problem sorting using member function as comparato

Question

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

---

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

Updating Graham Asher answer, as you don't need the compare but can use the less operator directly.

#include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

class Qaz {
public:
    Qaz(int aX): x(aX) { }

    bool operator<(const Qaz& aOther) const {
       return x < aOther.x;
    }

int x;
};

int main() {
    std::vector<Qaz> q;
    q.emplace_back(8);
    q.emplace_back(1);
    q.emplace_back(4);
    q.emplace_back(7);
    q.emplace_back(6);
    q.emplace_back(0);
    q.emplace_back(3);
    std::sort(q.begin(),q.end());
    for (auto& num : q)
        std::cout << num.x << "\n";

    char c;
    std::cin >> c;
    return 0;
}

Answer 2

A very simple way to effectively use a member function is to use operator<. That is, if you have a function called compare, you can call it from operator<. Here is a working example:

class Qaz
{
public:
Qaz(int aX): x(aX) { }

bool operator<(const Qaz& aOther) const
    {
    return compare(*this,aOther);
    }

static bool compare(const Qaz& aP,const Qaz& aQ)
    {
    return aP.x < aQ.x;
    }

int x;
};

Then you don't even need to give the function name to std::sort:

std::vector<Qaz> q;
q.emplace_back(8);
q.emplace_back(1);
q.emplace_back(4);
q.emplace_back(7);
q.emplace_back(6);
q.emplace_back(0);
q.emplace_back(3);
std::sort(q.begin(),q.end());

Answer 3

As Andreas Brinck says, doCompare must be static (+1). If you HAVE TO have a state in your comparator function (using the other members of the class) then you'd better use a functor instead of a function (and that will be faster):

class MyClass{

   // ...
   struct doCompare
   { 
       doCompare( const MyClass& info ) : m_info(info) { } // only if you really need the object state
       const MyClass& m_info;

       bool operator()( const int & i1, const int & i2  )
       { 
            // comparison code using m_info
       }
   };

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

Using a functor is always better, just longer to type (that can be unconvenient but oh well...)

I think you can also use std::bind with the member function but I'm not sure how and that wouldn't be easy to read anyway.

UPDATE 2014: Today we have access to c++11 compilers so you could use a lambda instead, the code would be shorter but have the exact same semantic.

Answer 4

There is a way to do what you want, but you need to use a small adaptor. As the STL doesn't write it for you, can can write it yourself:

template <class Base, class T>
struct adaptor_t
{
  typedef bool (Base::*method_t)(const T& t1, const T& t2));
  adaptor_t(Base* b, method_t m)
    : base(b), method(m)
  {}
  adaptor_t(const adaptor_t& copy) : base(copy.base), method(copy.method) {}
  bool operator()(const T& t1, const T& t2) const {
    return (base->*method)(t1, t2);
  }
  Base *base;
  method_t method;
}
template <class Base, class T>
adaptor_t<Base,T> adapt_method(Base* b, typename adaptor_t<Base,T>::method_t m)
{  return adaptor_t<Base,T>(b,m); }

Then, you can use it:

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

Answer 5

The solution proposed by Rob is now valid C++11 (no need for Boost):

void doSort()
{
  using namespace std::placeholders;
  std::sort(arr, arr+someSize, std::bind(&MyClass::doCompare, this, _1, _2));
}

Indeed, as mentioned by Klaim, lambdas are an option, a bit more verbose (you have to "repeat" that the arguments are ints):

void doSort()
{
  std::sort(arr, arr+someSize, [this](int l, int r) {return doCompare(l, r); });
}

C++14 supports auto here:

void doSort()
{
  std::sort(arr, arr+someSize, [this](auto l, auto r) {return doCompare(l, r); });
}

but still, you declared that arguments are passed by copy.

Then the question is "which one is the most efficient". That question was treated by Travis Gockel: Lambda vs Bind. His benchmark program gives on my computer (OS X i7)

                        Clang 3.5    GCC 4.9
   lambda                    1001        7000
   bind                3716166405  2530142000
   bound lambda        2438421993  1700834000
   boost bind          2925777511  2529615000
   boost bound lambda  2420710412  1683458000

where lambda is a lambda used directly, and lambda bound is a lambda stored in a std::function.

So it appears that lambdas are a better option, which is not too much of a surprise since the compiler is provided with higher level information from which it can make profit.

Answer 6

You can use boost::bind:

void doSort() {
  std::sort(arr,arr+someSize, boost::bind(&MyClass::doCompare, this, _1, _2));
}