Question

How can I customize the comparison of elements in a ptr_container? Using a ptr_set, I would like to define a function that checks for equality of elements. However, defining

bool operator==(const Foo& other) (or a friend function)

does not work. It just won’t be invoked, although boost’s unordered containers, on the other side, are aware of overloaded operator==s. Predicates a la:

struct FooEq
{
    bool operator()(const Foo& foo1, const Foo& foo2) const
};

don’t work either and I cannot find a reference that describes how to accomplish this.

struct Foo
{
    Foo(int i, int j) : _i(i), _j(j) {}

    bool operator<(const Foo& other) const { return this->_i < other._i; }

    bool operator==(const Foo& other) const
    {
        std::cout << "== involed";

        return this->_j == other._j;
    }

    int _i;
    int _j;
};

boost::ptr_set<Foo> foos;
std::cout << foos.size() << "\n";
foos.insert(new Foo(1, 2));
std::cout << foos.size() << "\n";
foos.insert(new Foo(2, 2));
std::cout << foos.size() << "\n";

The foos are supposed to be equal since for both j == 2 holds.

Thanks in advance

Answer 1

Set uses the < operator to insert items and test for equivalence (which is not quite the same as equality). This operator must define a strict weak ordering. Set will not use the == operator.

Quoted from the description for set:

Compare: Comparison class: A class
that takes two arguments of the same
type as the container elements and
returns a bool. The expression
comp(a,b), where comp is an object of
this comparison class and a and b are
elements of the container, shall
return true if a is to be placed at an
earlier position than b in a strict
weak ordering operation. This can
either be a class implementing a
function call operator or a pointer to
a function (see constructor for an
example). This defaults to less,
which returns the same as applying the
less-than operator (a

If you elaborated what you are trying to accomplish, I could offer suggestions for a better container.