Hi all,
I am reading Programming in D online book. There is a paragraph in the chapter More Templates.

typeof(return) generates the return type of a function, inside that function.
For example, instead of defining the calculate() function above as an auto function, we can be more explicit by replacing auto with LargerOf!(A, B) in its definition. (Being more explicit would have the added benefit of obviating at least some part of its function comment.)

And it shows this code.

LargerOf!(A, B) calculate(A, B)(A a, B b) {
    typeof(return) result;    // The type is either A or B
    // ...
    return result;
}

The author says LargerOf!(A, B) is used instead of auto keyword. How did compiler understands the return type from LargerOf!(A, B).

On Saturday, 26 March 2022 at 18:25:54 UTC, Vinod K Chandran wrote:

Oh Sorry !. I forgot the LargerOf!(A, B) definition which is in the same chapter. My fault. Sorry.

On 3/26/22 2:25 PM, Vinod K Chandran wrote:

typeof(return) generates the return type of a function, inside that function.
For example, instead of defining the calculate() function above as an auto function, we can be more explicit by replacing auto with LargerOf!(A, B) in its definition. (Being more explicit would have the added benefit of obviating at least some part of its function comment.)

LargerOf!(A, B) calculate(A, B)(A a, B b) {
     typeof(return) result;    // The type is either A or B
     // ...
     return result;
}

Not sure what the question here is, typeof(return) is the type that is returned from the function.

Where it is useful is if you don't really know what the return type is (i.e. an auto function) and you need to declare a variable of that type.

In this case, typeof(return) is really Larger!(A, B) which is likely aliased to either A or B.

The equivalent here is:

LargerOf!(A, B) result;

But the spirit of it is, "I don't want to have to think about whether it's A or B, just whatever it is, declare a variable of it here".

Hope that helps.

And yes, you can get into paradoxical problems like:

auto foo()
{
   typeof(return) x;
   return x;
}

which will not compile.

-Steve

On Sunday, 27 March 2022 at 01:11:02 UTC, Steven Schveighoffer wrote:

Thanks for the reply. Actually, my problem was this, I forgot the presence of LargerOf!(A, B) template function in that chapter. When I see it in a function, I thought where is the implementation of this function ? But later I found it.

auto foo()
{
   typeof(return) x;
   return x;
}

Yeah, I got the point. In fact, after reading the Template chapter in that book, I am amazed. There are lot of possibilities in D templates.