Re: Odd error I can't seem to recreate

From:

Victor Bazarov <v.bazarov@comcast.invalid>

Newsgroups:

comp.lang.c++

Date:

Fri, 04 Nov 2011 16:43:53 -0400

Message-ID:

<j91iqa$urv$1@dont-email.me>

On 11/4/2011 4:07 PM, Noah Roberts wrote:

[.. seems that auto_ptr is to blame ..]
I recreated the problem:

cat wtf.cpp
#include<memory>

struct base
{
         virtual ~base() {}
};

struct derived : base
{
   std::auto_ptr<int> i;
   derived() : i() {}
//private:
   //derived(derived const&);
};

struct base;
struct base2
{
         virtual ~base2() {}
         virtual void fun(base const&) const = 0;
};
struct derived2 : base2
{
         void fun(base const&) const {}
};

struct base3
{
         virtual ~base3() {}
         virtual std::auto_ptr<base2> get() const = 0;
};
struct derived3 : base3
{
         std::auto_ptr<base2> get() const { return
std::auto_ptr<base2>(new derived2); }
};

int main()
{
         derived3 d3;
         std::auto_ptr<base2> b2 = d3.get();
         b2->fun(derived());
}

Comeau online compiles it without even a blink. So does VC++ 2010 on my
machine.

Now I get all the errors I don't expect that are giving me crap in the
original code:

g++ wtf.cpp
wtf.cpp: In function ?int main()?:
wtf.cpp:42: error: no matching function for call to
?derived::derived(derived)?
wtf.cpp:9: note: candidates are: derived::derived(derived&)

Victor said:
So, 'test_service' copy constructor takes a non-const reference. To
initialize the argument to your 'run_query', a temporary of type
'location_service' has to be created (or has to be able to be
created),
and I am guessing there is no conversion that the compiler can use.
.................

As I understood it, because it takes a non-const reference the
temporary created by the "test_service()" call should be directly
used, rather than a copy of it attempted. Why is it necessary to make
a temporary of 'location_service', or 'base' in the above code?

I may just as well be wrong here.

So far I failed to reproduce your issue with another compiler. I don't
have access to gcc/g++ at this time.

Let's drop the auto_ptr member and try declaring our own copy-c-tor that
takes a non-const ref...

I've dropped the 'i' member from 'derived' and instead added a copy
c-tor with a non-const ref argument, both Comeau online and VC++ still
compiled it.

Something is funky with g++, I am guessing. Can it compile this:

    struct base
    {
       virtual ~base() {}
    };

    struct derived : base
    {
       derived() : base() {}
       derived(derived& d) : base(d) {}
    };

    void fun(base const& b) { return; }

    int main()
    {
       derived d;
       fun(d);
       fun(derived());
    }

?

V
--
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