如何在C / C ++中执行no-op？

Question

for the following:

( a != b ) ? cout<<"not equal" : cout<<"equal";

suppose I don't care if it's equal, how can I use the above statement by substituting cout<<"equal" with a no-op.

Answer 1

If it really is for a ternary operator that doesn't need a second action, the best option would be to replace it for an if:

if (a!=b) cout << "not equal";

it will smell a lot less.

Answer 2

Simple: I would code it as

if (a != b)
   cout << "not equal";

The ternary operator requires the two results to be of the same type. So you might also be able to get away with

(a != b) ? cout << "not equal" : cout;

because the stream operator (<<) just returns the ostream reference. That's ugly and unnecessary in my opinion though.

Answer 3

The only thing missing from the other answers is this: There is no way, directly, to code a "noop" in C/C++.

Also, doing: (a != b) ? : printf("equal\\n"); (a != b) ? : printf("equal\\n"); does actually compile for me (gcc -ansi in gcc 4.2.4).

Answer 4

The following will achieve what you're looking for, however, it may not be clear to people reading your code why it works:

(a != b) && (cout << "equal");

Personally, I agree with this answer from Vinko Vrsalovic.

Answer 5

(void)0; is noop. There is a lots of good reason to use expr?false:true form. Look how assert() is implemented.

So in your example, use ( a != b ) ? (void)0 : cout<<"equal"; ( a != b ) ? (void)0 : cout<<"equal";

Answer 6

In C++11 you can write ( in case of void ) :

somecondition ? foo() : [] {} () ;

So the NOP is actually an empty lambda. Besides void you could return any type and value.

This might look a bit overkill all by itself but suppose you have this :

somecondition1 ? foo1() :
somecondition2 ? foo2() :
somecondition3 ? foo3() :
                 flip_out_because_unhandled_condition() ;

Now if someone adds somecondition4, but forgets to include it in the handling code, the software will call the flip_out_... function causing all kinds of unwanted effects. But maybe somecondition4 doesn't need any special attention, it just needs to be ignored. Well then you could write :

somecondition1 ? foo1()   :
somecondition2 ? foo2()   :
somecondition3 ? foo3()   :
somecondition4 ? []{}() :
                 flip_out_because_unhandled_condition() ;

Answer 7

This is very confusing code. You could just write

cond ? cout << "equal" : cout;

but you won't (will you?) because you've got conventional if for that.

Answer 8

I think the problem here is that the operator : has two EXPRESSIONS as arguments. Let's say.. a = x ? y : z;

Expression by definition must have a value...that's why you cannot just "skip".

Answer 9

If the focus of the code is the output operation and not the condition, then something like this could be done:

cout << (cond ? "not equal" : "");

I suspect that's not the case, though, because you want to do nothing in the "else" clause.

Answer 10

if (a!=b) cout<<"not equal";

Answer 11

The syntax just requires a expression. You can just go: (a!=b)?cout<<"not equal":1;

Answer 12

Both statements compile:

( a != b ) ? cout<<"not equal" : NULL;

( a != b ) ? NULL : cout<<"equal";