RE: [Boost-users] [Q] shared_ptr c'tor

From: Tom Matelich (tmatelich_at_[hidden])
Date: 2003-10-17 13:25:47

The shared_ptr constructor from raw pointer is marked explicit (quick explanation: http://www.glenmccl.com/tip_023.htm).  This is because once you put a pointer into a shared_pointer, it's lifetime is now managed by the shared_ptr.  The authors feel it is very important that this is completely intentional, and rightly so.  It can have nasty side effects if you don't.

//untested code
void foo(boost::shared_ptr<int> p)
{
        (*p)++;
}

int main()
{
        int* p = new int(5);
        foo(p);
        std::cerr << "p is now " << *p << std::endl; //oops, p has been deleted!!!
        return 0;
}

> -----Original Message-----
> From: Marcel Loose [mailto:loose_at_[hidden]]
> Sent: Friday, October 17, 2003 4:39 AM
> To: boost-users_at_[hidden]
> Subject: [Boost-users] [Q] shared_ptr c'tor
> 
> 
> Hi,
> 
> I have a question related to the boost::shared_ptr constructor.
> I want to be able to store boost::shared_ptr<Base> objects in 
> a std::vector. However, I don't want to bother my users with 
> the fact (implementation detail) that I'm using shared_ptrs 
> instead of raw pointers. 
> 
> The simplified sample code below more or less clarifies what 
> I want. The code in line 27 is the code I would like to be 
> able to use. Below that line, several attempts to get the 
> code to compile can be found. I understand that line 32 won't 
> compile, because it is equivalent to line 27. However, line 
> 37 left me puzzled. I would expect that the conversion of B* 
> to shared_ptr<B> would be trivial. Not so.
> 
> Is it really true that the only way to use shared_ptr in the 
> way I want is as in lines 46, 47? Or am I overlooking 
> something? And is line 41 equivalent to line 46?
> Thanks in advance.
> 
> Regards,
> 
> 	Marcel Loose.
> 
> P.S.: I noticed that all code will compile if I omit the 
> "explicit" keyword in the constructor of shared_ptr, but I 
> don't consider that the proper way to solve the problem.
> 
> 
> 1:#include <boost/shared_ptr.hpp>
> 2:#include <vector>
> 3:
> 4:using namespace std;
> 5:using namespace boost;
> 6:
> 7:class Base
> 8:{
> 9:public:
> 10:  Base() { /* ... */ }
> 11:  virtual ~Base() {}
> 12:};
> 13:
> 14:class Derived : public Base
> 15:{
> 16:public:
> 17:  Derived() { /* ... */ }
> 18:  virtual ~Derived() {}
> 19:};
> 20:
> 21:
> 22:int main()
> 23:{
> 24:  vector<shared_ptr<Base> > v;
> 25:
> 26:  {                                        // failes to compile:
> 27:    v.push_back(new Derived());            // cannot 
> convert Derived*&
> 28:  }                                        // to 
> shared_ptr<Base>. Why?
> 29:
> 30:  {                                        // failes to compile:
> 31:    Derived* d(new Derived());             // cannot 
> convert Derived*&
> 32:    v.push_back(d);                        // to 
> shared_ptr<Base>. Why?
> 33:  }
> 34:
> 35:  {                                        // failes to compile:
> 36:    Base* b(new Derived());                // cannot convert Base*&
> 37:    v.push_back(b);                        // to 
> shared_ptr<Base>. Huh?
> 38:  }                                        // Seems a 
> trivial conversion.
> 39:
> 40:  {                                        // OK, this 
> compiles, but is
> 41:    shared_ptr<Derived> d(new Derived());  // it correct? 
> I.e. is conversion
> 42:    v.push_back(d);                        // 
> shared_ptr<Derived> to 
> 43:  }                                        // 
> shared_ptr<Base> trivial?
> 44:	
> 45:  {                                        // OK, this 
> compiles, and is
> 46:    shared_ptr<Base> b(new Derived());     // probably 
> what I meant.
> 47:    v.push_back(b);                        // However, I 
> dislike the
> 48:  }                                        // verbosity.
> 49:
> 50:  return 0;
> 51:}
> 
> Just to be complete, please find the compiler diagnostics below.
> 
> tBoost.cc: In function `int main()':
> tBoost.cc:27: no matching function for call to `
>    std::vector<boost::shared_ptr<Base>, 
> std::allocator<boost::shared_ptr<Base> 
>    > >::push_back(Derived*&)'
> /usr/local/gcc-3.2.1/include/c++/3.2.1/bits/stl_vector.h:490: 
> candidates are: 
>    void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = 
>    boost::shared_ptr<Base>, _Alloc = 
> std::allocator<boost::shared_ptr<Base> >]
> tBoost.cc:32: no matching function for call to `
>    std::vector<boost::shared_ptr<Base>, 
> std::allocator<boost::shared_ptr<Base> 
>    > >::push_back(Derived*&)'
> /usr/local/gcc-3.2.1/include/c++/3.2.1/bits/stl_vector.h:490: 
> candidates are: 
>    void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = 
>    boost::shared_ptr<Base>, _Alloc = 
> std::allocator<boost::shared_ptr<Base> >]
> tBoost.cc:37: no matching function for call to `
>    std::vector<boost::shared_ptr<Base>, 
> std::allocator<boost::shared_ptr<Base> 
>    > >::push_back(Base*&)'
> /usr/local/gcc-3.2.1/include/c++/3.2.1/bits/stl_vector.h:490: 
> candidates are: 
>    void std::vector<_Tp, _Alloc>::push_back(const _Tp&) [with _Tp = 
>    boost::shared_ptr<Base>, _Alloc = 
> std::allocator<boost::shared_ptr<Base> >]
>  
> _______________________________________________
> Boost-users mailing list
> Boost-users_at_[hidden]
> http://lists.boost.org/mailman/listinfo.cgi/boost-users
> 
-----------------------------------------------------------------------
DISCLAIMER:  Information contained in this message and/or
attachment(s) may contain confidential information of Zetec, Inc.
If you have received this transmission in error, please notify
the sender by return email.
-----------------------------------------------------------------------