Temporary objects

Temporary objects

[Aseem Rastogi]

Hi All,

I find following C++ behaviour regarding temporary objects quite
contradictory. Consider this:

class A
{
    ...

    public:
        void bar () { ... } // non-const member function

};

A getA () { ... } // return an object

void foo (A& a) { ... }

int main ()
{
    foo (getA ()); // error: not allowed to pass temporary object as
non-const reference. acceptable.

    getA ().bar (); // why is this allowed ?? calling a non-const
functrion on a temporary object ?

}

Any ideas why it could be so ?

If passing a temporary object as non-const reference is not allowed
because making changes to it doesn't make sense, doesn't same thing
apply to calling non-const member function on a temporary ?

Thanks in advance,

-Aseem.

Re: Temporary objects

[John Love-Jensen]

Hi Aseem,

> Any ideas why [passing temporary objects as non-const reference] could be [an
error] ?

C++ can generate unexpected temporaries, which when passed into functions
that are declared to mutate the temporary that can result in extremely
difficult to diagnose bugs.

As such, temporaries are not allowed to be passed to in/out and out
parameters.

> If passing a temporary object as non-const reference is not allowed
> because making changes to it doesn't make sense, doesn't same thing
> apply to calling non-const member function on a temporary ?

No.  It's not a const related issue.  It's an issue related to temporaries,
and C++'s rather promiscuous conversions.

If C++ required explicit conversions, it wouldn't be a problem.  But then
the code wouldn't be C++ as we have it today.  And we'd have a different set
of problems (such as problems regarding code verbosity, legibility,
maintainability).

> Thanks in advance,

You're welcome.

Sincerely,
--Eljay