# Constructors for Primitive types

**URL:** <https://discourse.julialang.org/t/constructors-for-primitive-types/19139>\
**Category:** General Usage\
**Tags:** question, faq, type\
**Created:** [December 30, 2018, 11:07pm UTC](https://discourse.julialang.org/t/constructors-for-primitive-types/19139 "2018-12-30T23:07:45Z")\
**Posts on this page:** 6\
**Page:** 1

<div class="post-metadata">

**Author:** ![chakravala](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chakravala/32/6832_2.png) [@chakravala](https://discourse.julialang.org/u/chakravala)\
**Post date:** [December 30, 2018, 11:07pm UTC](https://discourse.julialang.org/t/constructors-for-primitive-types/19139/1 "2018-12-30T23:07:45Z")

</div>

My goal is to have my own primitive integer type, for which I need to specialize dispatch.

```nohighlight
primitive type Basis{V,G} <: Unsigned 16 end

```

How do I create a constructor for it? The documentation does not explain how.

```nohighlight
julia> Basis{4,3}(0x0007)
ERROR: MethodError: no method matching Basis{4,3}(::UInt16)
Closest candidates are:
  Basis{4,3}(::T<:Number) where T<:Number at boot.jl:725
  Basis{4,3}(::Float16) where T<:Integer at float.jl:71
  Basis{4,3}(::Complex) where T<:Real at complex.jl:37
  ...

```

---

<div class="post-metadata">

**Author:** ![bennedich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bennedich/32/4894_2.png) [@bennedich](https://discourse.julialang.org/u/bennedich)\
**Post date:** [December 30, 2018, 11:52pm UTC](https://discourse.julialang.org/t/constructors-for-primitive-types/19139/2 "2018-12-30T23:52:41Z")

</div>

Try this:

```julia
primitive type Basis{V,G} <: Unsigned 16 end

Basis{V,G}(x::UInt16) where {V,G} = reinterpret(Basis{V,G}, x)

UInt16(x::Basis) = reinterpret(UInt16, x)

Base.show(io::IO, x::Basis) = print(io, UInt16(x))

Basis{4,3}(0x0007)

```

---

<div class="post-metadata">

**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:** [December 31, 2018, 10:00am UTC](https://discourse.julialang.org/t/constructors-for-primitive-types/19139/3 "2018-12-31T10:00:17Z")

</div>

> [@chakravala](#):
>
> The documentation does not explain how.

This question comes up very often; please consider contributing @bennedich’s answer to the docs as a PR.

---

<div class="post-metadata">

**Author:** ![chakravala](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chakravala/32/6832_2.png) [@chakravala](https://discourse.julialang.org/u/chakravala)\
**Post date:** [December 31, 2018, 8:22pm UTC](https://discourse.julialang.org/t/constructors-for-primitive-types/19139/4 "2018-12-31T20:22:21Z")

</div>

For now I created a repository which holds my basic concept for a primitive bit array…

> **[GitHub - chakravala/PrimitiveBits.jl: Static bit-vector with primitive byte...](https://github.com/chakravala/PrimitiveBits.jl)**
>
> Static bit-vector with primitive byte storage and bit-wise index - GitHub - chakravala/PrimitiveBits.jl: Static bit-vector with primitive byte storage and bit-wise index

However, the `reinterpret` thing will only work if the number of bits matches an existing type, so it does not work in general for arbitrary number of bits.

The question remains open for bit sizes such as 48 for example.

---

<div class="post-metadata">

**Author:** ![bennedich](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bennedich/32/4894_2.png) [@bennedich](https://discourse.julialang.org/u/bennedich)\
**Post date:** [December 31, 2018, 9:05pm UTC](https://discourse.julialang.org/t/constructors-for-primitive-types/19139/5 "2018-12-31T21:05:52Z")

</div>

Nice! You know about `BitArray`, right? It contains similar functionality, and generally has pretty good performance, e.g.:

```julia
julia> a = BitArray(bitrand(64));

julia> @btime a[2:63];
  63.278 ns (3 allocations: 160 bytes)

julia> b = PrimitiveBits64(a);

julia> @btime b[2:63];
  368.627 ns (2 allocations: 176 bytes)

```

One difference / disadvantage being that the minimum storage is 64 bits, so it’s not very space efficient for small types, and thus might not be what you need. Perhaps performance is not a concern for your implementation, but glancing over your code, I would propose the following trivial but impactful change:

```julia
- d = 2 ^ (i-1)
+ d = 1 << (i-1)

```

I.e. to use shifting rather than exponentiation. Running the same test as above:

```julia
julia> @btime b[2:63];
  106.148 ns (2 allocations: 176 bytes)

```

---

<div class="post-metadata">

**Author:** ![chakravala](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chakravala/32/6832_2.png) [@chakravala](https://discourse.julialang.org/u/chakravala)\
**Post date:** [December 31, 2018, 9:25pm UTC](https://discourse.julialang.org/t/constructors-for-primitive-types/19139/6 "2018-12-31T21:25:31Z")

</div>

Thanks for the suggestion. When using smaller bit sizes, the performance is more comparable

```nohighlight
julia> a = BitArray(rand(Bool,16));

julia> b = PrimitiveBits16(a);

julia> f(a,x) = a[x];

julia> @btime f($a,2:14);
  41.007 ns (2 allocations: 128 bytes)

julia> @btime f($b,2:14);
  46.182 ns (1 allocation: 96 bytes)

```

This type is more convenient than a `BitArray` if you need to convert to an integer more frequently.