Reputation: 3042

C++ short-circuiting of booleans

I'm new to c++ and am curious how the compiler handles lazy evaluation of booleans. For example,

if(A == 1 || B == 2){...}

If A does equal 1, is the B==2 part ever evaluated?

Upvotes: 12

Answers (8)

Robert Cartaino

Reputation: 28112

Unless the || operator is overloaded, the second expression will not be evaluated. This is called "short-circuit evaluation."

In the case of logical AND (&&) and logical OR (||), the second expression will not be evaluated if the first expression is sufficient to determine the value of the entire expression.

In the case you described above:

if(A == 1 || B == 2) {...}

...the second expression will not be evaluated because

TRUE || ANYTHING, always evaluates to TRUE.

Likewise,

FALSE && ANYTHING, always evaluates to FALSE, so that condition will also cause a short-circuit evaluation.

A couple of quick notes

Short circuit evaluation will not apply to overloaded && and || operators.
In C++, you are guaranteed that the first expression will be evaluated first. Some languages do not guarantee the order of evaluation and VB doesn't do short-circuit evaluation at all. That's important to know if you are porting code.

Upvotes: 22

James McNellis

Reputation: 355039

No, the B==2 part is not evaluated. This is called short-circuit evaluation.

Edit: As Robert C. Cartaino rightly points out, if the logical operator is overloaded, short-circuit evaluation does not take place (that having been said, why someone would overload a logical operator is beyond me).

Upvotes: 23

Conrad Meyer

Reputation: 2886

This is short-circuit evaluation, as James says. Lazy evaluation is something entirely different.

Upvotes: 1

Greg Hewgill

Reputation: 992955

The compiler handles this by generating intermediate jumps. For the following code:

if(A == 1 || B == 2){...}

compiled to pseudo-assembler, might be:

    load variable A
    compare to constant 1
    if equal, jump to L1
    load variable B
    compare to constant 2
    if not equal, jump to L2
L1:
    ... (complete body of if statement)
L2:
    (code after if block goes here)

Upvotes: 1