What’s the result of += in C and C++?

Semantics of the compound assignment operators is different in C and C++:

C99 standard, 6.5.16, part 3:

An assignment operator stores a value in the object designated by the left operand. An
assignment expression has the value of the left operand after the assignment, but is not an
lvalue.

In C++ 5.17.1:

The assignment operator (=) and the compound assignment operators all group right-to-left. All require a modifiable
lvalue as their left operand and return an lvalue with the type and value of the left operand after the assignment has taken place.

EDIT : The behavior of (i+=10)+=10 in C++ is undefined in C++98, but well defined in C++11. See this answer to the question by NPE for the relevant portions of the standards.