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

17 Oct 2003


      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@astron.nl]
Sent: Friday, October 17, 2003 4:39 AM
To: boost-users@lists.boost.org
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 
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 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
...
...
::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]
tBoost.cc:32: no matching function for call to `
   std::vector::push_back(const _Tp&) [with _Tp = 
   boost::shared_ptr<Base>, _Alloc = 
std::allocator]
tBoost.cc:37: no matching function for call to `
   std::vector::push_back(const _Tp&) [with _Tp = 
   boost::shared_ptr<Base>, _Alloc = 
std::allocator]
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RE: [Boost-users] [Q] shared_ptr c'tor

Tom Matelich