Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>You can do it with a template.</p> <p>Edit: new implementation based on the extra information. If <code>mymap</code> is a <code>std::map&lt;int, Shape*&gt;</code>:</p> <pre><code>template &lt;typename T&gt; void lookup(int idNum, T* &amp;ptr) { auto it = mymap.find(idNum); if (it == mymap.end()) { ptr = 0; } else { ptr = dynamic_cast&lt;T*&gt;(*it); // Shape must have a virtual member function } } </code></pre> <p>Or you might prefer:</p> <pre><code>template &lt;typename T&gt; T* lookup(int idNum) { auto it = mymap.find(idNum); if (it == mymap.end()) { return 0; } return dynamic_cast&lt;T*&gt;(*it); } </code></pre> <p>Then call it like <code>Circle *circle = database.lookup&lt;Circle&gt;(123);</code></p> <p>Obviously polymorphic containers are a whole heap of fun in themselves, but I'll assume you have that sorted. There may well be a <code>shared_ptr</code> in there somewhere that I've left out.</p> <p>Old implementation when I thought the DB might store copies of POD:</p> <pre><code>template &lt;typename T&gt; void lookup(int idNum, T* &amp;ptr) { void *theresult = // something based on idNum // some check needed here that theresult really is the right type. // how you do this depends on the database, but suppose that // the database gives us some integer "type" which indicates the type if (type != type_constant&lt;T&gt;::value) { ptr = 0; } else { ptr = static_cast&lt;T*&gt;(theresult); } } </code></pre> <p>type_constant is an example of "type traits", you implement it like:</p> <pre><code>template &lt;typename T&gt; struct type_constant {}; template &lt;&gt; struct type_constant&lt;Circle&gt; { static const int value = 1; }; template &lt;&gt; struct type_constant&lt;Rectangle&gt; { static const int value = 2; }; // etc... </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POIs it possible to take a type as an argument in a function?
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USSteve Jessop
    1. USSteve Jessop
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POIs it possible to take a type as an argument in a function?
      singulars
      1. PTQuestion
    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. COShould we not be using dynamic_cast<> to cast towards children types. (ie from Shape* to Circle*)?
      singulars
      1. PO
      1. USMartin York
    2. COMaybe. I don't know that this database actually returns - if it's a `Shape*` whose referand is a subclass, and if `Shape*` has any virtual member functions, then yes. If the database itself isn't object-based, or then someone somewhere has to do something based on a record that indicates type. I can't tell from the question whether `Shape*` is an actual base class, or just something that the questioner mentioned thinking it might be part of the solution.
      singulars
      1. PO
      1. USSteve Jessop
    3. COI didn't know about `type_constant`, and I'll likely just look at doing the `dynamic_cast` as Martin mentioned. Bit of a facepalm for not thinking templates right away.
      singulars
      1. PO
      1. USspbots
 

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