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22

3

Julia has the setter functions setproperty! and setfield! and the getter functions getproperty and getfield that operate on structs. What is the difference between properties and fields in Julia?

For example, the following seems to indicate that they do the same thing:

julia> mutable struct S
 a
 end

julia> s = S(2)
S(2)

julia> getfield(s, :a)
2

julia> getproperty(s, :a)
2

julia> setfield!(s, :a, 3)
3

julia> s
S(3)

julia> setproperty!(s, :a, 4)
4

julia> s
S(4)

julia

share|improve this question

asked Oct 1 at 8:25

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

add a comment
 | 

22

3

Julia has the setter functions setproperty! and setfield! and the getter functions getproperty and getfield that operate on structs. What is the difference between properties and fields in Julia?

For example, the following seems to indicate that they do the same thing:

julia> mutable struct S
 a
 end

julia> s = S(2)
S(2)

julia> getfield(s, :a)
2

julia> getproperty(s, :a)
2

julia> setfield!(s, :a, 3)
3

julia> s
S(3)

julia> setproperty!(s, :a, 4)
4

julia> s
S(4)

julia

share|improve this question

asked Oct 1 at 8:25

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

add a comment
 | 

Julia has the setter functions setproperty! and setfield! and the getter functions getproperty and getfield that operate on structs. What is the difference between properties and fields in Julia?

For example, the following seems to indicate that they do the same thing:

julia> mutable struct S
 a
 end

julia> s = S(2)
S(2)

julia> getfield(s, :a)
2

julia> getproperty(s, :a)
2

julia> setfield!(s, :a, 3)
3

julia> s
S(3)

julia> setproperty!(s, :a, 4)
4

julia> s
S(4)

julia

share|improve this question

asked Oct 1 at 8:25

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

julia> mutable struct S
 a
 end

julia> s = S(2)
S(2)

julia> getfield(s, :a)
2

julia> getproperty(s, :a)
2

julia> setfield!(s, :a, 3)
3

julia> s
S(3)

julia> setproperty!(s, :a, 4)
4

julia> s
S(4)

share|improve this question

asked Oct 1 at 8:25

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

share|improve this question

asked Oct 1 at 8:25

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

asked Oct 1 at 8:25

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

asked Oct 1 at 8:25

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

1 Answer
1

active

oldest

votes

26

fields are simply the "components" of a struct. The struct

struct A
 b
 c::Int
end

has the fields b and c. A call to getfield returns the object that is bound to the field:

julia> a = A("foo", 3)
A("foo", 3)

julia> getfield(a, :b)
"foo"

In early versions of Julia, the syntax a.b used to "lower", i.e. be the same as, writing getfield(a, :b). What has changed now is that a.b lowers to getproperty(a, :b) with the default fallback

getproperty(a::Type, v::Symbol) = getfield(a, v)

So by default, nothing has changed. However, authors of structs can overload getproperty (it is not possible to overload getfield) to provide extra functionality to the dot-syntax:

julia> function Base.getproperty(a::A, v::Symbol)
 if v == :c
 return getfield(a, :c) * 2
 elseif v == :q
 return "q"
 else
 return getfield(a, v)
 end
 end

julia> a.q
"q"

julia> getfield(a, :q)
ERROR: type A has no field q

julia> a.c
6

julia> getfield(a, :c)
3

julia> a.b
"foo"

So we can add extra functionality to the dot syntax (dynamically if we want). As a concrete example where this is useful is for the package PyCall.jl where you used to have to write pyobject[:field] while it is possible now to implement it such that you can write pyobject.field.

The difference between setfield! and setproperty! is analogous to the difference between getfield and getproperty, explained above.

In addition, it is possible to hook into the function Base.propertynames to provide tab completion of properties in the REPL. By default, only the field names will be shown:

julia> a.<TAB><TAB>
b c

But by overloading propertynames we can make it also show the extra property q:

julia> Base.propertynames(::A) = (:b, :c, :q)

julia> a.<TAB><TAB>
b c q

share|improve this answer

edited Oct 1 at 9:46

answered Oct 1 at 8:32

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So you cannot overload getfield?
  
