# Why NTuple{2, Ref{Int}} \<: NTuple get false?
**URL:** https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559
**Category:** General Usage
**Tags:** question, ntuple
**Created:** [October 11, 2021, 11:25am UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559 "2021-10-11T11:25:45Z")
**Posts on this page:** 8
**Page:** 1
### Author:  [@zwtechnb](https://discourse.julialang.org/u/zwtechnb)
#### Post date: [October 11, 2021, 11:25am UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559/1 "2021-10-11T11:25:45Z")
Code:
```julia
julia> NTuple{2, Int} <: NTuple
true
julia> NTuple{2, Ref{Int}} <: NTuple
false
julia> (NTuple{2, T} where T) <: NTuple
true
```
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [October 11, 2021, 11:55am UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559/2 "2021-10-11T11:55:23Z")
`Ref` is an abstract type - you’re probably looking for `RefValue`:
```julia
julia> NTuple{2, Base.RefValue{Int}} <: NTuple
true
julia> Ref(1) |> typeof
Base.RefValue{Int64}
```
Alternatively, if you want to be able to have an `NTuple` of any reference type that’s referencing an `Int`, you can use this:
```julia
julia> NTuple{2, <:Ref{Int}} <: NTuple
true
```
This specifies that the elements of the tuple have to be subtypes of `Ref{Int}`.
---
### Author:  [@zwtechnb](https://discourse.julialang.org/u/zwtechnb)
#### Post date: [October 11, 2021, 1:12pm UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559/3 "2021-10-11T13:12:48Z")
Same as above question. I am a bit of confusing the following:
```julia
julia> NTuple{2, Integer} <: NTuple{2, T} where T<:Number
false
```
I know Integer is an abstract and Integer \<: Number. What I am reasoning are : the `T` in above code can be any type including abstract type Integer, so above comparing result should be true!
Please help me to understand why result is false!
Thank you!
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [October 11, 2021, 1:15pm UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559/4 "2021-10-11T13:15:00Z")
Julia types are invariant, not covariant - see here for more information:
[https://docs.julialang.org/en/v1/manual/types/#man-parametric-composite-types](https://docs.julialang.org/en/v1/manual/types/#man-parametric-composite-types)
---
### Author:  [@zwtechnb](https://discourse.julialang.org/u/zwtechnb)
#### Post date: [October 11, 2021, 1:19pm UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559/5 "2021-10-11T13:19:12Z")
Thanks for your comment.
---
### Author:  [@simeonschaub](https://discourse.julialang.org/u/simeonschaub)
#### Post date: [October 11, 2021, 6:06pm UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559/6 "2021-10-11T18:06:51Z")
> [@Sukera](#):
>
> Julia types are invariant, not covariant - see here for more information:
Tuples are the exception here though. Tuples in Julia are indeed covariant.
The reason for this particular behavior is often referred to as the [diagonal rule](https://docs.julialang.org/en/v1/devdocs/types/#Diagonal-types). It says that for a tuple type to be a subtype of `Tuple{T, T} where T`, `T` always has to be a concrete type. This is the reason why you can write `f(x::T, y::T) where {T} = ...` to dispatch only on the case where `x` and `y` are of the same type.
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [October 11, 2021, 7:11pm UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559/7 "2021-10-11T19:11:56Z")
> [@simeonschaub](#):
>
> Tuples in Julia are indeed covariant.
I was not aware that this applied to `NTuple` as well, good to know!
---
### Author:  [@simeonschaub](https://discourse.julialang.org/u/simeonschaub)
#### Post date: [October 11, 2021, 7:23pm UTC](https://discourse.julialang.org/t/why-ntuple-2-ref-int-ntuple-get-false/69559/8 "2021-10-11T19:23:07Z")
Yes, `NTuple` is just an alias for `Tuple{Vararg{T, N}} where {N, T}`.