Databases
StackOverflow2013
Postsdbo
POConst correctness with temporary instances
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POConst correctness with temporary instances
    primarykey
    data
    text
    <p>Here is the example of the "scoped lock" idiom with common mistake: no local variable is created, so lock is not in effect. This code compiles flawlessly both with VC++ 2010 and Comeau C++ online:</p> <pre><code>class Mutex { public: void lock() {} }; class ScopedLock { public: ScopedLock() : m_pm(0) {} ScopedLock(Mutex&amp; m) : m_pm(&amp;m) { m_pm-&gt;lock(); } private: Mutex* m_pm; private: ScopedLock&amp; operator =(const ScopedLock&amp;); ScopedLock(const ScopedLock&amp;); }; class X { public: void foo() const { ScopedLock(m_mutex); } private: Mutex m_mutex; }; int main() { X x1; x1.foo(); } </code></pre> <p>If default constructor for ScopedLock is commented out, then both compilers give an error:</p> <blockquote> <p>error C2512: 'ScopedLock' : no appropriate default constructor available</p> </blockquote> <p>(When <code>ScopedLock</code> used correctly, i.e. local variable is created: <code>ScopedLock guard(m_mutex);</code>, then compilation fails as expected. Declaring <code>m_mutex</code> as mutable fixes the problem.)</p> <p>I have two questions:</p> <ol> <li><p>Why <code>X::foo</code> compiles? It seems that compiler was able to cast <code>const Mutex&amp;</code> to <code>Mutex&amp;</code> somehow.</p></li> <li><p>What role plays <code>ScopedLock</code> default constructor, so the compilation succeeds?</p></li> </ol> <p>Thanks.</p> <p><strong>Update:</strong> I found the answer. It appears that <code>ScopedLock(m_mutex);</code> statement creates a local variable <code>m_mutex</code> of type <code>ScopedLock</code>. Not a temporary. That's why <code>ScopedLock::ScopedLock</code> default constructor is required.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USAlex Blekhman
    1. USAlex Blekhman
    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. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POConst correctness with temporary instances
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POConst correctness with temporary instances
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POConst correctness with temporary instances
      1. This table or related slice is empty.
      1. VTUpMod
    1. This table or related slice is empty.
 

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