c++ - problem sorting using member function as comparator

Question

trying to compile the following code I get this compile error, what can I do?

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ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function.

class MyClass {
   int * arr;
   // other member variables
   MyClass() { arr = new int[someSize]; }

   doCompare( const int & i1, const int & i2 ) { // use some member variables } 

   doSort() { std::sort(arr,arr+someSize, &doCompare); }

}; 

Answer 1

doCompare must be static. If doCompare needs data from MyClass you could turn MyClass into a comparison functor by changing:

doCompare( const int & i1, const int & i2 ) { // use some member variables } 

into

bool operator () ( const int & i1, const int & i2 ) { // use some member variables } 

and calling:

doSort() { std::sort(arr, arr+someSize, *this); }

Also, isn't doSort missing a return value?

I think it should be possible to use std::mem_fun and some sort of binding to turn the member function into a free function, but the exact syntax evades me at the moment.

EDIT: Doh, std::sort takes the function by value which may be a problem. To get around this wrap the function inside the class:

class MyClass {
    struct Less {
        Less(const MyClass& c) : myClass(c) {}
        bool operator () ( const int & i1, const int & i2 ) {// use 'myClass'} 
        MyClass& myClass;
    };
    doSort() { std::sort(arr, arr+someSize, Less(*this)); }
}