Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>you could look at the visitor pattern. this is the general solution to 'double dispatch' type problems (dispatch to a virtual function based on the object <em>and</em> the message). </p> <p>That is, put <code>foo()</code> into a visitor, and rename <code>A::foo()</code> to <code>A::Visit(FooVisitor&amp; )</code>:</p> <p>edit: to clarify, it might help to disentangle the purposes of your hierarchies. If you think about it, you are trying to model the relationship of one hierarchy (<code>AA</code> and <code>BB</code>) in terms of another (<code>A</code> and <code>B</code>). This is pretty awkward to model, or even to think about conceptually. </p> <p>To remodel this as a visitor, you'd generally turn one of the hierarchies into a single class, and instead model the <em>operations</em> you can perform on that class in a hierarchy of algorithms via a visitor. This is more robust because it forces you to explicitly implement each combination of hierarchy relationships, and will break at compile-time (good) if you modify the hierarchy later on. </p> <pre><code>class A; class B; struct AVisitor { virtual ~AVisitor() { } virtual void Visit(A&amp; ) = 0; virtual void Visit(B&amp; ) = 0; }; class A { public: virtual ~A() { } virtual void Visit(AVisitor &amp; visitor) { visitor.Visit(*this); } }; class B : public A { public: virtual void Visit(AVisitor &amp; visitor) { visitor.Visit(*this); } }; struct PrintingVisitor : public AVisitor { void Visit(A&amp; a){cout&lt;&lt;"A::foo"&lt;&lt;endl;} void Visit(B&amp; b){cout&lt;&lt;"B::foo"&lt;&lt;endl;} }; int main() { B b; PrintingVisitor printer; b.Visit(printer); } </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POOverride contra-variance workaround needed
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USCechner
    1. USCechner
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThis solution doesn't translate well into the domain of my problem since in my case classes derived from A are in themselves algorithmic objects and foo is their main function which runs the algorithm on an input of type derived from AA. Removing the main algorithm from them nullifies the use of that class hierarchy completely. Further more there will be no more than a single implementation of foo for each child of A, so I will never create more than one child of AVisitor
      singulars
      1. PO
      1. USeladidan
    2. COfair enough - if you don't want to abstract away the foo operation the visitor loses much of its usefulness. If you don't want to treat the algorithms polymorphically though, inheritance is probably not the right approach... perhaps something template-y along the lines of grep's answer above is more suited
      singulars
      1. PO
      1. USCechner
    3. CO@eladian, have you considered making the `A` hierarchy [Strategies](http://en.wikipedia.org/wiki/Strategy_pattern)? That is, `FooStrategyA` and `FooStrategyB`, that are passed to the `BB` ctor and used directly by `BB`?
      singulars
      1. PO
      1. USCechner
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload