two classes with methods that return each other by value

This is a discussion on two classes with methods that return each other by value within the C++ Programming forums, part of the General Programming Boards category; i would like to achieve this: Code: class Row { public: static Vector transpose(const Row& r); }; class Vector { ...

  1. #1
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    two classes with methods that return each other by value

    i would like to achieve this:
    Code:
    class Row
    {
    public:
        static Vector transpose(const Row& r);
    };
    class Vector
    {
    public:
        static Row transpose(const Vector& v);
    };
    but i know of no way to do this. is it possible?

  2. #2
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    Where exactly are you experiencing an issue? If it's just with getting the compiler to process it, you could use a forward declaration of Vector above Row - although I think you'll then have to return a pointer/reference instead of a fully-fledged object. Alternatively, declare them as functions outside of (and below) the class definitions and you can have them both return objects.

    On another note - why have you declared both transpose() methods to be static? If they both act on a single instance of their own class, it would make sense to make them both instance methods... no?

  3. #3
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by m37h0d
    but i know of no way to do this. is it possible?
    Yes. Recall that you do not have to define member functions inline in the class definition.
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    Quote Originally Posted by laserlight View Post
    Yes. Recall that you do not have to define member functions inline in the class definition.
    right but if i want to return by value, the full type definition has to be available, correct? and it becomes a 'chicken-and-egg' problem where only 1 can have the definition of the other - that's the thrust of what i'm asking about.

    if i return by an indirect type, then i have to return dynamically allocated memory.

  5. #5
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by m37h0d
    right but if i want to return by value, the full type definition has to be available, correct? and it becomes a 'chicken-and-egg' problem where only 1 can have the definition of the other - that's the thrust of what i'm asking about.
    The "full type definition" does not need to include the definition of member functions. For example:
    Code:
    class Vector;
    
    class Row
    {
    public:
        static Vector transpose(const Row& r);
    };
    
    class Vector
    {
    public:
        static Row transpose(const Vector& v);
    };
    
    Vector Row::transpose(const Row& r)
    {
        return Vector();
    }
    
    Row Vector::transpose(const Vector& v)
    {
        return Row();
    }
    
    int main()
    {
        Vector v;
        Row r(Vector::transpose(v));
        v = Row::transpose(r);
    }
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  6. #6
    The larch
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    Yes, argument and return types may be incomplete in function declarations. For example, within a class definition the class itself is incomplete (but it is complete in methods defined inline):

    Code:
    template <unsigned> struct X {};
    
    struct Y
    {
       void foo(X<sizeof(Y)>); //error, Y is still incomplete 
       Y bar(Y); //OK, argument or return types may be incomplete
    };
    I might be wrong.

    Thank you, anon. You sure know how to recognize different types of trees from quite a long way away.
    Quoted more than 1000 times (I hope).

  7. #7
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    thanks anon and laserlight. this misconception had been kicking around for years, and it was only now that i had need to dispel it.

    Quote Originally Posted by JohnGraham View Post
    On another note - why have you declared both transpose() methods to be static? If they both act on a single instance of their own class, it would make sense to make them both instance methods... no?
    they are equivalent, i personally think the static function is cleaner syntactically, but it's just a matter of taste.

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