add method to a existing class and save to source file

Dear Clang Developers:
I have a class named classA in a.h and a.cpp file. it have some methods, I want to add a new method definition to a.h and add new method body after last method use clang . How to do it ?
I know the RecursiveASTVisitor::VisitCXXRecordDecl(…) can visit CXXRecordDecl and CXXMethodDecl , Rewriter be used to write code . but how to create new method definition and body to a.h and a.cpp?

//a.h file
classA {
int filed;
void method1();
void method2();
//insert a new method

//a.cpp file

void classA::method1()


void classA::method2()


// inset new method body

Yours truly