Is show() the correct way to return a String representation of a struct?

Admin 1
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In C# and Java, we have the toString() method to return a String representation of an object.

The closest I've seen in Julia is show(), but I'm not entirely sure if it's the correct function.

struct Person
   name::String
   age::Int32
end

amber = Person("Amber",22) 
println(amber) # Output: Person("Amber", 22)

If I want a little more control over the return String, I can do:

function Base.show(io::IO, person::Person)
    return "Name: $(person.name) Age: $(person.age)"
end

println(show(amber)) # Output: Name: Amber Age: 22

Do I have to call show() directly everytime? Can I pass it to println() and have it call show()? In C# and Java, if toString() is overridden, Console.writeLine() or System.out.println() will call toString(), no need to call it directly.

julia tostring
4 Answers

This works the same in Julia:

julia> struct Person
          name::String
             age::Int32
       end

julia>

julia> amber = Person("Amber",22)
Person("Amber", 22)

julia> Base.show(io::IO, person::Person) = println("Name: $(person.name) Age: $(person.age)")

julia> amber
Name: Amber Age: 22

This question is close to a duplicate of:

How to overload Base.show for custom array types?

user_24998920

Once you define a show(::IO, ::Person) method:

julia> Base.show(io::IO, person::Person) = print(io, "The name is $(person.name), and they are $(person.age) years old")

You can use the repr function to get that output as a String:

julia> repr(amber)
"The name is Amber, and they are 22 years old"

You can also use the string function

julia> string(amber)
"The name is Amber, and they are 22 years old"

which by default falls back to show and so returns the same result. However, you can customize that by defining a print(::IO, ::Person) method:

julia> Base.print(io::IO, person::Person) = print(io, "name: $(person.name)\nage: $(person.age)")

julia> string(amber)
"name: Amber\nage: 22"

julia> repr(amber)
"The name is Amber, and they are 22 years old"

Note that this doesn't change the repr output: repr depends directly on show, whereas string depends on print.

In practice, you can use this to, for eg., have string(amber) return a plain list of information about the struct that could be useful for debugging, by defining print to print that information. And meanwhile, repr(amber) can return a human readable interpretation of the struct useful to the end user of the type, by defining show to show such an interpretation.

sundar_81270

In addition to the answer above, if you want a String, we can always write our own to_string() function in addition to show():

function toString(person::Person)
  return "Name $(person.name) Age: $(person.age)"
end

amber = Person("Amber",22) 
println(toString(amber)) # Output: Name Amber Age: 22
Admin1

I wonder if you want to replicate toString() behavior if you can just define a Base.string() method

struct Person
   name::String
   age::Int32
end

Base.string(person::Person) = "Name: $(person.name) Age: $(person.age)"

p = Person("Julia", 5)

string(p)
"Name: Julia Age: 5"
user_138132190
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