Searching a vector with functionals

Hi folks,

I'm trying to implement a functor find_if, which takes a predicate, and
searches a container (for example, vector) and returns (a reference to)
the first element for which the predicate function returns true. I can't
get it to work. Maybe someone can give me a hint? From my understanding,
the call and the template-generated functions do match, typewise. The
problem appears to be that main()::f does not satisfy
std::unary_function<element_t&, bool>& but I don't understand why.

gcc (GCC) 4.1.2 20061115 (prerelease) (Debian 4.1.1-21) gives me the
following error message:

t.cc: In function 'int main()':
t.cc:35: error: no matching function for call to
'find_if(std::vector<foo, std::allocator<foo> >&, main()::f, foo&)'
t.cc:7: note: candidates are: element_t& find_if(container_t&,
std::unary_function<element_t&, bool>&, element_t&) [with container_t =
std::vector<foo, std::allocator<foo> >, element_t = foo]

from the following code:

------------------------------snip------------------------------

using namespace std;

#include <functional>
#include <vector>

template<class container_t, typename element_t>
element_t &find_if(container_t &cont, unary_function<element_t &, bool>
&fun, element_t &notfound)
{
    typename container_t::iterator iter = cont.begin(), iend =
cont.end();
    for (; iter != iend; iter++)
        if (fun(*iter))
            return *iter;
    return notfound;
}

struct foo { int a; foo(int aa = 0): a(aa) {}; };

int main()
{
    struct f: public unary_function<foo &, bool> {
        bool operator()(foo &x) {
            if (123 == x.a) return true;
            else return false;
        }
    };

    vector<foo> fv;

    fv.push_back(foo(1));
    fv.push_back(foo(123));
    fv.push_back(foo(2));

    foo notfound;

    foo &x = find_if(fv, f(), notfound);

    return 0;
}

------------------------------snap------------------------------

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