Transform a Sum into a List

I want to turn a sum like this

sum =a-b+c+d

Into a List like this:

sumToList[sum]={a,-b,c,d}

How can I achieve this?

Answer

List @@ sum

{a, -b, c, d}

From the docs on Apply (@@):

f@@expr replaces the head of expr by f.

So List@@sum replaces Head[sum] (that is, Plus) with List.

You can also get the same result by changing 0th Part of sum (which is its Head) to List:

sum[[0]] = List; sum

{a, -b, c, d}