C++ condition without selection statement

Question

Maybe because of Perl influence, I like using the following syntax

condition && do_something();

instead of

if ( condition )
{
  do_something();
}

(I know I could put the latter in one row without braces, but that's not the point.)

I've made several tests (below), which work. But I couldn't find a reference in the standard, stating that it's legal. It could be implied in

• 5.14 Logical AND operator
• 5.17 Assignment operators
• 6.4. Selection statements

but I'm not completely sure (I checked C++ 98, sorry to be old fashioned, but probably even C standard would suffice).

NB: I do realize that in order to work, the last expression (do_something) has also to be be convertible to bool, which is not necessary for the selection statement. This is a serious restriction of course.

So the questions are:

1. Is it perfectly legal?
2. Is there a reason not to use it?
3. Are there possible side effects (like optimizations leading to unexpected results)?

Apologies if turns out to be a duplicate, I couldn't find anything, but possibly because I didn't use the right keywords.

#include <cassert>

bool sayYes()
{
  return true;
}

bool sayNo()
{
  return false;
}

int main( int args, char* argv[] )
{
  bool a = true;
  bool b = false;
  int i = 0;

  a && ( i = 1 );
  assert( i == 1 );

  !a && ( i = 0 );
  assert( i == 1 );

  ( a || b ) && ( i = 0 );
  assert( i == 0 );

  sayYes() && ( i = 1 );
  assert( i == 1 );

  ( sayNo() || sayYes() ) && ( i = 0 );
  assert( i == 0 );

  return 0;
}

Answer 1

It's perfectly legal. From [expr.log.and]:

&& guarantees left-to-right evaluation: the second operand is not evaluated if the first operand is false.

expr() && fun() is equivalent to if (expr()) fun() (assuming fun() returns something convertible to bool). That's an extremely important property when it comes to writing compound conditional expressions. For example, only dereferencing a pointer after we know it's valid:

if (ptr && ptr->foo()) { ... }

That said, if what you really want is:

if (expr()) {
    fun();
}

just write that. The compiler will generate the same code either way, but the intent of what you're trying to do will be much clearer.

Answer 2

1. Yes it is legal. An expression followed by ; is a valid statement.

2. Some folk might find it obfuscating.

3. It cannot be optimised out if there are side effects.

Do be aware though that && can be overloaded in C++. If it's overloaded then then the short-cutting property of the operator is discarded. That will cause strange effects. It's chiefly for this reason that I would advise against this style.