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C++ Language/Expressions/ChainOfAssignment
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The "compound assignment" iVar += 2; is an abbreviation for iVar = iVar + 2;.
Programmers usually think about += only in terms of its effect (changing the value stored in iVar's memory), but that expression does also evaluate to a value.
In fact, it evaluates to a "lvalue" which means that iVar+=2 is allowed to occur on the left-hand-side of some other assignment (although it would be weird to do so).
Additional information about lvalues (includes interactive examples)