Re: Passing values by reference
On Jan 22, 4:25 pm, SG <s.gesem...@gmail.com> wrote:
On 22 Jan., 15:19, James Kanze <james.ka...@gmail.com> wrote:
More generally, C++ uses pass by value by default, and
that's what should be used unless there is a reason to do
otherwise:
-- polymorphic objects,
-- the callee modifies the object
-- (premature?) optimization
In many ways, passing even something like std::string by
const reference when you really mean value is premature
optimization. On the other hand, of course, it is so
ubiquious that you would seem wierd not to do it. (Note
that if the library uses the small string optimization, and
the compiler is really, really good, passing short
std::string's by reference may actually be more expensive
than passing them by value.)
The choice is not so easy IMHO -- especially if you want to
write generic code.
Library writers, who don't have the possibility of profiling the
user code and optimizing in consequence, do have a problem.
I use a lot of const references in generic code because
someone might instantiate it with types that can't be copied
cheaply.
Or not. The standard library is mixed in this regard, using
references for the value types in containers (although these are
going to have to be copied anyway), and values for iterators and
various predicates and functional objects (although polymorphism
would at times be very useful here).
For a library writer, there is no simple answer.
--
James Kanze (GABI Software) email:james.kanze@gmail.com
Conseils en informatique orient=E9e objet/
Beratung in objektorientierter Datenverarbeitung
9 place S=E9mard, 78210 St.-Cyr-l'=C9cole, France, +33 (0)1 30 23 00 34