Re: latest constructor changesets

[John W. Eaton]

On 9/10/19 11:40 AM, Rik wrote:

> One of the latest commit messages was "use default or deleted 
> ctors/dtors/assignment ops in liboctave/util".  This is just about code 
> readability, correct?  Replacing hand-rolled constructors with "= 
> default" where there was in fact nothing different from what the 
> compiler would automatically generate. There shouldn't be any 
> performance changes over what we had before?

Yes, I've been making changes like that because I think it makes the 
code easier to read and maintain.  I wouldn't expect any change in 
performance.  I think it's best to explicitly state that the compiler 
should generate these functions where possible.

As an example of the kind of error that can happen when writing them by 
hand, I noticed one case in which the existing copy constructor and 
assignment operator were not copying all class data members.  I couldn't 
see any reason to omit any of them but it took some time to try to 
determine whether the omission was intentional.  I eventually decided 
that it was more likely that some data members were added but the copy 
and assignment functions were not updated.  Having the compiler generate 
these functions avoids that error and makes it easier to understand 
immediately what the functions will do.  If they are written by hand, 
you have to scan the list of data members that are copied in the 
functions and compare with the class declaration to be sure that 
everything is properly copied.

I'm also switching to using "= delete" declarations instead of declaring 
copy constructors and assignment operators as private.  Using "= delete" 
will always give a compile time error when a copy is requested instead 
of a warning about "use of a private function in this context" or a 
linker error if a copy is requested inside another member function. 
Though we did try to mark these private declarations with a "No 
copying!" comment, the "= delete" declaration leaves no room for 
confusion about whether the function definitions were accidentally omitted.

jwe