# A function to add two flat structs
**URL:** https://discourse.julialang.org/t/a-function-to-add-two-flat-structs/6302
**Category:** New to Julia
**Created:** [October 7, 2017, 2:19am UTC](https://discourse.julialang.org/t/a-function-to-add-two-flat-structs/6302 "2017-10-07T02:19:07Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@tuckermcclure](https://discourse.julialang.org/u/tuckermcclure)
#### Post date: [October 7, 2017, 2:19am UTC](https://discourse.julialang.org/t/a-function-to-add-two-flat-structs/6302/1 "2017-10-07T02:19:07Z")
I’d like to be able to add two structs without knowing about the structs ahead of time, such as:
```julia
function Base.:+(a::T, b::T) where {T}
T((getfield(a, field) + getfield(b, field) for field in fieldnames(a))...)
end
```
I find that for a simple struct (with an int and a float) that this takes about a microsecond, which is much longer than I expected.
I also tried it this way with mutable structs:
```julia
function Base.:+(a::T, b::T) where {T}
s = deepcopy(a)
for field in fieldnames(a)
setfield!(s, field, getfield(a, field) + getfield(b, field))
end
return s
end
```
That takes about 2us.
If I hard-code the addition, like so:
```julia
addem(x::MyType, y::MyType) = MyType(x.a + y.a, x.b + y.b)
```
it only takes about 20ns. This is more like what I’m hoping for, but without hard-coding the addition.
Is there a better way to do this generally?
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [October 7, 2017, 3:00am UTC](https://discourse.julialang.org/t/a-function-to-add-two-flat-structs/6302/2 "2017-10-07T03:00:58Z")
> [@tuckermcclure](#):
>
> Is there a better way to do this generally?
Use a generated function to build the `MyType` addition statement using `fieldnames`.
Here’s an example of a function that copies all field of a generic abstract type. It’s similar to what you want and should be easy to modify.
[https://github.com/JuliaDiffEq/DiffEqBase.jl/blob/master/src/data\_array.jl#L57](https://github.com/JuliaDiffEq/DiffEqBase.jl/blob/master/src/data_array.jl#L57)
---
### Author:  [@tuckermcclure](https://discourse.julialang.org/u/tuckermcclure)
#### Post date: [October 7, 2017, 4:42am UTC](https://discourse.julialang.org/t/a-function-to-add-two-flat-structs/6302/3 "2017-10-07T04:42:00Z")
Wow, thanks @ChrisRackauckas. Here’s my first take at a generated function:
```julia
@generated function Base.:+(x::T, y::T) where T
expr = ( :(x.$f + y.$f) for f in fieldnames(T) ) # Make a tuple of expressions to add each field
:( T($(expr...)) ) # Splat them into an expression for a constructor for a T type
end
```
This only takes ~20ns or so, just like the hard-coded version.