2011/10/19 Steckmann, Sven <sven.steckmann@ziehm-eu.com>

Dear list,

I have a problem with the implicit conversion to bool in the shared_ptr class.

I want to use operator overloading for an easy to use interface. I have 2 Instances of a class with a common Interface and I want to define an operator-class (OpAnd of type K) which can handle the operator.

By compiling this little test below, I got: g++ -I/home/steckmann/proj/libs/boost -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"test.d" -MT"test.d" -o "test.o" "../test.cpp" ../test.cpp: In function 'int main()': ../test.cpp:49: error: ambiguous overload for 'operator&&' in 'a && b' ../test.cpp:49: note: candidates are: operator&&(bool, bool) <built-in> ../test.cpp:40: note: boost::shared_ptr<K> operator&&(boost::shared_ptr<K>, boost::shared_ptr<K>)

This is quite clear to me, but how can I avoid this without touching the shared_ptr?

Thanks for any comments, Sven

#include <boost/shared_ptr.hpp>

using namespace boost;

class K { public: virtual bool value() = 0; };

class Value :public K { public: Value( bool i ) : var( i ) {}

bool value() { return var; }

bool var; };

class OpAnd : public K { public: OpAnd( boost::shared_ptr<K> l, boost::shared_ptr<K> r ) :l(l), r(r) {}

bool value() { return l->value() && r->value(); }

boost::shared_ptr<K> l; boost::shared_ptr<K> r; };

boost::shared_ptr<K> operator&&( boost::shared_ptr<K> l, boost::shared_ptr<K> r) { return shared_ptr<K>( new OpAnd(l, r ) ); }

I'd really rethink the design ;-) that said, let's see: looks like shared_ptr<Value> is convertible to shared_ptr<K> and to some unspecified_bool_type. So what you need is a better matching template for operator &&, somthing like [warning: not tested]: template < class T > boost::shared_ptr<K> operator&&( boost::shared_ptr<T> l, boost::shared_ptr<T> r ) { return shared_ptr<K>( new OpAnd(l,r) ); } Also, you might want to enable_if that only for Ts that are derived from K. Hope this helps Regards Kris

int main() { boost::shared_ptr<Value> a( new Value(true) ); boost::shared_ptr<Value> b( new Value(false) ); boost::shared_ptr<K> c = a && b;

// Now use c here for some stuff c->value(); // returns false

b->var = true;

c->value(); // returns true

return 0; }