Question

I have a class which has one private member:

std::set<unsigned long> Sset;

And I have a problem with this function:

Prototype:

Set& Union (Set&, Set&);

Code:

    Set& Set::Union (Set& s1, Set& s2)
{
    set<unsigned long>::iterator a;
    set<unsigned long>::iterator j;

    for (a = s1.Sset.begin(); a!=s1.Sset.end(); ++a)
        for (j = s2.Sset.begin(); j!=s2.Sset.end(); ++j)
            if (*a = *j)
            {
                Sset.insert(*a);
                break;
            }

    return *this;
}

I get compiler error expression must be a modifiable lvalue at *a=*j

Everything is OK with iterator j, but it won’t accept *a

Any help, or explanation?
Thanks

Answer 1

You can’t assign to the value pointed to by a set::iterator.

Why?

Because sets keep their elements in whatever order it feels like (set, specifically, is sorted) to allow for quick retrieval, so you can’t manually tell it where to put elements.

Instead, insert the element at the right place.

Having said that — did you mean to use == instead of =?