std::set iterator automatically const

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C++ STL set update is tedious: I can't change an element in place

I have extracted the problem and changed names and so for simplicities sake.

Basically I instantiate a class and I stock it in a std::set, later on I'd like to have a reference to the class so that I can check its values and modify them...

The simplified code:

        MyClass tmpClass;
        std::set<MyClass > setMyClass;
        setMyClass.insert(tmpClass);
        std::set<MyClass >::iterator iter;
        iter=setMyClass.begin();
        MyClass &tmpClass2=*iter;

and the error:

error C2440: 'initializing' : cannot convert from 'const MyClass' to 'MyClass &'

(I removed parts of the error message, "MVB::Run::" to clear it up too.)

if I add a preceding 'const' to the last line of code then everything works well but then I can't change the value...

Is this normal behaviour and I have to, say, remove the data, change values and put it back again?

I have the feeling that this has something to do with the sorting of the set but I won't touch the variables that are used for the sorting.

user1055604 · Accepted Answer · 2012-03-16T11:36:23.080

20

I you are certain you won't touch the variables that are used for the sorting the you could work around this by using a const_cast like this:

    MyClass tmpClass;
    std::set<MyClass > setMyClass;
    setMyClass.insert(tmpClass);
    std::set<MyClass >::iterator iter;
    iter=setMyClass.begin();
    const MyClass &tmpClass2=*iter;

    MyClass &tmpClass3 = const_cast<MyClass&>(tmpClass2);

Alternatively, you could declare the members of the class that you intend to change as mutable.

edited Mar 16 '12 at 11:36

answered Mar 15 '12 at 19:21

user1055604

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3

You can do this, but it's bad practice. – Oliver Charlesworth Mar 15 '12 at 20:54
1

@Oliver: having const iterators is quite annoying in case you have heterogeneous lookup and you want to change the non key part. I agree that const_cast's are bad practice but what alternatives are there besides a similar bad practice of using mutable all over the place or taking a performance penalty using erase / insert. – gast128 Aug 03 '19 at 12:51
You can also use a set of MyClass pointers, which are hashable/sortable independent of its variables. (I.e., You can modify its variables through the pointer) – cid Oct 01 '19 at 21:49

score 14 · Answer 2 · answered Mar 15 '12 at 19:04

14

Yes, this is expected. If you were able to edit the objects already in the set, the applied sort order might no longer apply, leading to undefined behaviour.

answered Mar 15 '12 at 19:04

Oliver Charlesworth

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std::set iterator automatically const

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