In ancient versions of C, =+
was equivalent to +=
. Remnants of it have been found alongside the earliest dinosaur bones.
For example, B introduced generalized assignment operators, using
x+=y
to addy
tox
. The notation came from Algol 68 via McIlroy, who incorporated it in his version of TMG. (In B and early C, the operator was spelled=+
instead of+=
; this mistake, repaired in 1976, was induced by a seductively easy way of handling the first form in B’s lexical analyzer.)
[The Development of the C Language, Dennis Ritchie. Copyright ACM, 1993. Internal citations omitted.]
Since the mid-1970’s, it has no special meaning — it’s just a =
followed by a +
.