Issue with C++ map using member function pointers as keys

Question

I am writing in C++, trying to compile under Ubuntu, and I am experiencing some issues with a map using function pointers as keys. When I define the map, I get no compiling errors, but as soon as I try to insert an element, I get a rather wordy

In file included from /usr/include/c++/4.6/string:50:0,
                 from /usr/include/c++/4.6/bits/locale_classes.h:42,
                 from /usr/include/c++/4.6/bits/ios_base.h:43,
                 from /usr/include/c++/4.6/ios:43,
                 from /usr/include/c++/4.6/ostream:40,
                 from /usr/include/c++/4.6/iostream:40,
                 from main.cpp:1:
/usr/include/c++/4.6/bits/stl_function.h: In member function ‘bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = int (MyClass::*)()]’:
/usr/include/c++/4.6/bits/stl_tree.h:1277:4:   instantiated from ‘std::pair<std::_Rb_tree_iterator<_Val>, bool> std::_Rb_tree<_Key, _Val, _KeyOfValue, _Compare, _Alloc>::_M_insert_unique(const _Val&) [with _Key = int (MyClass::*)(), _Val = std::pair<int (MyClass::* const)(), std::vector<int> >, _KeyOfValue = std::_Select1st<std::pair<int (MyClass::* const)(), std::vector<int> > >, _Compare = std::less<int (MyClass::*)()>, _Alloc = std::allocator<std::pair<int (MyClass::* const)(), std::vector<int> > >]’
/usr/include/c++/4.6/bits/stl_map.h:518:41:   instantiated from ‘std::pair<typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::map<_Key, _Tp, _Compare, _Alloc>::value_type>::other>::iterator, bool> std::map<_Key, _Tp, _Compare, _Alloc>::insert(const value_type&) [with _Key = int (MyClass::*)(), _Tp = std::vector<int>, _Compare = std::less<int (MyClass::*)()>, _Alloc = std::allocator<std::pair<int (MyClass::* const)(), std::vector<int> > >, typename std::_Rb_tree<_Key, std::pair<const _Key, _Tp>, std::_Select1st<std::pair<const _Key, _Tp> >, _Compare, typename _Alloc::rebind<std::map<_Key, _Tp, _Compare, _Alloc>::value_type>::other>::iterator = std::_Rb_tree_iterator<std::pair<int (MyClass::* const)(), std::vector<int> > >, std::map<_Key, _Tp, _Compare, _Alloc>::value_type = std::pair<int (MyClass::* const)(), std::vector<int> >]’
main.cpp:36:51:   instantiated from here
/usr/include/c++/4.6/bits/stl_function.h:236:22: error: invalid operands of types ‘int (MyClass::* const)()’ and ‘int (MyClass::* const)()’ to binary ‘operator<’

Here is the example that caused the above error message:

#include <iostream>
#include <map>
#include <vector>

// class definition

class MyClass
{
    public:
            int f1(void);
            int f2(void);
};

int MyClass::f1(void)
{
    return 1;
}

int MyClass::f2(void)
{
    return 2;
}

using namespace std;

int main( int argc, char* argv[] )
{

    // define map
    map< int (MyClass::*)(void), vector<int> > myMap;
    vector<int> myVector;

    //myMap[ &MyClass::f1 ] = myVector;

    myMap.insert( make_pair( &MyClass::f1, myVector) );
    return 0;
}

What could be the issue? I tried with both insert and [] assign, and I get the same error. Browsing the forums, I found this; but could that be the issue? I don't think I need to define an operator "<" for function pointers (shouldn't they behave as regular pointers?) ...or do I?

Answer 1

The error is telling you all you need to know:

invalid operands of types ‘int (MyClass::* const)()’ and ‘int (MyClass::* const)()’ to binary ‘operator<’

You cannot compare member function pointers using standard operator<, so you must provide a custom comparator when declaring your map.

Unfortunately, pointers to member functions cannot be compared for inequality, so you cannot define a comparison operator or use a std::mapin this case. I suggest using std::unordered_map, which only needs a std::hash and equality comparison, which you can do. See here for hashing and here for equality comparison.