can isize and usize be different in rust?

Question

Can isize and usize be different? Both of them can be used for memory size, index, offset.

Since usize is used for arrays why don't we just have usize

I am new to Rust so this might be a basic question.

Update: On a 32 bit system they are both 32 bit long and on a 64 bit system they are both 64 bit long. Irrespective of the sign.

Answer 1

On a 32 bit system, isize is the same as i32 and usize is the same as u32. On a 64 bit system, isize is the same as i64 and usize is the same as u64.

• usize cannot be negative and is generally used for memory addresses, positions, indices, lengths (or sizes!).
• isize can be negative, and is generally used for offsets to addresses, positions, indices, or lengths.

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In all currently supported architectures usize and isize are the same size as each other, but this may not always be the case! Novel instruction sets incorporating CHERI need to include metadata in pointers for tracking provenance. On 64 bit systems, this scheme requires an extra 64 bits for metadata, making pointers 128-bit, but pointer offsets can still be 64-bit.

There is some discussion about how CHERI support would affect Rust here.

Answer 2

isize is architecture-based(e.g. 32bit/64bit) signed(negative/0/positive) integer type.

See here:

Primitive Type isize

The pointer-sized signed integer type.

See also the std::isize module.

The size of this primitive is how many bytes it takes to reference any location in memory. For example, on a 32 bit target, this is 4 bytes and on a 64 bit target, this is 8 bytes.

usize is architecture-based(e.g. 32bit/64bit) unsigned(0/positive) integer type.

See here:

Primitive Type usize

The pointer-sized unsigned integer type.

See also the std::usize module.

The size of this primitive is how many bytes it takes to reference any location in memory. For example, on a 32 bit target, this is 4 bytes and on a 64 bit target, this is 8 bytes.