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?

Answer 1

As addition, it seems you want just

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

...

theRadii[getSomeValue()]++;