如何评估'if（A && B）'语句？

Question

if( (A) && (B) )
{
  //do something
}
else
  //do something else

The question is, would the statement immediately break to else if A was FALSE. Would B even get evaluated?

I ask this in the case that B checking the validity of an array index say array[0] when the array is actually empty and has zero elements. Therefore throwing a segfault because we are trying to access something that is out of bounds of the array. Specifically

if( (array.GetElements() > 0) && (array[0]))
  array[0]->doSomething();
else
  //do nothing and return

This may be dangerous if array[0] actually gets evaluated because it segfaults without the first check to the left of the '&&'. Precedence tells me that the left side will definitely take precedence but it doesn't tell me that it won't evaluate the right side if the left is FALSE.

Answer 1

In C and C++, the && and || operators "short-circuit". That means that they only evaluate a parameter if required. If the first parameter to && is false, or the first to || is true, the rest will not be evaluated.

The code you posted is safe, though I question why you'd include an empty else block.

Answer 2

You are asking about the && operator, not the if statement.

&& short-circuits , meaning that if while working it meets a condition which results in only one answer, it will stop working and use that answer.

So, 0 && x will execute 0 , then terminate because there is no way for the expression to evaluate non-zero regardless of what is the second parameter to && .

Answer 3

Yes, it is called Short-circuit Evaluation .

If the validity of the boolean statement can be assured after part of the statement, the rest is not evaluated.

This is very important when some of the statements have side-effects.

Answer 4

for logical && both the parameters must be true , then it ll be entered in if {} clock otherwise it ll execute else {}. for logical || one of parameter or condition is true is sufficient to execute if {}.

if( (A) && (B) ){
     //if A and B both are true
}else{
}
if( (A) ||(B) ){
     //if A or B is true 
}else{
}

Answer 5

yes, if( (A) && (B) ) will fail on the first clause, if (A) evaluates false.

this applies to any language btw, not just C derivatives. For threaded and parallel processing this is a different story ;)