Re: as_ly_scm_list

[Jean Abou Samra]

> Le 25 juil. 2022 à 16:55, Dan Eble <dan@lyric.works> a écrit :
> 
> On Jul 24, 2022, at 19:04, Jean Abou Samra <jean@abou-samra.fr> wrote:
>> 
>> I understand that ly_scm_list takes an rvalue and disallows
>> lvalues for clarity, while as_ly_scm_list takes an lvalue,
>> and will fail on an rvalue by design, with the failure being
>> a compilation failure since e67154f7e330b61c5d9a973fbb89bd56866a148e.
>> 
>> But I don't understand the real difference that distinguishes
>> them. Before that commit, what would have been the difference
>> between as_ly_scm_list (lvalue) and ly_scm_list (lvalue)?
> 
> The difference is ownership of the head.
> 
> ly_scm_list has one data member:
> 
>    private:
>      SCM head_ = SCM_EOL;
> 
> Creating an instance of ly_scm_list creates this head.  If a list operation 
> modifies this head, no other SCM is modified.
> 
>    ly_scm_list x (my_things_);  // A. x.head_ = my_things_
>    ...
>    x.clear ();                  // B. my_things_ is unchanged
> 
> (B) might or might not be a problem.  Without a comment, it can require deep 
> review to tell whether leaving my_things_ alone was an error or not.
> 
> Forbidding (A) means that people can't write such code, but there are times 
> where we do want to work with a SCM like a ly_scm_list, including operations 
> that change the head SCM.
> 
> as_ly_scm_list casts a SCM to a ly_scm_list.  There is no separate head 
> because there is no new ly_scm_list, just a reinterpretation of the given SCM.
> 
>    auto& x = as_ly_scm_list (my_things_);  // C. 
>    …
>    x.clear ();                             // D. clears my_things_
> 
> Hope that helps.



Thanks, that helps.

Now, the next question. Suppose I just want to iterate over the list, nothing 
else. (This is actually the only case I have found myself in.) Suppose I get 
this list from an lvalue. The compiler won’t let me use ly_scm_list (lvalue). 
Is there a way to use ly_scm_list nevertheless and not as_ly_scm_list in order 
not to put the reader in a mode where they look for in-place modifications of 
the SCM lvalue?

(Maybe that’s related to std::move? I can’t test what it does, as I am in 
vacation, and I never understood all the rules around lvalues, rvalues, 
xvalues, prvalues etc)