# Typeof on Any vs Typeof on Union?

**URL:** <https://discourse.julialang.org/t/typeof-on-any-vs-typeof-on-union/106806>\
**Category:** Performance\
**Created:** [November 27, 2023, 3:35pm UTC](https://discourse.julialang.org/t/typeof-on-any-vs-typeof-on-union/106806 "2023-11-27T15:35:37Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![hlbnet](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hlbnet/32/204549_2.png) [@hlbnet](https://discourse.julialang.org/u/hlbnet)\
**Post date:** [November 27, 2023, 3:35pm UTC](https://discourse.julialang.org/t/typeof-on-any-vs-typeof-on-union/106806/1 "2023-11-27T15:35:37Z")

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Hi everybody,

Is there a general explanation of why typeof comparisons on Any is 20 times slower than on Union in the following example ?  
Especially, the huge memory allocation in one case but not in the other ?

Here is the output of the test:  
First run ------------------------------

- Typeof on union:  
1.301423 seconds  
Total=7898919599
- Typeof on any:  
21.113947 seconds (657.92 M allocations: 16.339 GiB, 7.15% gc time, 0.05% compilation time)  
Total=7898919599

Second run ------------------------------

- Typeof on union:  
1.055775 seconds (2 allocations: 32 bytes)  
Total=7898919599
- Typeof on any:  
21.103012 seconds (657.92 M allocations: 16.339 GiB, 7.05% gc time)  
Total=7898919599

Here is the code:

```julia
struct Person
    name::String
    age::UInt8
end

struct Employee
    person::Person
    salary::Real
end

function sum_age(elements, repeat)
    total = 0
    for _ in 1:repeat, element in elements
        if typeof(element) === Person
            total += element.age
        elseif typeof(element) === Employee
            total += element.person.age
        end
    end
    return total
end

function test(size, repeat)
    person = Person("John", 41)
    employee = Employee(Person("Jane", 31), 1001.0)
    elements_any = Vector{Any}()
    elements_union = Vector{Union{Person,Employee}}()
    for _ in 1:size
        element = rand([person, employee])
        push!(elements_any, element)
        push!(elements_union, element)
    end

    println("First run ------------------------------")
    println("- Typeof on union:")
    @time total = sum_age(elements_union, repeat)
    println(" Total=", total)

    println("- Typeof on any:")
    @time total = sum_age(elements_any, repeat)
    println(" Total=", total)

    println()
    println("Second run ------------------------------")
    println("- Typeof on union:")
    @time total = sum_age(elements_union, repeat)
    println(" Total=", total)

    println("- Typeof on any:")
    @time total = sum_age(elements_any, repeat)
    println(" Total=", total)
end

test(123457, 1777)

```

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<div class="post-metadata">

**Author:** ![jameson](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jameson/32/23_2.png) [@jameson](https://discourse.julialang.org/u/jameson)\
**Post date:** [November 27, 2023, 3:52pm UTC](https://discourse.julialang.org/t/typeof-on-any-vs-typeof-on-union/106806/2 "2023-11-27T15:52:16Z")

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> [@hlbnet](#):
>
> ```julia
> if typeof(element) === Person
> 
> ```

FWIW, this may not answer your question exactly, but this pattern doesn’t have optimization support, as the expected way to write this is `element isa Person`

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<div class="post-metadata">

**Author:** ![hlbnet](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/hlbnet/32/204549_2.png) [@hlbnet](https://discourse.julialang.org/u/hlbnet)\
**Post date:** [November 27, 2023, 4:12pm UTC](https://discourse.julialang.org/t/typeof-on-any-vs-typeof-on-union/106806/3 "2023-11-27T16:12:04Z")

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Useful advice @jameson  
I will keep in mind to always prefer **isa** to **typeof** and **===** (am I right ?).  
I checked that using **isa** in my example actually removes the issue and is faster.  
Thanks

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<div class="post-metadata">

**Author:** ![Oscar\_Smith](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oscar_smith/32/25343_2.png) [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)\
**Post date:** [November 27, 2023, 4:48pm UTC](https://discourse.julialang.org/t/typeof-on-any-vs-typeof-on-union/106806/4 "2023-11-27T16:48:10Z")

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that is correct.