Blog / Go Language Spec

Variable vs Constant size

January 15, 2025

You’ll likely remember that certain built-in functions can sometimes evaluate to constants, rather than variables. For example:

var x [10]int
var y []int

len(x) // evaluates to a constant at compile time
len(y) // evaluates to a variable at runtime

Package unsafe

…

A (variable of) type T has variable size if T is a type parameter, or if it is an array or struct type containing elements or fields of variable size. Otherwise the size is constant. Calls to Alignof, Offsetof, and Sizeof are compile-time constant expressions of type uintptr if their arguments (or the struct s in the selector expression s.f for Offsetof) are types of constant size.

So here we have a few other special case examples of functions that can evaluate to constants at compile time, depending on the arguments passed to them.

Quotes from The Go Programming Language Specification Language version go1.23 (June 13, 2024)

Share this

Direct to your inbox, daily. I respect your privacy .

Unsure? Browse the archive .

Related Content

Variable Alignment

Package unsafe … Computer architectures may require memory addresses to be aligned; that is, for addresses of a variable to be a multiple of a factor, the variable’s type’s alignment. The function Alignof takes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variable x: uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0 I don’t think I’m qualified to really talk much more about this.

Offsetof

Package unsafe … The function Offsetof takes a (possibly parenthesized) selector s.f, denoting a field f of the struct denoted by s or *s, and returns the field offset in bytes relative to the struct’s address. If f is an embedded field, it must be reachable without pointer indirections through fields of the struct. For a struct s with field f: uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f)) I expect this is neither too confusing, nor too useful for most of us.

Determining the size of a variable

Package unsafe … The functions Alignof and Sizeof take an expression x of any type and return the alignment or size, respectively, of a hypothetical variable v as if v was declared via var v = x. Grammar nit! That should be “…as if v were…” But nobody cares about that. While Alignof is unlikely to be used outside of CGO, or low-level optimizations, Sizeof can be useful for us mere mortals.

Get daily content like this in your inbox!

Subscribe