C + +: The difference between a member function implementation and an outside class definition in a class definition

//A.cppclassa{ Public:      intFunintx) {      return(x*x+ +);  }  }; voidtt () {}//B.cppclassa{ Public:      intFunintx);  }; voidtt (); intyy () {TT ();      A; returnA.fun (3); }
Compile them and then link them separately:

A link error was displayed because a reference to A::fun (int) could not be found in B.cpp (B.O).

Change the a.cpp above to read as follows:
Class  a{      public:      int fun (intint A:: Fun (int  x) {      return (x*x+);      }   void tt ()  {  }  

Compile the a.cpp again without compiling the B.CPP,A.O and B.O after the link to display the link successfully.

Thus, the reason for the first link error is obvious.

Conclusion: the class member function in the class definition has a file inner scope, while the class implementation outside the class definition has some global scope.

C + +: member function implementation differs from class definition in class definition