Databases
StackOverflow2013
Postsdbo
POForce a compile time error if std::move will result in an unintended copy?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POForce a compile time error if std::move will result in an unintended copy?
    primarykey
    data
    text
    <p>In his GoingNative 2013 talk, Scott Meyers pointed out that <code>std::move</code> is no guarantee that the generated code will actually perform a move.</p> <p>Example:</p> <pre><code>void foo(std::string x, const std::string y) { std::string x2 = std::move(x); // OK, will be moved std::string y2 = std::move(y); // compiles, but will be copied } </code></pre> <p>Here, the move constructor cannot be applied but because of overload resolution, the normal copy constructor will be used instead. This fallback option may be crucial for backward compatibility with C++98 code, but in the example above it is most likely not what the programmer intended.</p> <p><strong>Is there a way to enforce that a move constructor will be called?</strong></p> <p>For example, assume that you want to move a huge matrix. If your application really depend on the Matrix to be moved, it would be great to immediately get a compile error if a move is not possible. (Otherwise, you the performance problem may slip easily through unit tests and you will only find out after some profiling.)</p> <p>Lets call this guaranteed move <code>strict_move</code>. I would like to be able to write code like this:</p> <pre><code>void bar(Matrix x, const Matrix y) { Matrix x2 = strict_move(x); // OK Matrix y2 = strict_move(y); // compile error } </code></pre> <p>Is it possible?</p> <p><strong>Edit:</strong></p> <p>Thanks for the great answers! There were some legitimate requests to clarify my question:</p> <ul> <li>Should <code>strict_move</code> fail if the input is const?</li> <li>Should <code>strict_move</code> fail if the result will not lead to an actual move operation (even though the copy might be as fast as a move, e.g., <code>const complex&lt;double&gt;</code>)?</li> <li>Both?</li> </ul> <p>My original idea was very vague: I considered Scott Meyers examples quite alarming, so I wondered if it is possible to have the compiler prevent such unintended copies.</p> <p>Scott Meyers mentioned in his talk that a general compiler warning is not an option as it would result in a huge number a false positives. Instead I want to communicate to the compiler something like "I'm 100% sure that this must always resulting in a move operation and a copy is too expensive for this specific type".</p> <p>Thus, I would have offhandedly said that <code>strict_move</code> should fail in both cases. Meanwhile I'm not sure what would be best. Another aspects that I didn't consider is <code>noexcept</code>.</p> <p>From my side, the exact semantics of <code>strict_move</code> are open. Everything that helps to prevent some dumb mistakes at compile time without having serious drawbacks is fine.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USPhilipp Claßen
    1. USPhilipp Claßen
    plurals
    1. This table or related slice is empty.
    1. 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. POForce a compile time error if std::move will result in an unintended copy?
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POForce a compile time error if std::move will result in an unintended copy?
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POForce a compile time error if std::move will result in an unintended copy?
      1. This table or related slice is empty.
      1. VTUpMod
    1. COMake your own version of `move` that checks the return type?
      singulars
      1. POForce a compile time error if std::move will result in an unintended copy?
      1. USKerrek SB
    2. COWait, did you want to check that the input isn't const, or did you want to check that an actual move will happen. Because the second is a bad idea, and the answers below ignore that interpretation.
      singulars
      1. POForce a compile time error if std::move will result in an unintended copy?
      1. USMooing Duck
    3. CO@MooingDuck Interesting, I haven't thought about that difference in detail, I just wanted to have a safety net against unintentional behavior. I agree that preventing moving `const` object is safer, but what are the arguments against additionally checking that a move constructor is available and would be called? I can understand that using such a construct in library code could make the code inflexible as it excludes all classes without move constructors and might cause problems with conversions. But what are the downside of using it only in a performance critical part of your own code base?
      singulars
      1. POForce a compile time error if std::move will result in an unintended copy?
      1. USPhilipp Claßen
 

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