Re: [Boost-users] [lambda] instantiation in bind

20 Jul 2009

      On Mon, Jul 20, 2009 at 7:24 PM, Igor R  wrote:
...
In VC9.0 the following code gives the error message:
1>...\boost\tuple\detail\tuple_basic.hpp(419) : error C2259: 'Base' :
cannot instantiate abstract class
If the Base is not abstract, the code compiles and works as expected.
Unfortunately, I'm unable to realise why it tries to instantiate the
Base, and how to overcome this issue, so any enlightenment would be
greatly appreciated!
class Base
{
public:
 virtual void close() = 0;
};
class Derived : public Base
{
public:
 void close()
 {}
};
#include 
typedef boost::shared_ptr<Base> BasePtr;
typedef boost::shared_ptr<BasePtr> BasePtrPtr;
#include 
#include 
namespace bl = boost::lambda;
BasePtrPtr makeHolder(Base *base)
{
 return BasePtrPtr(new BasePtr(base), bl::bind(&Base::close, **bl::_1));
}
int main()
{
 Base *b = new Derived();
 makeHolder(b);
 return 0;
}
The **bl::_1 (I suspect) results in a Base (eventually) rather than a
Base& (which I guess is what you wanted). Using the fact that bind
will take an object pointer (as opposed to an object/object
reference), you can use this:

   return BasePtrPtr(new BasePtr(base), bl::bind(&Base::close,
bl::bind(&BasePtr::get, bl::_1)));

For some reason, *bl::_1 didn't want to compile. I'm making no
guesses, so I just tried BasePtr::get, which I've used before (and
compiles!)

Stuart Dootson