# User-Defined Struct Constructors similar to Named Tuples

**URL:** <https://discourse.julialang.org/t/user-defined-struct-constructors-similar-to-named-tuples/35596>\
**Category:** Internals & Design\
**Created:** [March 5, 2020, 5:27pm UTC](https://discourse.julialang.org/t/user-defined-struct-constructors-similar-to-named-tuples/35596 "2020-03-05T17:27:37Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Deduction42](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/deduction42/32/9206_2.png) [@Deduction42](https://discourse.julialang.org/u/Deduction42)\
**Post date:** [March 5, 2020, 5:27pm UTC](https://discourse.julialang.org/t/user-defined-struct-constructors-similar-to-named-tuples/35596/1 "2020-03-05T17:27:37Z")

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When initializing user-define objects or structs, I find that he positional argument approach isn’t very clean. For example, let us say that we first start with

mutable struct MyPoint  
x::Float64  
z::Float64  
end

Then as we continue to work, we find that we need a new field “y”  
mutable struct MyPoint  
x::Float64  
y::Float64  
z::Float64  
end

If we use the constructor method of MyPoint(x,y,z), we then have to change the code whenever there is an assignment to make sure z is not assigned to y. A cheap workaround I’ve used is the following

mutable struct MyPoint  
x::Float64  
z::Float64  
MyPoint(T) = new(T.x, T.z)  
end

This allows you to assign the point using a Named Tuple like this  
MyPoint((x=1,z=2))

My question is that, would it break anything if we made the default behaviour of struct construction something like  
MyPoint(x=1, z=2)

We already have that behavior for Named Tuples, is there something that inhibits us from making user defined structs behave this way too?

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**Author:** ![tbeason](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tbeason/32/15898_2.png) [@tbeason](https://discourse.julialang.org/u/tbeason)\
**Post date:** [March 5, 2020, 5:34pm UTC](https://discourse.julialang.org/t/user-defined-struct-constructors-similar-to-named-tuples/35596/2 "2020-03-05T17:34:12Z")

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You should check out [https://github.com/mauro3/Parameters.jl](https://github.com/mauro3/Parameters.jl) or `@kwdef` which is in Base.

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**Author:** ![CameronBieganek](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/cameronbieganek/32/6915_2.png) [@CameronBieganek](https://discourse.julialang.org/u/CameronBieganek)\
**Post date:** [March 5, 2020, 5:51pm UTC](https://discourse.julialang.org/t/user-defined-struct-constructors-similar-to-named-tuples/35596/3 "2020-03-05T17:51:26Z")

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Note that the only way one can find the documentation on `@kwdef` is to type `?Base.@kwdef` into the REPL. I can’t seem to find `Base.@kwdef` in the online docs. Also note that `@kwdef` is not exported from Base, so you will need to use `Base.@kwdef` or bring it into scope via

`using Base: @kwdef`

or

`import Base.@kwdef`

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**Author:** ![Deduction42](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/deduction42/32/9206_2.png) [@Deduction42](https://discourse.julialang.org/u/Deduction42)\
**Post date:** [March 6, 2020, 6:08pm UTC](https://discourse.julialang.org/t/user-defined-struct-constructors-similar-to-named-tuples/35596/4 "2020-03-06T18:08:03Z")

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Oh my goodness! `Base.@kwdef` is EXACTLY what I was looking for. It was MORE than I was looking for! This definitely needs to show up in the Composite Types ([Types · The Julia Language](https://docs.julialang.org/en/v1/manual/types/#Composite-Types-1)) and Constructors ([Constructors · The Julia Language](https://docs.julialang.org/en/v1/manual/constructors/)) section in the manual.

Goodnes, you have no idea how much this helps me. I’m trying to run a customized variant of the Extended Kalman Filter, and state space models have at least 6 parameters; two of which are functions.