declaration order

Hi,

I've reached a deadlock with shared_ptr and my next step will be a
complete re-think of the problem but before I'd like to share with you
my problem as probably some of you had the very problem and solved
better than me...

I want an std::set < shared_ptr<Foo> > to check duplicates and find
elements by one of Foo's attributes:

#include <iostream>
#include <set>
#include <map>
#include <string>
#include <boost/shared_ptr.hpp>

using namespace std;

struct Foo {
    char c;
    Foo(char s) : c(s) { cout << "ctor: " << c << endl; }
    ~Foo() { cout << "dtor: " << c << endl; }
};
typedef boost::shared_ptr<Foo> ptr;
struct less_by_value
{
  bool operator()(const ptr f1, const ptr f2) const
  {
    return f1.get()->c < f2.get()->c;
  }
};

int main () {
    int i;
    set<ptr, less_by_value> list;

    // Populate map (double insert to test multiplicity)
    for (i=0; i<5; i++) {
        ptr foo (new Foo('A' + i));
        list.insert(foo);
        list.insert(foo);
    }
    cout << "map size: " << list.size() << " == 5 ?" << endl;

    // Test find in set
    set<ptr, less_by_value>::iterator it;
    ptr needle (new Foo('A'));
    if ((it = list.find(needle)) != list.end())
        cout << (*it)->c << " == " << needle->c << endl;
    else
        cout << needle->c << " not found" << endl;
    return 0;
}

This code works fine:
$ g++ ptr.cpp && ./a.out
ctor: A
ctor: B
ctor: C
ctor: D
ctor: E
map size: 5 == 5 ?
ctor: A
A == A
dtor: A
dtor: E
dtor: D
dtor: C
dtor: B
dtor: A

Now, here comes the problem... I want to keep a list of Foo pointers
inside Foo itself. Like in a graph, I need to keep all other nodes
connected and the distance, so I need to put the typedef before:

struct Foo;
typedef boost::shared_ptr<Foo> ptr;
struct Foo {
    char c;
    map<ptr, double> connections;
    Foo(char s) : c(s) { cout << "ctor: " << c << endl; }
    ~Foo() { cout << "dtor: " << c << endl; }
};

So far, so good, but my map need also to use the same 'less' for
finding elements and I need to declare the struct 'less_by_value'
_before_ my struct:

struct Foo;
typedef boost::shared_ptr<Foo> ptr;
struct less_by_value
{
  bool operator()(const ptr s1, const ptr s2) const
  {
    return s1.get()->c < s2.get()->c;
  }
};
struct Foo {
    char c;
    map<ptr, double, less_by_value> connections;
    Foo(char s) : c(s) { cout << "ctor: " << c << endl; }
    ~Foo() { cout << "dtor: " << c << endl; }
};

And because 's1.get()->c' is using the structure of 'Foo' before it's
own full declaration, GCC gives me this error message:

ptr.cpp: In member function 'bool less_by_value::operator()(ptr, ptr)
const':
ptr.cpp:14: error: invalid use of undefined type 'struct Foo'
ptr.cpp:8: error: forward declaration of 'struct Foo'
ptr.cpp:14: error: invalid use of undefined type 'struct Foo'
ptr.cpp:8: error: forward declaration of 'struct Foo'

Even if not using shared_ptr I would still need to write my own less
to compare by value in the main set to reuse the same entries
regardless of it's connections at any time.

I thought of writing my own "search_by_value" functor class with
std::find_if but I'd still not have uniqueness within the std::set and
the std::map I want.

Is there an elegant way out of this deadlock?

cheers,
--renato