Throwing with the ternary operator

Question

This compiles:

struct A{};
struct B{};

int main(){
  if(true)
    throw A();
  else
    throw B();  
}

, but

struct A{};
struct B{};

int main(){
  throw( true ?  A() : B() );
}

won't.

Can I throw with the ternary operator?

`A` and `B` are different types, so the expression `true ? A() : B()` is ill-typed (it'd have to be either an A or a B). — Colonel Thirty Two, Nov 30 '15 at 16:07
The compiler error should have called out that `A` and `B` are incompatible. gcc gives `error: incompatible operand types ('A' and 'B')` — NathanOliver, Nov 30 '15 at 16:09
@Almo Matter of taste. I find the ternary operator version easier to read. — Petr Skocik, Nov 30 '15 at 16:09
This isn't a dupe; the dupe is referring to a compiler crash when `throw` is inside the ternary operator, but this question has it outside (and it's about a type error). — Colonel Thirty Two, Nov 30 '15 at 16:10
Is there a means to retract a close vote? You're right, it's not a dupe. — Almo, Nov 30 '15 at 16:13
What geniuses voted to close this as a duplicate of an obviously unrelated question? — 15ee8f99-57ff-4f92-890c-b56153, Nov 30 '15 at 16:15
@EdPlunkett Somebody who only skimmed the other question, voted to close, realised their mistake, and reopened it. That idiot. ;-) — Konrad Rudolph, Nov 30 '15 at 16:16
@KonradRudolph Sounds like I'm in a pretty bad position to criticize people for acting rashly...! — 15ee8f99-57ff-4f92-890c-b56153, Nov 30 '15 at 16:17
You also can't `std::cout << (true? 1 : "nope")`. Hardly a surprise. — n. m. could be an AI, Nov 30 '15 at 16:18
Possible duplicate of [Can an exception be thrown from the ternary operator?](http://stackoverflow.com/questions/20848132/can-an-exception-be-thrown-from-the-ternary-operator) — Paolo M, Nov 30 '15 at 16:19
@PaoloM We already went thourgh this once. It is not a duplicate of that. If anything it is a duplicate of http://stackoverflow.com/questions/29842095/incompatible-operand-types-when-using-ternary-conditional-operator — NathanOliver, Nov 30 '15 at 16:25
@NathanOliver Yeah, I've noticed too late :P This question does not have **anything** to do with *thowing*... — Paolo M, Nov 30 '15 at 16:27

score 7 · Accepted Answer · answered Nov 30 '15 at 16:14

7

A and B are different, incompatible types, so the expression true ? A() : B() is ill-typed (it'd have to be either an A or a B).

answered Nov 30 '15 at 16:14

Colonel Thirty Two

23,953
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score 2 · Answer 2 · answered Nov 30 '15 at 16:14

2

Ternary operator needs to have the same type(or something convertible to the same type) on both paths otherwise the compiler can't reason about type safety.

answered Nov 30 '15 at 16:14

Sorin

11,863
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score 0 · Answer 3 · answered Nov 30 '15 at 16:25

If there is a result from the conditional operator, the types needs to be somehow compatible. If the types are not compatible, you can still throw from a ternary operator, though:

condition? throw A(): throw B();

Although all compilers I tried do compile the above statement, it seems to be illegal: according to 5.16 [expr.cond] paragraph 2, first bullet "The second or the third operand (but not both) is a (possibly parenthesized) throw-expression (15.1); ...". To counter that constraint it is possible to use something like

condition? (throw A()), true: throw B()

Throwing with the ternary operator

3 Answers3