How to define a map from function pointers to strings - c++

Question

I am trying to define a map from member function pointers to strings in C++. But the the compiler complains that operator < is not defined for member function pointers. Is there a way to do this?
Example code:
#include <map>
#include <string>
class foo;
typedef void (foo::*fooFunc)();
class foo {
public:
foo() {
myMap[func]="example function";
}
void func() {};
std::map<fooFunc,std::string> myMap;
};
Resulting error message:
In file included from C:/msys64/mingw64/include/c++/6.2.0/bits/stl_tree.h:65:0,
from C:/msys64/mingw64/include/c++/6.2.0/map:60,
from grr.cpp:1:
C:/msys64/mingw64/include/c++/6.2.0/bits/stl_function.h: In instantiation of 'constexpr bool std::less<_Tp>::operator()(const _Tp&, const _Tp&) const [with _Tp = void (foo::*)()]':
C:/msys64/mingw64/include/c++/6.2.0/bits/stl_map.h:501:32: required from 'std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](std::map<_Key, _Tp, _Compare, _Alloc>::key_type&&) [with _Key = void (foo::*)(); _Tp = std::__cxx11::basic_string<char>; _Compare = std::less<void (foo::*)()>; _Alloc = std::allocator<std::pair<void (foo::* const)(), std::__cxx11::basic_string<char> > >; std::map<_Key, _Tp, _Compare, _Alloc>::mapped_type = std::__cxx11::basic_string<char>; std::map<_Key, _Tp, _Compare, _Alloc>::key_type = void (foo::*)()]'
grr.cpp:9:15: required from here
C:/msys64/mingw64/include/c++/6.2.0/bits/stl_function.h:386:20: error: invalid operands of types 'void (foo::* const)()' and 'void (foo::* const)()' to binary 'operator<'
{ return __x < __y; }
~~~~^~~~~

Answer 1

std::map requires a comparison operator for its keys. By default, it uses operator <. Since operator < is not defined for pointer-to-member types, you are getting this error.
To fix it you'll need to define a custom comparison operator and provide it as a parameter to std::map. Alternatively, use a different container.
From cppreference:
std::map is a sorted associative container that contains key-value
pairs with unique keys. Keys are sorted by using the comparison
function Compare. Search, removal, and insertion operations have
logarithmic complexity. Maps are usually implemented as red-black
trees
If you want options on how to write such a comparison operator, it has already been answered here.