DIP 1018--The Copy Constructor--Community Review Round 1
ismailsiege at gmail.com
Fri Dec 21 14:43:50 UTC 2018
On Tuesday, 18 December 2018 at 14:51:39 UTC, Mike Parker wrote:
> This is the feedback thread for the first round of Community
> Review for DIP 1018, "The Copy Constructor"
1). I do not like the ability to specify a mutable copy source.
Under no circumstance should the code like
return a; // lowered to return tmp.copyCtor(a)
A b = fun(); // the return value of fun() is moved to the
location of b
be allowed to modify the value of a. This is an absolute pain to
read\debug, and I would instead like to see a mandatory
const\immutable on the source reference.
Copy operation should not modify the source, or it should not be
called a copy. If we are talking about a typical smart pointer
struct (Refcounted), copy still should not modify the source. It
is D's transitive const\immutable that is a problem here and it
must be explicitly casted away by the developer.
Only const also mitigates the combinatorial mess caused by 4
combinations of mutable\immutable copy constructors, since
immutable is implicitly converted to const.
2). "A declaration is a copy constructor declaration if it is a
constructor declaration that takes only one non-default parameter
by reference that is of the same type as typeof(this), followed
by any number of default parameters..."
If you need other parameters, you are not performing a copy. Copy
constructor needs no additional parameters. If the semantics of
your domain problem involve parametrized post-copy operations,
the code should state that explicitly - by using specialized
properly-named methods, that notify the reader about this
3). Section "Copy constructor usage", I don't understand the
A b = a; // copy constructor gets called
b = a; // assignment, not initialization
a = fun(); // NRVO - no copy constructor call
A b; // b is initialized after this semicolon.
// why is this one not an assignment, but initialization?
// do we have the difference formally and consistently
defined for structs?
b = gun(); // NRVO cannot be performed, copy constructor
More information about the Digitalmars-d