类的static函数成员在定义时不用要static

否则出错：

 cannot declare member function ‘static int Foo::bar()’ to have static linkage

参考：http://cplusplus
WRONG
 Foo.h: class Foo  {   public:      static int bar(); }; Foo.cc: static int Foo::bar()  {   // stuff }
 

WORKS
 Foo.h: class Foo  {   public:      static int bar(); }; Foo.cc: int Foo::bar()  {   // stuff }
 
ALSO WORKS
 Foo.h: class Foo  {   public:      static int bar()       {         // stuff       }; };