# Custom hash not called for field in another struct – expected?
**URL:** https://discourse.julialang.org/t/custom-hash-not-called-for-field-in-another-struct-expected/130869
**Category:** General Usage
**Tags:** question
**Created:** [July 19, 2025, 2:21pm UTC](https://discourse.julialang.org/t/custom-hash-not-called-for-field-in-another-struct-expected/130869 "2025-07-19T14:21:40Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@MilesCranmer](https://discourse.julialang.org/u/MilesCranmer)
#### Post date: [July 19, 2025, 2:21pm UTC](https://discourse.julialang.org/t/custom-hash-not-called-for-field-in-another-struct-expected/130869/1 "2025-07-19T14:21:40Z")
The Julia docs say:
> [@](#):
>
> New types should implement the 2-argument form, typically by calling the 2-argument hash method recursively in order to mix hashes of the contents with each other (and with `h`).
But when I do this, it seems to ignore my custom hash function in certain contexts:
```julia-repl
julia> struct Foo
end
julia> Base.hash(::Foo, h::UInt) = error()
```
The following calls are what I would expect:
```julia-repl
julia> hash(Foo())
ERROR:
Stacktrace:
[1] error()
@ Base ./error.jl:44
[2] hash(::Foo, h::UInt64)
@ Main ./REPL[2]:1
julia> hash((Foo(),))
ERROR:
Stacktrace:
[1] error()
@ Base ./error.jl:44
[2] hash(::Foo, h::UInt64)
@ Main ./REPL[2]:1
```
But this one is not expected:
```julia
julia> struct Bar
x::Foo
end
julia> hash(Bar(Foo()))
0xb4bfb67e8815404c
```
Is there a reason why the default `hash` on structs does not recursively travel through fields? Does this mean if I implement a custom hashing function for my type, I also need to overload hash for any type that takes it as a property to ensure my hash gets hit?
---
### Author:  [@Oscar\_Smith](https://discourse.julialang.org/u/Oscar_Smith)
#### Post date: [July 19, 2025, 2:24pm UTC](https://discourse.julialang.org/t/custom-hash-not-called-for-field-in-another-struct-expected/130869/2 "2025-07-19T14:24:52Z")
The default hash is `objectid` (matching the default `==` which is `===`) which doesn’t look at the fields.
---
### Author:  [@adienes](https://discourse.julialang.org/u/adienes)
#### Post date: [July 19, 2025, 2:26pm UTC](https://discourse.julialang.org/t/custom-hash-not-called-for-field-in-another-struct-expected/130869/3 "2025-07-19T14:26:26Z")
see also: [Convenient ways to define == and hash · Issue #53565 · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/issues/53565)
---
### Author:  [@MilesCranmer](https://discourse.julialang.org/u/MilesCranmer)
#### Post date: [July 19, 2025, 2:54pm UTC](https://discourse.julialang.org/t/custom-hash-not-called-for-field-in-another-struct-expected/130869/4 "2025-07-19T14:54:59Z")
Thanks!
Maybe it’s just me but the default `==` and `hash` feel kinda unintuitive, no? I know it won’t change, just wondering why my mental model is broken
```julia-repl
julia> mutable struct A
a::Float64
end
julia> x = A(1.0)
A(1.0)
julia> hash(deepcopy(x))
0x9e99cc3918a450f2
julia> hash(x)
0x3f7020c38a5b2e12
julia> deepcopy(x) == x
false
```
I guess I just never think of a “hash” as being sensitive to the particular location in memory.
Obviously this doesn’t matter for immutables but still
---
### Author:  [@MilesCranmer](https://discourse.julialang.org/u/MilesCranmer)
#### Post date: [July 19, 2025, 2:56pm UTC](https://discourse.julialang.org/t/custom-hash-not-called-for-field-in-another-struct-expected/130869/5 "2025-07-19T14:56:16Z")
Ah, there’s even an issue for it. Good to know it’s not just me.
>
>
> This doesn't make much sense:
>
> \`\`\` julia
> julia\> immutable Foo{T}
> bar::T…
> end
>
> julia\> Foo("baz") == Foo("baz")
> false
>
> julia\> Foo("baz").bar == Foo("baz").bar
> true
>
> julia\> Foo(1) == Foo(1)
> true
> \`\`\`
>
> If the fields are \`==\` to each other then the objects should be \`==\` to each other.