Databases
StackOverflow2013
Postsdbo
PO
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PO
    primarykey
    data
    text
    <p>The practical reason why this code often works is that anything which breaks this tends to be optimized out in release/optimized-for-performance builds. However, any compiler setting that focuses on finding errors (such as debug builds) is more likely to trip on this.</p> <p>In those cases, your assumption ("note, that <code>Derived</code> shouldn't carry any additional data anyway") doesn't hold. It definitely should, to facilitate debugging.</p> <p>A slightly more complicated example is even trickier:</p> <pre><code>class Base { public: int data_; virtual void bar() { std::cout &lt;&lt; "Base\n"; } }; class Derived : public Base { public: void fun() { ::std::cout &lt;&lt; "Hi, I'm " &lt;&lt; this &lt;&lt; ::std::endl; } virtual void bar() { std::cout &lt;&lt; "Derived\n"; } }; int main() { Base base; Derived *derived = static_cast&lt;Derived*&gt;(&amp;base); // Undefined behavior! derived-&gt;fun(); derived-&gt;bar(); } </code></pre> <p>Now a reasonable compiler may decide to skip the vtable and statically call <code>Base::bar()</code> since that's the object you're calling <code>bar()</code> on. Or it may decide that <code>derived</code> must point to a real <code>Derived</code> since you called <code>fun</code> on it, skip the vtable, and call <code>Derived::bar()</code>. As you see, both optimizations are quite reasonable given the circumstances.</p> <p>And in this we see why Undefined Behavior can be so surprising: compilers can make incorrect assumptions following code with UB, even if the statement itself is compiled right.</p>
    singulars
    1. This table or related slice is empty.
    1. POHow can C++ compiler optimize incorrect hierarchical downcast to cause real undefined behavior
      singulars
      1. PTQuestion
    1. PTAnswer
    1. This table or related slice is empty.
    1. USMSalters
    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. This table or related slice is empty.
    1. COI haven't seen any problems with debug configurations as well.
      singulars
      1. PO
      1. USanxieux
    2. COIt works because the vtable pointers land in the same place. Memory layout of Base and Derived is identical, so they're "unsafely compatible". If derived is virtual but Base not then it will crash, but break the "Derived shouldn't carry any additional data" requirement.
      singulars
      1. PO
      1. USAgent_L
    3. CO@Agent_L: That assumes the compiler uses the vtable. As I noted, a smart compiler skips the vtable access since it's unnecessary.
      singulars
      1. PO
      1. USMSalters
 

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