"Conditional Expressions" on this page covers the ternary operator as well:

  http://digitalmars.com/d/2.0/expression.html#ConditionalExpression

It says "the second and third expressions are implicitly converted to a common type which becomes the result type of the conditional expression."

How "common" should the "common type" be? Wouldn't you expect the following ternary operator's result be I, instead of Object?

interface I {}
class A : I {}
class B : I {}

void foo(I) {}

void main()
{
    bool some_condition;
    foo(some_condition ? new A : new B);  // <-- compiler error
}

Compiler error:

Error: function deneme.foo (I _param_0) is not callable using argument types (Object)
Error: cannot implicitly convert expression (some_condition ? new A : new B) of type object.Object to deneme.I

Ali

Ali Çehreli wrote:
> "Conditional Expressions" on this page covers the ternary operator as well:
> 
>   http://digitalmars.com/d/2.0/expression.html#ConditionalExpression
> 
> It says "the second and third expressions are implicitly converted to a common type which becomes the result type of the conditional expression."
> 
> How "common" should the "common type" be? Wouldn't you expect the following ternary operator's result be I, instead of Object?
> 
> interface I {}
> class A : I {}

My expectation is that the hierarchy of A should look like this:

Object
 |
 I
 |
 A

For me, Object should always be at the top. I know that interfaces can not inherit from classes; but as now the interfaces may have implementations, perhaps it's time to make Object an interface, as opposed to a class?

> class B : I {}
> 
> void foo(I) {}
> 
> void main()
> {
>     bool some_condition;
>     foo(some_condition ? new A : new B);  // <-- compiler error
> }
> 
> Compiler error:
> 
> Error: function deneme.foo (I _param_0) is not callable using argument types (Object)
> Error: cannot implicitly convert expression (some_condition ? new A : new B) of type object.Object to deneme.I

Thank you,
Ali

On 05/24/2010 06:36 PM, Ali Çehreli wrote:
> Ali Çehreli wrote:
>> "Conditional Expressions" on this page covers the ternary operator as
>> well:
>>
>> http://digitalmars.com/d/2.0/expression.html#ConditionalExpression
>>
>> It says "the second and third expressions are implicitly converted to
>> a common type which becomes the result type of the conditional
>> expression."
>>
>> How "common" should the "common type" be? Wouldn't you expect the
>> following ternary operator's result be I, instead of Object?
>>
>> interface I {}
>> class A : I {}
>
> My expectation is that the hierarchy of A should look like this:
>
> Object
> |
> I
> |
> A

interfaces are not objects, so crush your expectations.

Question: what should happen with

class A : I, J {}
class B : I, J {}

?

Ellery Newcomer wrote:
> On 05/24/2010 06:36 PM, Ali Çehreli wrote:
>> Ali Çehreli wrote:
>>> "Conditional Expressions" on this page covers the ternary operator as
>>> well:
>>>
>>> http://digitalmars.com/d/2.0/expression.html#ConditionalExpression
>>>
>>> It says "the second and third expressions are implicitly converted to
>>> a common type which becomes the result type of the conditional
>>> expression."
>>>
>>> How "common" should the "common type" be? Wouldn't you expect the
>>> following ternary operator's result be I, instead of Object?
>>>
>>> interface I {}
>>> class A : I {}
>>
>> My expectation is that the hierarchy of A should look like this:
>>
>> Object
>> |
>> I
>> |
>> A
>
> interfaces are not objects, so crush your expectations.

I guess this is how it actually is:

I Object
\ /
 A

> Question: what should happen with
>
> class A : I, J {}
> class B : I, J {}

Good point. I think the error message threw me off:

Error: cannot implicitly convert expression (some_condition ? new A : new B) of type object.Object to deneme.I

The compiler apparently does go up in the hierarchy, but it favors the Object branch. (C++ does not try to find a common ancestor; stays with the types A and B.)

Ali