Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p><code>make_shared</code> allocates a new <code>int</code>, and wraps a <code>shared_ptr</code> around it. This means that your two <code>shared_ptr&lt;int&gt;</code>s point to <em>different memory</em>, and since you're creating a hash table keyed on pointer value, they are distinct keys.</p> <p>For the same reason, this will result in a size of 2:</p> <pre><code>boost::unordered_set&lt;int *&gt; s3; s3.insert(new int(5)); s3.insert(new int(5)); assert(s3.size() == 2); </code></pre> <p>For the most part you can consider <code>shared_ptr</code>s to act just like pointers, including for comparisons, except for the auto-destruction.</p> <p>You could define your own hash function and comparison predicate, and pass them as template parameters to <code>unordered_map</code>, though:</p> <pre><code>struct your_equality_predicate : std::binary_function&lt;boost::shared_ptr&lt;int&gt;, boost::shared_ptr&lt;int&gt;, bool&gt; { bool operator()(boost::shared_ptr&lt;int&gt; i1, boost::shared_ptr&lt;int&gt; i2) const { return *i1 == *i2; } }; struct your_hash_function : std::unary_function&lt;boost::shared_ptr&lt;int&gt;, std::size_t&gt; { std::size_t operator()(boost::shared_ptr&lt;int&gt; x) const { return *x; // BAD hash function, replace with somethign better! } }; boost::unordered_set&lt;int, your_hash_function, your_equality_predicate&gt; s4; </code></pre> <p>However, this is probably a bad idea for a few reasons:</p> <ol> <li>You have the confusing situation where <code>x != y</code> but <code>s4[x]</code> and <code>s4[y]</code> are the same.</li> <li><p>If someone ever changes the value pointed-to by a hash key <strong>your hash will break</strong>! That is:</p> <pre><code>boost::shared_ptr&lt;int&gt; tmp(new int(42)); s4[tmp] = 42; *tmp = 24; // UNDEFINED BEHAVIOR </code></pre></li> </ol> <p>Typically with hash functions you want the key to be immutable; it will always compare the same, no matter what happens later. If you're using pointers, you usually want the pointer identity to be what is matched on, as in <code>extra_info_hash[&amp;some_object] = ...</code>; this will normally always map to the same hash value whatever <code>some_object</code>'s members may be. With the keys mutable after insertion is it all too easy to actually do so, resulting in undefined behavior in the hash.</p>
    singulars
    1. This table or related slice is empty.
    1. POC++: shared_ptr as unordered_set's key
      singulars
      1. PTQuestion
    1. PTAnswer
    1. USbdonlan
    1. USbdonlan
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. POC++: shared_ptr as unordered_set's key
      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. VTAcceptedByOriginator
    1. CORight, ofcourse. So I should be nevertheless able to achieve the same semantics with the use of (smart) pointers as well? Meaning that by defining the hash function in some specific way the size would be 1 instead of 2. How would I do that?
      singulars
      1. PO
      1. USJay
    2. CO@Jay: updated with an example, and a warning not to do so
      singulars
      1. PO
      1. USbdonlan
 

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