Re: returning references

James Kanze <>
Sun, 6 Jan 2008 04:52:10 -0800 (PST)
On Jan 6, 12:14 pm, Michael DOUBEZ <> wrote:

Daniel T. a =E9crit :


When James said the above, I took it to mean something like
the example below.

class Foo {
   const Bar& get() const;

void bad_client( Foo& foo ) {
   const Bar* b = &foo.get();
   // then do things, assuming that 'b' will continue to be
   // valid no matter what I may do to 'foo'.

I consider such code to be inherently dangerous... When what
is returned is a const-reference to internal state, the
calling code shouldn't assume 'b's validity even past the
';' of the get call.

That depends on the contract you have with Foo. If it is a
singleton or a scoped_ptr<> by example, there is nothing

More generally: part of the problem may be just one of
semantics: what is meant by "internal state". But my fealing is
that the statement as presented by Daniel T is an over
simplification---the issues are just too complicated to be
covered by simple cliches. I'd be interesting in hearing his
comments on the following (which is a very frequent idiom in
servers in business applications):

    class EntityObject : private boost::uncopiable
        virtual ~EntityObject() {}
        virtual std::string id() const = 0 ;
        // ...
    } ;

    class EntityObjectDataBase
        struct EntityObjectOrder
            bool operator()(
                EntityObject* lhs,
                EntityObject* rhs ) const
                return lhs->id() < rhs->id() ;
        typedef std::set< EntityObject*, EntityObjectOrder >
                            DataBase ;
        DataBase myData ;

        EntityObject* get( std::string const& id ) const
                                entry = myData.find( id ) ;
            return entry != myData.end()
                ? *entry
                : NULL ;
        // functions for insertion and removal...
    } ;

Note that the object pointed to by the return value of
EntityObjectDataBase::get() definitely contains state relevant
to the class invariants of the myData member of
EntityObjectDataBase; presumable, one could even write a derived
EntityObject in which non-const member functions modified the
value returned by EntityObject::id(). Obviously, of course, one
doesn't want to do that, but there's nothing in the language
which would prevent it. The problem here is simple: the
language (and simple additional rules) can only go so far in
protecting against stupid errors. The requirement that I call
get() (and check for null) in every single statement which uses
an object is simply not tenable, and saving the pointer returned
by get() is seems to be a case of the sort of thing Daniel T
seems to be saying one should never do.

James Kanze (GABI Software)
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