Question

Can someone explain to me why this code prints 14? I was just asked by another student and couldn’t figure it out.

int i = 5; i = ++i + ++i; cout<<i; 

Answer 1

The order of side effects is undefined in C++. Additionally, modifying a variable twice in a single expression has no defined behavior (See the C++ standard, §5.0.4, physical page 87 / logical page 73).

Solution: Don’t use side effects in complex expression, don’t use more than one in simple ones. And it does not hurt to enable all the warnings the compiler can give you: Adding -Wall(gcc) or /Wall /W4(Visual C++) to the command line yields a fitting warning:

test-so-side-effects.c: In function 'main': test-so-side-effects.c:5: warning: operation on 'i' may be undefined test-so-side-effects.c:5: warning: operation on 'i' may be undefined 

Obviously, the code compiles to:

i = i + 1; i = i + 1; i = i + i;