From: Bill Buklis (boostuser_at_[hidden])
Date: 2007-08-14 13:57:14
> -----Original Message-----
> From: boost-users-bounces_at_[hidden] [mailto:boost-users-
> bounces_at_[hidden]] On Behalf Of e r
> Sent: Monday, August 13, 2007 6:23 PM
> To: boost-users_at_[hidden]
> Subject: [Boost-users] ptr_map<int, T> where T has no default constructor
> because its private members are const
>
> hi,
>
> i can build a map<int,A*>, but not a boost::ptr_map<unsigned int,A>.
> could someone please help?
>
> thank you,
>
> e.
>
>
> #include <boost/ptr_container/ptr_map.hpp>
>
> using std::map;
> using std::cout;
> using std::endl;
>
> class A{
> public:
> A(double x_):x(x_){};
> const double x;
> };
>
> int main(){
>
> map<int,A*> map_A; //cannot use map<int,A>
> //because A() not defined
> //why? because A has const members.
>
> A a(0.0);
> map_A[0]=new A(0.0);//fine
>
>
> boost::ptr_map<unsigned int,A> ptr_map_A;
> ptr_map_A[0]=new A(0.0);//not fine
>
> return 0;
> };
I don't know for sure, but this may be disallowed for exception safety. In
any case this should work:
unsigned int key = 0;
ptr_map_A.insert(key, new A(0.0));
"insert" also has the advantage in that the map doesn't have to create an
empty value or null-pointer entry) before the assignment.
-- Bill --