OSDev.org

Solar wrote:Generally speaking (and this isn't with regards to C++ specifically, but any language) as soon as you are at the point of "if only this language X were more like language Y", you're halfway down the wrong direction of a one-way road. Define clearly what it is you want to achieve in language X. Don't try to specify it in terms of language Y, because X isn't Y. Ask yourself (or someone familiar with X) how to best achieve what you want in X. Don't be halfway down the wrong road, because that adds several things to helping you -- telling you that you are going the wrong direction, why it is the wrong direction, and leading you back to the starting point.

Instead of just telling you the best way from your starting point to where you want to go. The "languages" variant of the XY problem, so to speak.

AndrewAPrice wrote:This has annoyed me for a while. In C++ it's not possible to do this:

Code: Select all

class Dog {
 Cat TransformIntoACat();
};

class Cat {
 Dog TransformIntoADog();
};

I can't declare "class Cat;" at the top, because TransformIntoACat() returns Cat as a value.

class Cat;

class Dog {
public:
    Cat TransformIntoACat();
};

class Cat {
public:
    Dog TransformIntoADog();
};

Cat Dog::TransformIntoACat() {
    return Cat();
}

Dog Cat::TransformIntoADog() {
    return Dog();
}

int main(int argc, char **argv) {
    return 0;
}

kzinti wrote:You probably don't want to return Cats and Dogs by value, this would be inefficient for any non-trivial class.

class Cat
{
    Cat(Dog&& dog);
};

PeterX wrote: Very true. I once tried to implement a loop in Scheme... (It's impossible, solution: tail recursion.)

(define-syntax while
  ; a simple while loop
  (syntax-rules ()
    ((while condition expr1 expr2 ...)
     (let loop ()
       (if condition
           (begin expr1 expr2 ... (loop)))))))

> (define x 1)

> (while (< x 5)
    (display x)
    (newline)
    (set! x (+ 1 x)))

--> 1 2 3 4

Velko wrote:That's what move constructors are for. (..)You ensure that you will not keep the original Dog. Note that it should take a non-const reference, as the source might need modifications to be "finalized" properly.

vvaltchev wrote:Let's forget about design patterns etc. Actually, in C++ you can do that.
Check the example on compiler explorer: https://gcc.godbolt.org/z/KKfhTT

Code: Select all

class Cat;

class Dog {
public:
    Cat TransformIntoACat();
};

class Cat {
public:
    Dog TransformIntoADog();
};

Cat Dog::TransformIntoACat() {
    return Cat();
}

Dog Cat::TransformIntoADog() {
    return Dog();
}

int main(int argc, char **argv) {
    return 0;
}

It compiles without any problems. Just, don't implement the methods in-line.

AndrewAPrice wrote:You are right! Now I'm very embarrased.

PeterX wrote:Very true. I once tried to implement a loop in Scheme... (It's impossible, solution: tail recursion.)