why does std::set<>::find return a const?

Question

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

I want to use a std::set<> to count the number occurences of a certain value and simultaneosly sort the objects. For this I created a class RadiusCounter

class RadiusCounter
{
public:
    RadiusCounter(const ullong& ir) : r(ir) { counter = 1ULL; }
    void inc() { ++counter; }
    ullong get() const { return counter;}
    ullong getR() const { return r;}
    virtual ~RadiusCounter();
protected:
private:
    ullong r;
    ullong counter;
};

(the destructor does nothing) together with comparison operators:

const inline bool operator==(const RadiusCounter& a, const RadiusCounter& b) {return  a.getR() == b.getR();}
const inline bool operator< (const RadiusCounter& a, const RadiusCounter& b) {return a.getR() < b.getR();}
const inline bool operator> (const RadiusCounter& a, const RadiusCounter& b) {return a.getR() > b.getR();}
const inline bool operator!=(const RadiusCounter& a, const RadiusCounter& b) {return a.getR() != b.getR();}
const inline bool operator<=(const RadiusCounter& a, const RadiusCounter& b) {return a.getR() <= b.getR();}
const inline bool operator>=(const RadiusCounter& a, const RadiusCounter& b) {return a.getR() >= b.getR();}

now I want to use it like this:

set<RadiusCounter> theRadii;
....
ullong r = getSomeValue();

RadiusCounter ctr(r);
set<RadiusCounter>::iterator itr = theRadii.find(ctr);

// new value -> insert
if (itr == theRadii.end()) theRadii.insert(ctr);

// existing value -> increase counter
else itr->inc();

But now the compiler complains at the line with the call to itr->inc():

error: passing 'const RadiusCounter' as 'this' argument of 'void RadiusCounter::inc()' discards qualifiers

Why is the instance in *itr a const here?

score 10 · Answer 1 · answered Feb 26 '12 at 17:04

Because you can't modify elements in a std::set. If you could, it would allow the possibility that it would break the strict-weak ordering invariant, resulting in undefined behaviour.

If you want to modify an element, then you should erase the element, and insert a new one.

score 3 · Accepted Answer · answered Feb 26 '12 at 17:12

3

As addition, it seems you want just

typedef int Radius;
typedef int Counter
std::map<Radius, Conunter>theRadii;

...

theRadii[getSomeValue()]++;

answered Feb 26 '12 at 17:12

Lol4t0

12,444
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score 1 · Answer 3 · edited May 23 '17 at 10:29

1

As it happens I already answered this question a few hours ago: https://stackoverflow.com/a/9452445/766580. Basically, you can't change the value of a set element because the set has no way of knowing what you changed. You will need to remove and reinsert your modified value if you want to do this.

edited May 23 '17 at 10:29

Community

1
1

answered Feb 26 '12 at 17:04

spencercw

3,320
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Please flag the question as a duplicate if you find something like this. As it stands, your answer isn't really an answer, it's just a link to some place else. – Mat Feb 26 '12 at 17:07

why does std::set<>::find return a const?

3 Answers3