Re: operator==

James Kanze <>
Thu, 16 Dec 2010 08:46:05 -0800 (PST)
On Dec 16, 11:07 am, Andrea Crotti <> wrote:

James Kanze <> writes:

On Dec 15, 5:22 pm, Andrea Crotti <> wrote:

But how do you know that all the fields are equal?

If you want to support comparison for equality in all of the
derived classes, you have to implement some function which
defines equality for those classes. There is no other way;
there is no implicit definition of equality in C++.

If you want to compare for equality over a hierarchy, you also
need to implement the case where the two classes are not the
same. Most of the time (but not always), you can get by with
something like the above, and assume that if the two objects
have different types, they aren't equal. (When you can't, you
need something more complex; some form of double dispatch.)

Yes sure that I know, in theory every subclass of Serializable also
redefines operator==.
The problem is here that the signatures are different so unless I change
all of them with

bool operator==(const Serializable& other)
it doesn't work.

That's why I used a separate function.

But if you support == in the base class, why do you want to
support it in the derived class?

I was trying with a templated function maybe that works...
    template<typename T>
    bool packetEqualTo(const T& first, const T& second) {
        assert(first.fields.size() > 0);
        assert(first.fields.size() == second.fields.size());

        for (size_t i=0; i < first.fields.size(); ++i) {
            if (! ((*first.fields[i]) == (*second.fields[i]))) {
                return false;
        return true;

and then I call packetEqualTo in all the subclasses...

Which supposes that you have some mechanism to get the fields,
and that you only use the static type of the object. (But if
you're only using the static type, what's the point of

James Kanze

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