# Question about internal representation of Union{Missing, Float64}

**URL:** https://discourse.julialang.org/t/question-about-internal-representation-of-union-missing-float64/17000
**Category:** Performance
**Tags:** question, array, memory
**Created:** [October 31, 2018, 10:01am UTC](https://discourse.julialang.org/t/question-about-internal-representation-of-union-missing-float64/17000 "2018-10-31T10:01:19Z")
**Posts on this page:** 1
**Showing post:** 2

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### Author: ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [October 31, 2018, 11:07am UTC](https://discourse.julialang.org/t/question-about-internal-representation-of-union-missing-float64/17000/2 "2018-10-31T11:07:57Z")

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cf

> [@sizeof(::Vector{Union{missing,T}}), and hypothetical size](https://discourse.julialang.org/t/sizeof-vector-union-missing-t-and-hypothetical-size/15271):
>
> Two (related) questions: julia\> sizeof(vcat(ones(10), fill(missing, 10))) 160 but I expected 20\*9 because of the type tag, what am I missing? v"1.1.0-DEV.298". If I have a type T which is a Union, can I calculate the size of a Vector{T} of n elements without actually creating one?

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