Problem with function not being visible

Guys, I have the following piece of code. Could you please help me understand why
b.ToString( ) cannot be called while b.foo( ) can? When I compile I get (gcc, but
visual studio gives the same pretty much). Thanks

$ g++ -Wall foo.cpp
foo.cpp: In function `int main(int, char**)':
foo.cpp:21: error: no matching function for call to `Bar::ToString()'
foo.cpp:14: note: candidates are: virtual std::string Bar::ToString(std::string&) const

#include <stdlib.h>
#include <string>
using namespace std;

template <typename _T> class Foo {
public:
  virtual std::string ToString (std::string& pfx) const = 0;
  std::string ToString ( ) const { return(ToString(std::string( ))); }
  virtual void* _foo ( ) const = 0;
  void* foo( ) const { return (_foo( )); }
};

class Bar: public Foo<int> {
public:
  std::string ToString (std::string& pfx) const { return (std::string("test")); }
  void* _foo ( ) const { return(NULL); }
};

int main (int argc, char **argv) {
  Bar b;
  b.foo( );
  b.ToString( );
  return (0);
}

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