Re: default parameter value for a reference from a constructor

[Marc Glisse <marc.glisse@xxxxxxxx>]

On Tue, 5 Oct 2010, gongweigang@xxxxxxxxx wrote:

> Hi, all,
>
> I have following code:
>
> class A { };
>
> class B
> {
> public:
>    void foo( A& ra = A() ) { }
> };
>
> int main() {
>   return 0;
> }
>
> If I compile with g++, it has following error message: invalid type 'A' for 
> default argument to 'A&'.
>
> But when I compile it using SunStudio compiler, it works.

With the latest version:
man CC
[...]
    -features=a
        Enables/disables various C++ language features.
[...]
        [no%]rvalueref [Do not] Allow binding a non-const reference to
                       an rvalue or temporary.
                       Default: -features=no%rvalueref
                       The C++ compiler has traditionally been lax in
                       enforcing the rule that a non-const reference
                       cannot be bound to a temporary or rvalue.  Beginâ
                       ning with Studio Express 6/10 the C++ compiler
                       will not allow the invalid code by default.  To
                       restore the old compiler behavior, use the option
                       -features=rvalueref.


> I suspect the error is due to set the default value for reference "ra" with a 
> constructor.
>
> Can anyone explain whether the above code is valid in C++ standard ? If not, 
> how to make it work by g++ ?

void foo( A& ra ) { }
void foo( ) { A a; foo(a); }

or:

void foo( A const& ra = A() ) { }

--
Marc Glisse