Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>You said that "creating one a second time would damage the whole application.". My response is: so don't make more then one. In C++, the type system is too weak to easily ensure this at compile-time. We can still write up a utility to approximate it at run-time, though.</p> <p>Note, though, that this in no way implies you should use a singleton. (You have zero need for a global; it's unfortunate the drones have associated single-instance with global). What you want is this:</p> <pre><code>#include &lt;stdexcept&gt; // inherit from this class (privately) to ensure only // a single instance of the derived class is created template &lt;typename D&gt; // CRTP (to give each instantiation its own flag) class single_instance { protected: // protected constructors to ensure this is used as a mixin single_instance() { if (mConstructed) throw std::runtime_error("already created"); mConstructed = true; } ~single_instance() { mConstructed = false; } private: // private and not defined in order to // force the derived class be noncopyable single_instance(const single_instance&amp;); single_instance&amp; operator=(const single_instance&amp;); static bool mConstructed; }; template &lt;typename T&gt; bool single_instance&lt;T&gt;::mConstructed = false; </code></pre> <p>Now you get an exception if the class is constructed more than once:</p> <pre><code>class my_class : private single_instance&lt;my_class&gt; { public: // usual interface (nonycopyable) }; int main() { my_class a; // okay my_class b; // exception } </code></pre>
    singulars
    1. This table or related slice is empty.
    1. POA Singleton that is not globally accessible
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USGManNickG
    1. USGManNickG
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POA Singleton that is not globally accessible
      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. VTAcceptedByOriginator
    3. VO
      singulars
      1. PO
      1. This table or related slice is empty.
      1. VTUpMod
    1. COVery nice! I think that fits my need pretty perfectly. Thanks a lot!
      singulars
      1. PO
      1. UStyrondis
    2. CO@Benjamin: No problem.
      singulars
      1. PO
      1. USGManNickG
    3. CO@GMan: don't bother with `atomic`, the code is not thread-safe anyway. You can still create multiple instances with the right timing when in a multi-threaded situation (you would need some form of `test-and-set` to prevent this).
      singulars
      1. PO
      1. USMatthieu M.
 

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