Undefined behaviour when copying dynamic_format_arg_store

[lucpelletier]

The copy constructor of dynamic_format_arg_store completely ignores the dynamic_args_ private member variable. This can unexpectedly cause undefined behaviour when a dynamic_format_arg_store instance is copied, since anything stored in dynamic_args_ will not get copied over. See the following for an example:

https://godbolt.org/z/8n7rnM55E

I can think of a couple of ways of fixing this problem, and I'd be happy to provide a PR with a fix, but I first wanted to get input from library maintainers on what would be the best solution.

[vitaut]

The easiest solution is to mark the copy ctor as deleted. A PR would be welcome.

[lucpelletier]

#2432 added the copy constructor and seemed to have a use case for it. Are you saying this is no longer required and the copy ctor should now be removed?

[vitaut]

I think we should disable the copy ctor because it's currently broken. If someone has a use case they can contribute a fix later.

[vitaut]

Disable the broken copy ctor in be51ee1. I don't think there is much reason to copy dynamic_format_arg_store at all, it's better to copy the actual arguments and construct the argument list only before a formatting function call.

[vitaut]

Thanks for reporting.

[spyridon97]

@vitaut The actual arguments cannot be copied from an dynamic_format_arg_store (there is no public fuction for such functionality) so i believe the copy constructor is the way to go.

I did not copy the dynamic_args_ inside dynamic_format_arg_store because dynamic_arg_list also does not have a copy constructor. I don't think it's possible to add one, am i right?

[spyridon97]

@vitaut Any suggested solution?

[vitaut]

Any suggested solution?

I see you've submitted a PR (#2832) so let's discuss there.