  – Alfaizkhan
  Oct 1 at 9:51
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  No, getfield is a special (built-in) function. Trying to overload it will give the error cannot add methods to a builtin function.
  
  – Kristoffer Carlsson
  Oct 1 at 11:07
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe add that info to the answer somewhere?
  
  – StefanKarpinski
  Oct 1 at 13:29
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  The answer already explicitly says " (it is not possible to overload getfield)" so, in a sense, it is already there.
  
  – Kristoffer Carlsson
  Oct 1 at 14:13
  

  
  
  
  

add a comment
 | 

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26

fields are simply the "components" of a struct. The struct

struct A
 b
 c::Int
end

has the fields b and c. A call to getfield returns the object that is bound to the field:

julia> a = A("foo", 3)
A("foo", 3)

julia> getfield(a, :b)
"foo"

In early versions of Julia, the syntax a.b used to "lower", i.e. be the same as, writing getfield(a, :b). What has changed now is that a.b lowers to getproperty(a, :b) with the default fallback

getproperty(a::Type, v::Symbol) = getfield(a, v)

So by default, nothing has changed. However, authors of structs can overload getproperty (it is not possible to overload getfield) to provide extra functionality to the dot-syntax:

julia> function Base.getproperty(a::A, v::Symbol)
 if v == :c
 return getfield(a, :c) * 2
 elseif v == :q
 return "q"
 else
 return getfield(a, v)
 end
 end

julia> a.q
"q"

julia> getfield(a, :q)
ERROR: type A has no field q

julia> a.c
6

julia> getfield(a, :c)
3

julia> a.b
"foo"

So we can add extra functionality to the dot syntax (dynamically if we want). As a concrete example where this is useful is for the package PyCall.jl where you used to have to write pyobject[:field] while it is possible now to implement it such that you can write pyobject.field.

The difference between setfield! and setproperty! is analogous to the difference between getfield and getproperty, explained above.

In addition, it is possible to hook into the function Base.propertynames to provide tab completion of properties in the REPL. By default, only the field names will be shown:

julia> a.<TAB><TAB>
b c

But by overloading propertynames we can make it also show the extra property q:

julia> Base.propertynames(::A) = (:b, :c, :q)

julia> a.<TAB><TAB>
b c q

share|improve this answer

edited Oct 1 at 9:46

answered Oct 1 at 8:32

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So you cannot overload getfield?
  
  – Alfaizkhan
  Oct 1 at 9:51
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  No, getfield is a special (built-in) function. Trying to overload it will give the error cannot add methods to a builtin function.
  
  – Kristoffer Carlsson
  Oct 1 at 11:07
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe add that info to the answer somewhere?
  
  – StefanKarpinski
  Oct 1 at 13:29
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  The answer already explicitly says " (it is not possible to overload getfield)" so, in a sense, it is already there.
  
  – Kristoffer Carlsson
  Oct 1 at 14:13
  

  
  
  
  

add a comment
 | 

26

fields are simply the "components" of a struct. The struct

struct A
 b
 c::Int
end

has the fields b and c. A call to getfield returns the object that is bound to the field:

julia> a = A("foo", 3)
A("foo", 3)

julia> getfield(a, :b)
"foo"

In early versions of Julia, the syntax a.b used to "lower", i.e. be the same as, writing getfield(a, :b). What has changed now is that a.b lowers to getproperty(a, :b) with the default fallback

getproperty(a::Type, v::Symbol) = getfield(a, v)

So by default, nothing has changed. However, authors of structs can overload getproperty (it is not possible to overload getfield) to provide extra functionality to the dot-syntax:

julia> function Base.getproperty(a::A, v::Symbol)
 if v == :c
 return getfield(a, :c) * 2
 elseif v == :q
 return "q"
 else
 return getfield(a, v)
 end
 end

julia> a.q
"q"

julia> getfield(a, :q)
ERROR: type A has no field q

julia> a.c
6

julia> getfield(a, :c)
3

julia> a.b
"foo"

So we can add extra functionality to the dot syntax (dynamically if we want). As a concrete example where this is useful is for the package PyCall.jl where you used to have to write pyobject[:field] while it is possible now to implement it such that you can write pyobject.field.

