# Is this conversion+store safe?
**URL:** https://discourse.julialang.org/t/is-this-conversion-store-safe/133533
**Category:** Performance
**Tags:** question
**Created:** [October 30, 2025, 2:07am UTC](https://discourse.julialang.org/t/is-this-conversion-store-safe/133533 "2025-10-30T02:07:49Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Tortar](https://discourse.julialang.org/u/Tortar)
#### Post date: [October 30, 2025, 2:07am UTC](https://discourse.julialang.org/t/is-this-conversion-store-safe/133533/1 "2025-10-30T02:07:49Z")
I’m wondering if this is a correct usage of these unsafe functions:
```julia-auto
julia> using StaticArrays
julia> a = MVector{4,UInt}(1,2,3,4);
julia> b = UInt.((5,6,7,8));
julia> function f(a, b)
dst = Base.unsafe_convert(Ptr{MVector{4, UInt64}}, a)
unsafe_store!(dst, b)
return a
end
julia> f(a, b)
4-element MVector{4, UInt64} with indices SOneTo(4):
0x0000000000000005
0x0000000000000006
0x0000000000000007
0x0000000000000008
```
is it?
Should I use instead
```julia-auto
julia> function f(a, b)
GC.@preserve a begin
dst = Base.unsafe_convert(Ptr{MVector{4, UInt64}}, a)
unsafe_store!(dst, b)
end
return a
end
```
maybe?
---
### Author:  [@Sukera](https://discourse.julialang.org/u/Sukera)
#### Post date: [October 30, 2025, 5:37am UTC](https://discourse.julialang.org/t/is-this-conversion-store-safe/133533/2 "2025-10-30T05:37:06Z")
Since this is an `MVector`, why use unsafe functions at all?
---
### Author:  [@Keno](https://discourse.julialang.org/u/Keno)
#### Post date: [October 30, 2025, 5:39am UTC](https://discourse.julialang.org/t/is-this-conversion-store-safe/133533/3 "2025-10-30T05:39:41Z")
> [@Tortar](#):
>
> is it?
No, it’ll crash
> [@Tortar](#):
>
> ```julia-auto
> julia> function f(a, b)
> GC.@preserve a begin
> dst = Base.unsafe_convert(Ptr{MVector{4, UInt64}}, a)
> unsafe_store!(dst, b)
> end
> return a
> end
>
> ```
Certainly better, although these things can be very tricky and I don’t think we’re guaranteeing the memory layout here. If at all possible, try to use the APIs provided by the package (which may do similar things, but will likely get caught in PkgEval if we break an implicit assumption). If the issue is performance, file an issue.
---
### Author:  [@Mason](https://discourse.julialang.org/u/Mason)
#### Post date: [October 30, 2025, 9:28am UTC](https://discourse.julialang.org/t/is-this-conversion-store-safe/133533/4 "2025-10-30T09:28:23Z")
> [@Sukera](#):
>
> Since this is an `MVector`, why use unsafe functions at all?
To expand on this, you can just use `a.data = b` here:
```julia-auto
julia> using StaticArrays
julia> a = MVector{4, UInt}(1,2,3,4);
julia> b = UInt.((5,6,7,8));
julia> a.data = b;
julia> a
4-element MVector{4, UInt64} with indices SOneTo(4):
0x0000000000000005
0x0000000000000006
0x0000000000000007
0x0000000000000008
```
* * *
If you’re exploring unsafe pointer shenanigans though, it’s perhaps also worth noting if you’re going down this path that you can’t `unsafe_load` a `Ptr{T}` when `T` is a mutable type (like `MVector`), since the GC will incorrectly reason about the loaded object.
---
### Author:  [@Tortar](https://discourse.julialang.org/u/Tortar)
#### Post date: [October 30, 2025, 12:52pm UTC](https://discourse.julialang.org/t/is-this-conversion-store-safe/133533/5 "2025-10-30T12:52:11Z")
thank you all! Indeed, I can use the straightforward version of @Mason, I thought the only other option was `a[1]=b[1], a[2]=b[2], ...`