An offsetof expression returns the byte offset of an instance variable in a struct or class type. It accepts the type as first argument and the instance variable name prefixed by an @ as second argument:

offsetof(Type, @ivar)

This is a low-level primitive and only useful if a C API needs to directly interface with the data layout of a Crystal type.


struct Foo
  @x = 0_i64
  @y = 34_i32

offsetof(Foo, @y) # => 8

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