Question

Option 1:

//A.h
class A
{
   void foo()
   {
   }
};

Option 2:

//A.h
class A
{
    void foo();
};

inline void A::foo()
{
}

Note that in option 2, the method is also implemented in the header (marked inline to prevent multiple definition).

Answer 1

A method defined inside the class definition is implicitly inline, so, functionally, there is no difference.

Rob provides a good reason on one difference. Here’s another, IMO.

Say you have:

class DBConnection
{
public:
   DBConnection(const Config& config)
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   void reconnect()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   void checkIntegrity()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   void runQuery()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   void dropTables()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   void disconnected(Callback callback)
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   void selectTables()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   Results getResults()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
        //YES, I copy-pasted these
   }
   //MANY MORE
}

Tell me, is there a way to get notified when the database is disconnected?

Now, try again with:

class DBConnection
{
public:
   DBConnection(const Config& config);
   void reconnect();
   void checkIntegrity();
   void runQuery();
   void dropTables();
   void disconnected(Callback callback);
   void selectTables();
   Results getResults();
   //MANY MORE
};

inline DBConnection::DBConnection(const Config& config)
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   inline void DBConnection::reconnect()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   inline void DBConnection::checkIntegrity()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   inline void DBConnection::runQuery()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   inline void DBConnection::dropTables()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   inline void DBConnection::disconnected(Callback callback)
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   inline void DBConnection::selectTables()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
   }
   inline Results DBConnection::getResults()
   {
        //some
        //really
        //complicated
        //logic
        //and
        //checks
        //and
        //stuff
        //I dunno
        //not
        //a
        //db
        //guy
        //possibly
        //exceptions
        //thrown?
        //YES, I copy-pasted these
   }

Easier, right?