C++ Logo

STD-PROPOSALS

Advanced search

Subject: Re: [std-proposals] user copy constructor
From: connor horman (chorman64_at_[hidden])
Date: 2019-12-12 15:03:09


I would agree. As it stands, the way you would do this now would lead
to many special cases, with a class created solely so you can
manipulate the copy-behavior of a single field in a single class for a
single special case.
Having some method to specify that only a particular set of fields
would not be copied, and instead be constructed in some specific way
would be useful.

On Thu, 12 Dec 2019 at 15:55, Jake Arkinstall via Std-Proposals
<std-proposals_at_[hidden]> wrote:
>
> Even then, the flexibility of the initial suggestion is that a custom value can be set upon copy. For example, the user could specify not just a default value for the no-copy value, but also refer to the copied object, e.g.
>
> struct x{
> // other vars
> foo* f;
> x(x const& other)
> :!
> f{new foo(*other.f)}
> {}
> };
>
> I'd prefer a wildcard syntax, though, e.g. *{*} (but prettier), so there is something there telling the reader that other members automatically copy.
>
> An alternative, though one that would only work with pointers, is a version of unique_ptr that allows copies when it holds any object with a copy constructor. That isn't much use for other resources, mind.
>
>
> On Thu, 12 Dec 2019, 20:27 Stephan Reiter via Std-Proposals, <std-proposals_at_[hidden]> wrote:
>>
>> Can you not just mark that no_copy field as ... no copy?
>>
>> The following code is not complete, but illustrates the idea.
>>
>> #include <iostream>
>> #include <string>
>>
>> template <typename T>
>> struct NoCopy : public T
>> {
>> template <typename... Args>
>> NoCopy(Args &&... args) : T(std::forward<Args>(args)...) {}
>>
>> NoCopy(const NoCopy &) : NoCopy() {}
>> };
>>
>> struct Class
>> {
>> std::string a;
>> NoCopy<std::string> b;
>> };
>>
>> int main()
>> {
>> Class x;
>> x.a = "foo";
>> x.b = "bar";
>> Class y = x;
>> std::cout << x.a << x.b << "\n";
>> std::cout << y.a << y.b << "\n";
>> }
>>
>> Am Do., 12. Dez. 2019 um 21:17 Uhr schrieb Mario Charest via
>> Std-Proposals <std-proposals_at_[hidden]>:
>> >
>> > Hello Everyone,
>> >
>> > Sorry if this has been discuss previously.
>> >
>> > Imagine a class with 20 variables, out of these variable one should not be copied by the copy constructor
>> >
>> > class Foo {
>> > std::vector<int> first;
>> > std::list<int> second;
>> > ...
>> > void *no_copy;
>> >
>> > Foo (const Foo &);
>> > }
>> >
>> > Writing the Foo copy constructor is detious, error prone and leads to higher maintenance. But what if one could create the copy constructor in such a way that it would tell the compiler to default copy construct all variables, except one (or more)
>> >
>> > Foo::Foo(const Foo &) :! no_copy(nullptr)
>> > {
>> > }
>> >
>> > Notice the:!
>> >
>> > Did not put much though into the details, but wanted to first get a feel if this make sense.
>> >
>> > - Regards
>> >
>> > --
>> > Std-Proposals mailing list
>> > Std-Proposals_at_[hidden]
>> > https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>> --
>> Std-Proposals mailing list
>> Std-Proposals_at_[hidden]
>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals


STD-PROPOSALS list run by std-proposals-owner@lists.isocpp.org

Standard Proposals Archives on Google Groups