How to overload +=?

General Usage
1

Is it possible to overload +=?

mutable struct A
    a::Int
end

function Base.:+=(a::A, b::A)
  a.a += b.a
  a
end

EDIT: As pointed out by @giordano in post 7, the struct A should be defined immutable:

struct A
    a::Int
end
2

You can’t overload assignment, but I think it’s enough in this case to overload + and += will come for free.

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3

Thank you for the quick reply but it does not seem to work:

julia> Base.:+(a::A, b::A) = A(a.a+b.a)

julia> A(2) + A(3)
A(5)

julia> A(2) += A(3)
ERROR: syntax: invalid assignment location "A(2)" around REPL[12]:1
Stacktrace:
 [1] top-level scope
   @ REPL[12]:1
4 
x = A(2)
x += A(3)
A(5)
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5

This is basically the same as

julia> 1 += 2

which of course doesn’t work.

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6

:man_facepalming: Thank you.

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7

Note that this wouldn’t mutate the argument (which one?), in case this is what you were expecting, since you marked the type as mutable.

8

But the sum should not mutate neither argument: it is only += that should. Am I wrong?

9

Assignment, including +=, never mutates. Is this what you were expecting?

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