Is short-circuiting logical operators mandated? And evaluation order?

Joe Pineda picture Joe Pineda · Mar 10, 2009 · Viewed 33.9k times · Source

Does the ANSI standard mandate the logical operators to be short-circuited, in either C or C++?

I'm confused for I recall the K&R book saying your code shouldn't depend on these operations being short circuited, for they may not. Could someone please point out where in the standard it's said logic ops are always short-circuited? I'm mostly interested on C++, an answer also for C would be great.

I also remember reading (can't remember where) that evaluation order isn't strictly defined, so your code shouldn't depend or assume functions within an expression would be executed in a specific order: by the end of a statement all referenced functions will have been called, but the compiler has freedom in selecting the most efficient order.

Does the standard indicate the evaluation order of this expression?

if( functionA() && functionB() && functionC() ) cout<<"Hello world";

Answer

Alex B picture Alex B · Mar 10, 2009

Yes, short-circuiting and evaluation order are required for operators || and && in both C and C++ standards.

C++ standard says (there should be an equivalent clause in the C standard):

1.9.18

In the evaluation of the following expressions

a && b
a || b
a ? b : c
a , b

using the built-in meaning of the operators in these expressions, there is a sequence point after the evaluation of the first expression (12).

In C++ there is an extra trap: short-circuiting does NOT apply to types that overload operators || and &&.

Footnote 12: The operators indicated in this paragraph are the built-in operators, as described in clause 5. When one of these operators is overloaded (clause 13) in a valid context, thus designating a user-defined operator function, the expression designates a function invocation, and the operands form an argument list, without an implied sequence point between them.