Ternary operator and function signature

Question

Let's say I have a C++ class with two functions like

class MyClass
{
    bool Foo(int val);
    bool Foo(string val);
}

Is it possible to use the ternary operator like this

MyClassInstance->Foo(booleanValue?24:"a string");

and have a different function of MyClass invoked depending on the value of booleanValue?

Have you tried compiling and running it to see what happens? — Dan F, Oct 24 '12 at 13:22
This doesn't even compile. The compiler will complain about the different types of the ternary operator. — Olaf Dietsche, Oct 24 '12 at 13:26
I do not understand the upvotes here. Sure, it's a fun experiment, but the question could have easily been avoided if the OP only *tried it*. The OP has not provided any research information at all. — default, Oct 24 '12 at 13:27
I came from a PHP background and I got 3 downvotes immediately. damn some languages are just stubborn isn't it? — mauris, Oct 24 '12 at 13:33
@Default I should have phrased my question differently: I have that code that does not compile, is there a way to get it compiling or is this notation forbidden? I will be more careful next time... — Yannick Blondeau, Oct 24 '12 at 13:55

score 25 · Accepted Answer · answered Oct 24 '12 at 13:23

Not with the ternary operator. The type of a ternary expression is the common type of its second and third operands; if they have no common type you can't use it. So just use an ordinary if statement:

if (booleanValue)
    MyClassInstance->Foo(24);
else
    MyClassInstance->Foo("a string");

Kerrek SB · Answer 2 · 2012-10-24T13:33:13.950

The type of a ternary conditional expression is the common type two which both operands are convertible. You can definitely not perform "dynamic overload resolution", as you seem to be suggesting.

Since there is no common type for int and char const *, the code won't even compile (as you surely noticed when you tested this).

(You may be thrilled to know that the ternary conditional is used precisely because of those semantics in the implementation of the std::common_type trait class template, together with decltype.)

(If the condition is known statically, such as sizeof(int) != 7, then you can use template specialization to write similar-looking code that does perform conditional overload resolution, but of course statically.)

score 5 · Answer 3 · answered Oct 24 '12 at 13:23

5

No. To perform overload resolution the compiler will ask "what is the type of booleanValue?24:"a string"?". That question cannot be answered.

answered Oct 24 '12 at 13:23

R. Martinho Fernandes

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Cannot be answered *at compile-time* (not my down-vote BTW - just making a small suggestion to improve the answer) – Paul R Oct 24 '12 at 13:23
@Paul In C++ expressions only have types at compile-time. It does not make sense to talk about something else. – R. Martinho Fernandes Oct 24 '12 at 13:24
Yes, I know, the point is that it is *conceivable* that you could determine which function to call dynamically, i.e. at run-time, but for C++ you need to know the type at compile-time. – Paul R Oct 24 '12 at 13:26

CashCow · Answer 4 · 2012-10-24T13:32:16.790

No, this is not permitted.

Overloads are compile-time, so it cannot work in runtime that way.

It is not common in code you would want to do exactly that, however sometimes with iostream there is a desire to do something like:

os << ( condition ? var1 : var2 )

where var1 and var2 have different types. That also doesn't work.

You could do:

MyClassInstance->Foo( booleanValue ? boost::any(24) : boost::any("a string") );

Ternary operator and function signature

4 Answers4