# \`Core.Box\` around captured variable
**URL:** https://discourse.julialang.org/t/core-box-around-captured-variable/91299
**Category:** General Usage
**Tags:** closure, corebox
**Created:** [December 6, 2022, 11:39am UTC](https://discourse.julialang.org/t/core-box-around-captured-variable/91299 "2022-12-06T11:39:17Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@jlbosse](https://discourse.julialang.org/u/jlbosse)
#### Post date: [December 6, 2022, 11:39am UTC](https://discourse.julialang.org/t/core-box-around-captured-variable/91299/1 "2022-12-06T11:39:17Z")
If I define a variable inside an outer function that generates a closure, this variable gets boxed inside the closer. Does this affect performance and if yes, how do I best avoid it?
Here is a simple MWE:
```julia
julia> function Adder(a)
ncalled = 0
function f(b)
ncalled += 1
@info "calling this adder for the $ncalled time"
return a + b
end
return f
end
julia> plus1 = Adder(1)
(::var"#adder#1"{Int64}) (generic function with 1 method)
julia> plus1.a
1
julia> plus1.ncalled
Core.Box(0)
```
---
### Author:  [@roflmaostc](https://discourse.julialang.org/u/roflmaostc)
#### Post date: [December 6, 2022, 12:31pm UTC](https://discourse.julialang.org/t/core-box-around-captured-variable/91299/2 "2022-12-06T12:31:54Z")
That’s unfortunately a [known issue](https://github.com/JuliaLang/julia/issues/15276)
You can fix it by using a let
```julia
function f(a)
g = let a=a
function g(b)
a + b
end
end
end
```
I’m on my phone, sorry for brevity
---
### Author:  [@jlbosse](https://discourse.julialang.org/u/jlbosse)
#### Post date: [December 6, 2022, 1:24pm UTC](https://discourse.julialang.org/t/core-box-around-captured-variable/91299/3 "2022-12-06T13:24:33Z")
Ah, I thought that `let` should be the solution but didn’t know how exactly it went. Thanks!