You put your class definition in the header file. This must contain the prototypes of the methods in the class, and the definitions of the data members of the class, e.g.
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void InterfaceMethod2(int arg1);
#endif // MYCLASS_H
Normally, the actual method definitions go into an associated source code file. This source code file should include the header, because otherwise the compiler will have no way of knowing the class definition, e.g:
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// Do some stuff and return a value
void MyClass::InterfaceMethod2(int arg1)
// Do some stuff here
Then if any other file needs to know about MyClass - for example, if the code calls a method of MyClass, or instatiates a MyClass object, or a class definition inherits from MyClass - then it should include the header file to get the definition of the class.