# Keep default constructor when adding a zero-argument constructor
**URL:** https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014
**Category:** New to Julia
**Created:** [August 26, 2019, 10:08pm UTC](https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014 "2019-08-26T22:08:41Z")
**Posts on this page:** 7
**Page:** 1
### Author:  [@milesf](https://discourse.julialang.org/u/milesf)
#### Post date: [August 26, 2019, 10:08pm UTC](https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014/1 "2019-08-26T22:08:42Z")
If I add a zero-argument constructor `Foo()`, then I lose the ability to use the default constructor (e.g. `Foo(1, 2)`). This behavior is expected, and described in the [documentation](https://docs.julialang.org/en/v1/manual/constructors/index.html#Inner-Constructor-Methods-1): _If any inner constructor method is defined, no default constructor method is provided: it is presumed that you have supplied yourself with all the inner constructors you need._
If I want to keep the default constructor, I can create it by listing all arguments (or at least the correct number of arguments), but it seems like there should be a less-tedious way to maintain this part of the code. I tried using vargs `Foo(args...) = new(args)`, but calling `Foo(1, 2)` with that strategy produces `Foo((1, 2), #undef)`.
```julia
mutable struct Foo
Foo() = new()
#Foo(a, b) = new(a, b) # This works, but is not ideal
#Foo(args...) = new(args) # Something like this would be better
a
b
end
```
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [August 26, 2019, 10:11pm UTC](https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014/2 "2019-08-26T22:11:08Z")
> [@milesf](#):
>
> If I add a zero-argument constructor `Foo()` , then I lose the ability to use the default constructor
Only if you add it as an inner constructor. Why can’t you add it as an outer constructor?
---
### Author:  [@milesf](https://discourse.julialang.org/u/milesf)
#### Post date: [August 26, 2019, 10:19pm UTC](https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014/3 "2019-08-26T22:19:13Z")
Outer constructor does not have access to `new()`.
So this doesn’t work:
```julia
mutable struct Foo
a
b
end
Foo() = new()
```
```julia
Foo()
ERROR: UndefVarError: new not defined
```
> An inner constructor method is like an outer constructor method, except for two differences:
> 1. …
> 2. It has access to a special locally existent function called new that creates objects of the block’s type.
> Outer constructor methods can only ever create a new instance by calling another constructor method, such as the automatically provided default ones.
---
### Author:  [@DNF](https://discourse.julialang.org/u/DNF)
#### Post date: [August 26, 2019, 10:27pm UTC](https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014/4 "2019-08-26T22:27:33Z")
So you want to be able to instantiate them with `undef` field values? OK.
```julia
Foo(args...) = new(args) # Something like this would be better
```
I guess you mean
```julia
Foo(args...) = new(args...)
```
That should work. The three dots after the input argument slurps them into a tuple. You have to splat them out again.
---
### Author:  [@martink](https://discourse.julialang.org/u/martink)
#### Post date: [August 26, 2019, 10:44pm UTC](https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014/5 "2019-08-26T22:44:49Z")
> [@DNF](#):
>
> I guess you mean
>
> ```julia
> Foo(args...) = new(args...)
>
> ```
This appears to be not allowed (in Julia-1.1.0)
```julia
ERROR: syntax: ... is not supported inside "new"
```
---
### Author:  [@milesf](https://discourse.julialang.org/u/milesf)
#### Post date: [August 26, 2019, 10:46pm UTC](https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014/6 "2019-08-26T22:46:01Z")
Lucky Julia release timing for me then. I’m testing on Julia-1.2.0
---
### Author:  [@yuyichao](https://discourse.julialang.org/u/yuyichao)
#### Post date: [August 26, 2019, 10:50pm UTC](https://discourse.julialang.org/t/keep-default-constructor-when-adding-a-zero-argument-constructor/28014/7 "2019-08-26T22:50:45Z")
[https://github.com/JuliaLang/julia/pull/30577](https://github.com/JuliaLang/julia/pull/30577)