Override some methods in wxF(F)ileStream to resolve ambiguities.

Override virtual methods IsSeekable(), GetLength() and OnSysSeek/Tell() to
forward to wxF(F)InputStream base class as otherwise it's impossible to use
them at all because of ambiguity between the versions inherited from this
class and wxF(F)OutputStream (even though the two versions should do the same
thing as they operate on the same file descriptor/handle).

Also improve documentation of these classes: provide a brief description,
correct the base classes.

Closes #11577.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62995 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/wfstream.h

@@ -118,6 +118,31 @@ public:
     wxFileStream(const wxString& fileName);
     virtual bool IsOk() const;
 
+    // override (some) virtual functions inherited from both classes to resolve
+    // ambiguities (this wouldn't be necessary if wxStreamBase were a virtual
+    // base class but it isn't)
+
+    virtual bool IsSeekable() const
+    {
+        return wxFileInputStream::IsSeekable();
+    }
+
+    virtual wxFileOffset GetLength() const
+    {
+        return wxFileInputStream::GetLength();
+    }
+
+protected:
+    virtual wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode)
+    {
+        return wxFileInputStream::OnSysSeek(pos, mode);
+    }
+
+    virtual wxFileOffset OnSysTell() const
+    {
+        return wxFileInputStream::OnSysTell();
+    }
+
 private:
     wxDECLARE_NO_COPY_CLASS(wxFileStream);
 };
@@ -193,8 +218,33 @@ class WXDLLIMPEXP_BASE wxFFileStream : public wxFFileInputStream,
 {
 public:
     wxFFileStream(const wxString& fileName, const wxString& mode = "w+b");
+
+    // override some virtual functions to resolve ambiguities, just as in
+    // wxFileStream
+
     virtual bool IsOk() const;
 
+    virtual bool IsSeekable() const
+    {
+        return wxFFileInputStream::IsSeekable();
+    }
+
+    virtual wxFileOffset GetLength() const
+    {
+        return wxFFileInputStream::GetLength();
+    }
+
+protected:
+    virtual wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode)
+    {
+        return wxFFileInputStream::OnSysSeek(pos, mode);
+    }
+
+    virtual wxFileOffset OnSysTell() const
+    {
+        return wxFFileInputStream::OnSysTell();
+    }
+
 private:
     wxDECLARE_NO_COPY_CLASS(wxFFileStream);
 };