STL iterator won't work? Can't figure out why

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

你应该在if语句中使用==吗？

Answer 2

经典错误:-) - 比较需要两个等号

if (*a == *j)

Answer 3

if (*a = *j)

是一项任务

if (*a == *j)

Answer 4

You can just insert one set into another in one go:

std::set<int> s1, s2;

s1.insert(s2.begin(), s2.end());

Set members are unique, so there's nothing else to worry about.

Answer 5

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 = ?

Answer 6

#include <algorithm>

...
std::set_union(s1.begin(), s1.end(), s2.begin(), s2.end() ... )

http://en.cppreference.com/w/cpp/algorithm/set_union