`clear`ing a dynamic array

[Shriramana Sharma via Digitalmars-d-learn]

Jonathan M Davis via Digitalmars-d-learn wrote:

> Appender really isn't intended to be used as a
> container - just as a way to make appending more efficient or to have an
> output range which is an array

I get the part about Appender helping to make an output range of a regular 
array, but I'm not sure how it is supposed to make appending "more 
efficient". I just had a look at the std.array code for appender and 
couldn't figure what was so special – obviously it's my limited knowledge. 

http://dlang.org/arrays.html#resize says: """Also, you may wish to utilize 
the phobos reserve function to pre-allocate array data to use with the 
append operator."""

I presume this means 
http://dlang.org/phobos/std_array.html#.Appender.reserve but how `append` is 
considered an operator is beyond me.

Anyhow, is `reserve` the only thing that makes this more efficient? How is 
this more efficient than setting the .length of the dynamic array directly? 
I see there's a lot of logic going into ensureAddable() but doesn't this 
logic happen within druntime for the regular dynamic arrays itself?

-- 
Shriramana Sharma, Penguin #395953