavior with architecture-dependent widths are central to writing portable, low-level code that interacts with memory layouts and system interfaces.