Any gain using a global const std::string for empty strings?

Hi,

In some of our wast set of C++ source files I've seen some code
which uses a global const std::string as illustrated in the following
code:

  // file: strutil.hpp
  #include <string>
  extern const std::string EmptyString;

  // file: strutil.cpp
  #include "strutil.hpp"
  const std::string EmptyString;

  // file: file1.hpp
  #include "strutil.hpp"

  class A
  {
  public:
    void foo( int money, const std::string & name = EmptyString );
    virtual std::string boo() { return EmptyString; }
  };
  ...

Are the use of the global EmptyString constant really necessary?
Are there any performance/optimization gain of doing this, instead
of simply using std::string() as illustrated by:

  class A
  {
  public:
    void foo( int money, const std::string & name = std::string() );
    virtual std::string boo() { return std::string(); }
  };
  ...


Thanks in advance for all replies,
/Michael


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