The difference between setfield! and setproperty! is analogous to the difference between getfield and getproperty, explained above.

In addition, it is possible to hook into the function Base.propertynames to provide tab completion of properties in the REPL. By default, only the field names will be shown:

julia> a.<TAB><TAB>
b c

But by overloading propertynames we can make it also show the extra property q:

julia> Base.propertynames(::A) = (:b, :c, :q)

julia> a.<TAB><TAB>
b c q

share|improve this answer

edited Oct 1 at 9:46

answered Oct 1 at 8:32

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  So you cannot overload getfield?
  
  – Alfaizkhan
  Oct 1 at 9:51
  

  
  
  
  


• 
  
  
  

  
  3
  

  
  
  
  
  
  

  
  
  No, getfield is a special (built-in) function. Trying to overload it will give the error cannot add methods to a builtin function.
  
  – Kristoffer Carlsson
  Oct 1 at 11:07
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Maybe add that info to the answer somewhere?
  
  – StefanKarpinski
  Oct 1 at 13:29
  

  
  
  
  


• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  The answer already explicitly says " (it is not possible to overload getfield)" so, in a sense, it is already there.
  
  – Kristoffer Carlsson
  Oct 1 at 14:13
  

  
  
  
  

add a comment
 | 

fields are simply the "components" of a struct. The struct

struct A
 b
 c::Int
end

has the fields b and c. A call to getfield returns the object that is bound to the field:

julia> a = A("foo", 3)
A("foo", 3)

julia> getfield(a, :b)
"foo"

In early versions of Julia, the syntax a.b used to "lower", i.e. be the same as, writing getfield(a, :b). What has changed now is that a.b lowers to getproperty(a, :b) with the default fallback

getproperty(a::Type, v::Symbol) = getfield(a, v)

So by default, nothing has changed. However, authors of structs can overload getproperty (it is not possible to overload getfield) to provide extra functionality to the dot-syntax:

julia> function Base.getproperty(a::A, v::Symbol)
 if v == :c
 return getfield(a, :c) * 2
 elseif v == :q
 return "q"
 else
 return getfield(a, v)
 end
 end

julia> a.q
"q"

julia> getfield(a, :q)
ERROR: type A has no field q

julia> a.c
6

julia> getfield(a, :c)
3

julia> a.b
"foo"

So we can add extra functionality to the dot syntax (dynamically if we want). As a concrete example where this is useful is for the package PyCall.jl where you used to have to write pyobject[:field] while it is possible now to implement it such that you can write pyobject.field.

The difference between setfield! and setproperty! is analogous to the difference between getfield and getproperty, explained above.

In addition, it is possible to hook into the function Base.propertynames to provide tab completion of properties in the REPL. By default, only the field names will be shown:

julia> a.<TAB><TAB>
b c

But by overloading propertynames we can make it also show the extra property q:

julia> Base.propertynames(::A) = (:b, :c, :q)

julia> a.<TAB><TAB>
b c q

share|improve this answer

edited Oct 1 at 9:46

answered Oct 1 at 8:32

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

struct A
 b
 c::Int
end

julia> a = A("foo", 3)
A("foo", 3)

julia> getfield(a, :b)
"foo"

getproperty(a::Type, v::Symbol) = getfield(a, v)

julia> function Base.getproperty(a::A, v::Symbol)
 if v == :c
 return getfield(a, :c) * 2
 elseif v == :q
 return "q"
 else
 return getfield(a, v)
 end
 end

julia> a.q
"q"

julia> getfield(a, :q)
ERROR: type A has no field q

julia> a.c
6

julia> getfield(a, :c)
3

julia> a.b
"foo"

julia> a.<TAB><TAB>
b c

julia> Base.propertynames(::A) = (:b, :c, :q)

julia> a.<TAB><TAB>
b c q

share|improve this answer

edited Oct 1 at 9:46

answered Oct 1 at 8:32

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

answered Oct 1 at 8:32

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges

answered Oct 1 at 8:32

Kristoffer CarlssonKristoffer Carlsson

1,01022 silver badges99 bronze badges