Question

The following code fails to compile in Visual C++ 11 with this error:

1>c:\users\tony\documents\visual studio 11\projects\cpp11_ex1\cpp11_ex1\main.cpp(52): error C2440: ” : cannot convert from ‘Foo *const ‘ to ‘std::weak_ptr<_Ty>’

#include <stdio.h>
#include <memory>

using namespace std;

class Foo;

class Bar
{
public:
    Bar( weak_ptr<Foo> foo ) : _foo(foo) { printf("Bar(%p)\n",this); }
    ~Bar() { printf("~Bar(%p)\n",this); }
private:
    weak_ptr<Foo> _foo;
};

class Foo
{
public:
    Foo() : _bar() { _bar = make_shared<Bar>( weak_ptr<Foo>(this) );  printf("Foo(%p)\n",this); }
    ~Foo() { printf("~Foo(%p)\n",this); }
private:
    shared_ptr<Bar> _bar;
};

int main( int argc, char* argv[] )
{
    shared_ptr<Foo> instance = make_shared<Foo>();

    return 0;
}

It seems that I can’t create a weak_ptr from a raw this pointer. This causes an interesting series of problems.

1. Since I am attempting this in Foo‘s ctor, Foo‘s reference count is 0 (i.e. the make_shared<> in main hasn’t returned yet).

2. I’ve discovered that I can create weak_ptrs from shared_ptrs… But if I change Bar ctor to take a shared_ptr, I the act of calling Bar‘s constructor ends up destroying Foo! (Since Foo‘s reference count is still 0, creating (and then destroying) a shared_ptr to Foo via a call to Bar‘s ctor ).

All I really want to do is create Foo, have Foo create and own a Bar, but have Bar have a weak reference back to Foo. I really don’t want to be forced into 2 part initialization here!

Answer 1

Since Foo will be pointed at by a shared_ptr, and Bar will always be owned by a shared_pointer of Bar, then if Bar exists, Foo exists. Ergo, you don’t need a smart pointer in Bar. (If I understand the problem correctly)

#include <stdio.h>
#include <memory>

using namespace std;

class Foo;

class Bar
{
public:
    Bar( Foo* foo ) : _foo(foo) { printf("Bar(%p)\n",this); }
    ~Bar() { printf("~Bar(%p)\n",this); }
private:
    Foo* _foo;
};

class Foo
{
public:
    Foo() : _bar(new Bar(this)) { printf("Foo(%p)\n",this); }
    ~Foo() { printf("~Foo(%p)\n",this); }
private:
    shared_ptr<Bar> _bar;
};

int main( int argc, char* argv[] )
{
    shared_ptr<Foo> instance = make_shared<Foo>();
    return 0;
}