CODEWITHSUNDEEP
c++containers
user44184
user44184

Reputation: 27

C++: set::insert with iterators and sets of sets

I have a set of sets of ints called nets which I'm trying to iterate through to determine if either of the ints to or from have been added to an existing set; if so, I add them to the existing set (this is to keep track of all combinations of shorts on an electrical network).

However, I CANNOT get the set::insert function to work! I'm able to use (*it).count just fine to determine if either int has already been added to a set, but (*it).insert doesn't work (I always receive the error No matching member function call to 'insert'). However, if I just create a set of ints, setName.insert(to) works fine.

What's wrong with my code?

for (std::set<std::set<int>>::iterator it=nets.begin(); it!=nets.end(); ++it) {  
    if ((*it).count(to) || (*it).count(from)) {
        currentSet = (*it);
        (*it).insert(to);
        (*it).insert(from);
        addedToExistingSet = true;    
    }
}

Upvotes: 0

Views: 1399

Answers (2)

Pierre Fourgeaud
Pierre Fourgeaud

Reputation: 14510

As a std::set is an associative container that contains a sorted set of unique objects of type T, you cannot modify an object inside it because you would invalidate the set.

If it was allowed, you'd be allowed to change the items value out from under the container, potentially altering the ordering.

The standard says :

23.2.4 Associative containers [associative.reqmts]

iterator of an associative container is of the bidirectional iterator category. For associative containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators.

You should consider using another container for the outer one like std::vector.

Upvotes: 2

dunc123
dunc123

Reputation: 2713

The problem here is that you are trying to modify an object inside a set, which isn't a valid operation as you would invalidate the set. Is there a reason you need the outer container to be a set? Could it be a std::vector for example?

Upvotes: 0

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