Function pointer problem

Example A fails to compile with
exampleA.cpp: In member function 'void A::a() const':
exampleA:10: error: no matching function for call to 'A::b(const
std::string&) const'
exampleA:17: note: candidates are: virtual void (* A::b(const
std::string&))()const <near match>

Example B compiles fine.

Now I assume that the const is not binding to what I think it should
in the example A

Adding the typedef fixes this

Can anyone tell me what is going on

Adrian

+++++++++++++ Example A +++++++++++++++
#include <iostream>
#include <string>

class A
{
   public:
      void a() const
      {
         const std::string arg("test");
         void(*func_ptr)(void)=b(arg);
         if(func_ptr)
         {
         }
      };
      virtual ~A() throw();

      virtual void (*b(const std::string &str))(void) const=0;
};

int main(int argc, char *argv[])
{
   return 0;
}

+++++++++++++ Example B +++++++++++++++
#include <iostream>
#include <string>

class A
{
   public:
      typedef void(*func_ptr)(void);
      void a() const
      {
         const std::string arg("test");
         func_ptr func_ptr=b(arg);
         if(func_ptr)
         {
         }
      };
      virtual ~A() throw();

      virtual func_ptr b(const std::string &str) const=0;
};

int main(int argc, char *argv[])
{
   return 0;
}