int qempty() { return (f == r ? 1 : 0); } In the above snippet, what does "?" mean? What can we replace it with?
- 25In this particular case of course, you can just replace it with return f==r;Eclipse– Eclipse2009-04-27 21:15:50 +00:00Commented Apr 27, 2009 at 21:15
- 7@Eclipse: I wouldn't rely on an implicit conversion bool->int if I can avoid it.Daniel Daranas– Daniel Daranas2009-06-23 07:10:59 +00:00Commented Jun 23, 2009 at 7:10
- 2@DanielDaranas why not? (This is kind of a beginner question- an explanation of your comments for beginners would be very helpful and appreciated.)Michael Hoffmann– Michael Hoffmann2015-02-05 04:08:32 +00:00Commented Feb 5, 2015 at 4:08
- 4@MichaelHoffmann The behaviour of the implicit conversion in this case is well defined, so using it is perfectly correct; see this answer for a reference to the standard. Personally, I avoid using implicit type conversions because I think the code is more readable and maintainable and less error prone without them. I wrote in more detail about it in this blog post.Daniel Daranas– Daniel Daranas2015-02-05 09:10:59 +00:00Commented Feb 5, 2015 at 9:10
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8 Answers
This is commonly referred to as the conditional operator, and when used like this:
condition ? result_if_true : result_if_false ... if the condition evaluates to true, the expression evaluates to result_if_true, otherwise it evaluates to result_if_false.
It is syntactic sugar, and in this case, it can be replaced with
int qempty() { if(f == r) { return 1; } else { return 0; } } Note: Some people refer to ?: it as "the ternary operator", because it is the only ternary operator (i.e. operator that takes three arguments) in the language they are using.
answered Apr 27, 2009 at 21:09
Daniel LeCheminant
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4 Comments
JohnMcG
In regular code, it's syntactic sugar, but it does enable you to do conditional initialization in the initialization list of the constructot.
JohnMcG
Foo(Bar* y) pMember (y == NULL ? NULL : y->pMember) -- Here, we initialize pMember to y's pMember, or NULL if it's not there. Can't put if-else in a constructor initialization, so ternary operator makes it possible.
celtschk
@JohnMcG: Well, in C++11 you could consider
a? b : c syntactic sugar for [&]() -> Type { if (a) return b; else return c; }().JohnMcG
Probably, but the question, answer, and comment were written in 2009.
This is a ternary operator, it's basically an inline if statement
x ? y : z works like
if(x) y else z except, instead of statements you have expressions; so you can use it in the middle of a more complex statement.
It's useful for writing succinct code, but can be overused to create hard to maintain code.
2 Comments
Johannes Schaub - litb
worthwhile to know that there is a sequence point at the '?'. That means the following is valid: ++x ? x : y;
Richard
@Daniel, that's what I meant by having expressions rather than statements. I probably wasn't explicit enough about the difference, so thanks for adding some clarification.
Just a note, if you ever see this:
a = x ? : y; It's a GNU extension to the standard (see https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html#Conditionals).
It is the same as
a = x ? x : y; 1 Comment
Vinícius A. Jorge
in CLang (at least the most recent versions) this extension is also available. It's available even with C++11 flag turned off in a qmake project. So an expression like int
x = 1+1 ? : 0 ; correctly returns 2 , in my compiler and this didn't complain anything.You can just rewrite it as:
int qempty(){ return(f==r);} Which does the same thing as said in the other answers.
1 Comment
Don Cheadle
this would perform implicit conversion from boolean to int
It is called the conditional operator.
You can replace it with:
int qempty(){ if (f == r) return 1; else return 0; } 
Comments
It's the conditional operator.
a ? b : c
It's a shortcut for IF/THEN/ELSE.
means: if a is true, return b, else return c. In this case, if f==r, return 1, else return 0.
Comments
The question mark is the conditional operator. The code means that if f==r then 1 is returned, otherwise, return 0. The code could be rewritten as
int qempty() { if(f==r) return 1; else return 0; } which is probably not the cleanest way to do it, but hopefully helps your understanding.
