Re: lambda expressions (was: Re: error with d28016d16e9a ...)

On 10/02/2020 05:23 AM, John W. Eaton wrote:
> On 10/1/20 8:13 PM, Rik wrote:
>> On 10/01/2020 12:42 PM, John W. Eaton wrote:
>
>>> We could also decide to always use "[=]" when we need to capture
>>> something.  Then all variables that are needed, including "this" will
>>> be captured using the default rules for capture by value.
>>
>> I don't think this is recommended practice.  The temporary anonymous
>> struct that is created to represent the lambda expression will then have
>> an argument in the constructor for every existing variable in the
>> surrounding function.  The compiler *might* then optimize out all of the
>> additional unused captures, but I think it would be better to just
>> capture what you need either by value "[=variable_name]" if small like a
>> built-in type or a pointer or by reference "[&variable_name]" if it is
>> something large like the instance of a class.  For reference, I was
>> using this
>> https://dzone.com/articles/all-about-lambda-functions-in-cfrom-c11-to-c17.
>
> As I understand it, the lambda expression only captures variables that
> are used.  Using a capture default specification actually seems better to
> me because the variables are already listed in the lambda expression, so
> explicitly listing them again is redundant.  To me, this seems quite
> similar to using "auto" to avoid writing out something that the compiler
> can figure out.  I don't know why the article you linked lists [=] and
> [&] as not recommended.  I didn't see an explanation and I'm not finding
> other similar recommendations.

jwe,

Just to fully close this issue, I created a function with a 1000 local
variables and a lambda expression that used one of the variables.  I
defined the capture expression as either "[=]" or "[=var995]" which
captures just one variable.  I then compiled with gcc and compared the
resulting binaries.  At least with gcc, there is no particular binary
difference between using either expression which indicates it is safe to
capture all variables as the compiler is smart enough to optimize out
anything that is unused.  So, I think we're fine with adopting the
conventions you proposed.

--Rik