The sizeof expression returns an Int32 with the size in bytes of a given type. For example:

sizeof(Int32) # => 4
sizeof(Int64) # => 8

For Reference types, the size is the same as the size of a pointer:

# On a 64 bits machine
sizeof(Pointer(Int32)) # => 8
sizeof(String)         # => 8

This is because a Reference's memory is allocated on the heap and a pointer to it is passed around. To get the effective size of a class, use instance_sizeof.

The argument to sizeof is a type and is often combined with typeof:

a = 1
sizeof(typeof(a)) # => 4

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