Question

I’ve got a set of pointers which I would like to iterate over in a deterministic manner. Obviously if I use the default sort order for set, this will be based on the pointers memory addresses which can be different each time the program runs. So I define a custom comparator which I want to use, but I don’t want to change the templated type of the set (because it’s used in a million places in the code), so I want to pass in a comparator object to the set constructor which is derived from std::less

class TestClass
{
public:
    TestClass(int id_) : id(id_)    {}
    ~TestClass()                    {}
    int  getId() const              { return id;}
    void setId(int id_)             { id = id_; }
private: 
    int id; 
};

struct TestClassLessThan : public std::less<TestClass*>
{   // functor for operator<
    bool operator()(const TestClass* &_Left, const TestClass* &_Right) const
    {   // apply operator< to operands
        return (_Left->getId() < _Right->getId());
    }
};


int main(void)
{
    TestClassLessThan comp; 
    set<TestClass*> testSet(comp), testSet2(comp);

    TestClass* obj1 = new TestClass(1);
    TestClass* obj2 = new TestClass(2);
    testSet.insert(obj1);
    testSet.insert(obj2);

    TestClass* obj = *(testSet.begin());

    cout << "First run" << endl;
    BOOST_FOREACH(TestClass* o, testSet)  // expecting 1,2 - get 1,2
        cout << o->getId() << endl;

    // now change the ordering (based on id) and insert into a new set in the same order
    obj1->setId(3);
    testSet2.insert(obj1);
    testSet2.insert(obj2);

    cout << "Second run" << endl;
    BOOST_FOREACH(TestClass* o, testSet2) // expecting 2,3 - get 3,2
        cout << o->getId() << endl;

    delete obj1;
    delete obj2;
}

So my question is, what have I forgotten to do?

Answer 1

All the above is valid.
A possible solution is to customise std::less itself using template specialisation:

namespace std
{
    template<>
    struct less< TestClass*>
    {   // functor for operator<
    public:
        bool operator()(  TestClass* const  &_Left,  TestClass* const  &_Right) const
        {   // apply operator< to operands      
            return (_Left->getId() < _Right->getId());
        }
    };
}

You then get your custom behaviour using the default template comparator for std::set.

Depending on how you build your system, you may have problems if the set is “used in a million places ” and the custom std::less is not consistently available.