construct two shared_ptr objects from the same pointer

Question

hello, I have a problem from "The C++ Standard Library Extensions":

Exercise 6
I said in Section 2.4.2 that you shouldn't construct two shared_ptr objects from the same pointer. The danger is that both shared_ptr objects or their progeny will eventually try to delete the resource, and that usually leads to trouble. In fact, you can do this if you're careful. It's not particularly useful, but write a program that constructs two shared_ptr objects from the same pointer and deletes the resource only once.

below is my answer:

template <typename T>
void nonsence(T*){}
struct SX {
     int data;
     SX(int i = 0) :
              data(i) {
              cout << "SX" << endl;
     }
     ~SX() {
              cout << "~SX" << endl;
     }
};
int main(int argc, char **argv) {
    SX* psx=new SX;
    shared_ptr<SX> sp1(psx),sp2(psx,nonsence<SX>);
    cout<<sp1.use_count()<<endl;
    return 0;
}

but I don't think it is a good solution--because i don't want solving it by use constructor. can anyone give me a better one? thx, forgive my bad english.

Answer 1

You can look at how boost solves it with shared_from_this. Here's the code.

Answer 2

The trick you found is valid, though useless. The central feature of shared_ptr is reference-counting, which you subvert here. The deleter (the second constructor argument) is there for using shared_ptr with resources other then plain pointers. You could use it with files, e.g:

typedef boost::shared_ptr FilePtr;
void FileClose( FILE* pf ) { if ( pf ) fclose( pf ); }
FilePtr pfile( fopen( "filename" ), FileClose );

Same with database connections, sockets, etc. etc. and RAII in general.

Answer 3

All you need to do is construct the second shared_ptr from the first shared_ptr.

shared_ptr<SX> sp1( new SX );
shared_ptr<SX> sp2( sp1 );

The created SX will then properly be deleted only when all shared pointers to it are destroyed.

Answer 4

I got the "STANDARD" answer from boost doc : http://www.boost.org/doc/libs/1%5F38%5F0/libs/smart%5Fptr/sp%5Ftechniques.html#another_sp