# Port C++ union to Julia

**URL:** <https://discourse.julialang.org/t/port-c-union-to-julia/950>\
**Category:** General Usage\
**Tags:** question\
**Created:** [December 14, 2016, 5:30pm UTC](https://discourse.julialang.org/t/port-c-union-to-julia/950 "2016-12-14T17:30:09Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![peter.mancini](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/peter.mancini/32/334_2.png) [@peter.mancini](https://discourse.julialang.org/u/peter.mancini)\
**Post date:** [December 14, 2016, 5:30pm UTC](https://discourse.julialang.org/t/port-c-union-to-julia/950/1 "2016-12-14T17:30:09Z")

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I am porting some complex C++ code to Julia. The one part that has me really perplexed is how to port the C++ concept of a union. If you aren’t familiar with union, its like a structure, but all of the members share the exact same memory space. So an array of two UInt32s and a Float64 are supposed to be in the same 8 bytes. The code I am looking at loads the array with a couple of Ints, does some bit shifting and then reads the resultant Float.

In Julia I have been doing the manipulation of the ints, getting the bit patterns, and concatenating them and parsing as a float. That doesn’t work. I guess I need to talk to someone that understands the internal representation of data in a union to help me figure out a way of replicating the results.

I fully admit this is straight up sorcery. No one should write an int array to a union and then read it as a float. However, it’s the code I have to port so… have pity on me. 🙂

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**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [December 14, 2016, 5:54pm UTC](https://discourse.julialang.org/t/port-c-union-to-julia/950/2 "2016-12-14T17:54:22Z")

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You can store it as `UInt64` and use `i64 % UInt32` to get a `UInt32` and `reinterpret(Float64, i64)` to get a `Float64`. Julia is type safe by default so it’s non-trivial to come up with a representation of C compatible `union`.

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**Author:** ![peter.mancini](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/peter.mancini/32/334_2.png) [@peter.mancini](https://discourse.julialang.org/u/peter.mancini)\
**Post date:** [December 14, 2016, 6:51pm UTC](https://discourse.julialang.org/t/port-c-union-to-julia/950/3 "2016-12-14T18:51:01Z")

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I think I understand you, though I am not sure.

```julia
A = UInt32(1293080650)
B = UInt64(1074340347) << 32
C = A + B
reinterpret(Float64, C)

```

That should in theory be the same as what is happening in a union, right?

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**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [December 14, 2016, 8:09pm UTC](https://discourse.julialang.org/t/port-c-union-to-julia/950/4 "2016-12-14T20:09:08Z")

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Yes. There are multiple ways to shuffle bits around in a `UInt64`. What fits you the best depends on your application.

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<div class="post-metadata">

**Author:** ![peter.mancini](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/peter.mancini/32/334_2.png) [@peter.mancini](https://discourse.julialang.org/u/peter.mancini)\
**Post date:** [December 14, 2016, 9:26pm UTC](https://discourse.julialang.org/t/port-c-union-to-julia/950/5 "2016-12-14T21:26:06Z")

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I tested it out and it worked perfectly. In my own application there were some other, distracting, issues, but the code snippet I put above is a great simplification and general solution. Thanks for the help @yuyichao!