Databases
StackOverflow2013
Postsdbo
POtemplate metaprogramming: (trait for?) dissecting a specified template into types T<T2,T3 N,T4, ...>
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POtemplate metaprogramming: (trait for?) dissecting a specified template into types T<T2,T3 N,T4, ...>
    primarykey
    data
    text
    <p>I'm trying to deduce the underlying template type <code>T</code> from a type <code>E = T&lt;T2,T3&gt;</code>. This would for example make it possible to make a template function pair_maker(const E &amp; a) which can be used with one of several similar types of containers. Rough meta code:</p> <pre><code>template &lt;typename T&gt; auto pairmaker(const E &amp; a) -&gt; PairContents&lt;E,std::string&gt;::type { ContainerPairMaker&lt;E,std::string&gt;::type output; ... some code ... return output; } </code></pre> <p><code>PairContents&lt;E,std::string&gt;</code> </p> <p>would transform the type <code>vector&lt;int&gt;</code> into <code>vector&lt;pair(int,std::string)&gt;</code> or <code>whatever&lt;T1&gt;</code> into <code>whatever&lt;pair(T1,std::string)&gt;</code>.</p> <p>Another similar example of type dissection is for std::array (or similar containers) where I like to figure out the <em>container</em> type to make a new similar array. For example for these kind of functions (this is actual working code now)</p> <pre><code>template &lt;typename T &gt; auto make_some3(const T &amp; a) -&gt; std::array&lt;typename T::value_type,10*std::tuple_size&lt;T&gt;::value&gt;{ return std::array&lt;typename T::value_type,10*std::tuple_size&lt;T&gt;::value&gt;{} ; } </code></pre> <p>This works fine but what I'm after is to make the explicit use of 'std::array' automatic. </p> <p>For std::array there's the tuple_size trait which helps, and a similar thing can be used to find the <code>type</code> for any second argument, but again I can't think of anything for finding the container type.</p> <p>To summarize: what kind of machinery (if any) can be used for cases like these. To which extent is it possible to deal with mixes of template arguments, template-template arguments, any number of arguments, and non-template arguments of unknown types.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USJohan Lundberg
    1. USJohan Lundberg
    plurals
    1. This table or related slice is empty.
    1. PL
      singulars
      1. LTLinked
    2. PL
      singulars
      1. LTLinked
    3. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POtemplate metaprogramming: (trait for?) dissecting a specified template into types T<T2,T3 N,T4, ...>
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POtemplate metaprogramming: (trait for?) dissecting a specified template into types T<T2,T3 N,T4, ...>
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POtemplate metaprogramming: (trait for?) dissecting a specified template into types T<T2,T3 N,T4, ...>
      1. USJohan Lundberg
      1. VTFavorite
    1. COProblem is, if you *only* rebind the first parameter, standard containers will have a mismatching allocator, aka `PairContent<vector<int, allocator<int>>,float>::type` will be `vector<pair<int,float>, allocator<int>>`.
      singulars
      1. POtemplate metaprogramming: (trait for?) dissecting a specified template into types T<T2,T3 N,T4, ...>
      1. USXeo
    2. COAs a point of terminology, in `T<T2, T3>` `T` is not a type. What you're looking for is the underlying class template `T` of which `T<T2, T3>` is a specialization (and a type). Hence you can't have `extract<std::vector<int>>::type` be `std::vector`.
      singulars
      1. POtemplate metaprogramming: (trait for?) dissecting a specified template into types T<T2,T3 N,T4, ...>
      1. USLuc Danton
    3. COXeo, Ohh... That's a huge issue. Why don't you bring that into your now deleted answer?
      singulars
      1. POtemplate metaprogramming: (trait for?) dissecting a specified template into types T<T2,T3 N,T4, ...>
      1. USJohan Lundberg
 

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