diff --git a/include/wx/generic/dvrenderers.h b/include/wx/generic/dvrenderers.h
index 6f80c773a5..60d431f84e 100644
--- a/include/wx/generic/dvrenderers.h
+++ b/include/wx/generic/dvrenderers.h
@@ -50,8 +50,8 @@ public:
                             wxDataViewCellMode mode = wxDATAVIEW_CELL_INERT,
                             int align = wxDVR_DEFAULT_ALIGNMENT );
 
-    bool SetValue( const wxVariant &value );
-    bool GetValue( wxVariant &value ) const;
+    virtual bool SetValue( const wxVariant &value );
+    virtual bool GetValue( wxVariant &value ) const;
 
     virtual bool Render(wxRect cell, wxDC *dc, int state);
     virtual wxSize GetSize() const;
@@ -80,11 +80,11 @@ public:
                               wxDataViewCellMode mode = wxDATAVIEW_CELL_INERT,
                               int align = wxDVR_DEFAULT_ALIGNMENT );
 
-    bool SetValue( const wxVariant &value );
-    bool GetValue( wxVariant &value ) const;
+    virtual bool SetValue( const wxVariant &value );
+    virtual bool GetValue( wxVariant &value ) const;
 
-    bool Render( wxRect cell, wxDC *dc, int state );
-    wxSize GetSize() const;
+    virtual bool Render( wxRect cell, wxDC *dc, int state );
+    virtual wxSize GetSize() const;
 
 private:
     wxIcon m_icon;
@@ -105,11 +105,11 @@ public:
                               wxDataViewCellMode mode = wxDATAVIEW_CELL_INERT,
                               int align = wxDVR_DEFAULT_ALIGNMENT );
 
-    bool SetValue( const wxVariant &value );
-    bool GetValue( wxVariant &value ) const;
+    virtual bool SetValue( const wxVariant &value );
+    virtual bool GetValue( wxVariant &value ) const;
 
-    bool Render( wxRect cell, wxDC *dc, int state );
-    wxSize GetSize() const;
+    virtual bool Render( wxRect cell, wxDC *dc, int state );
+    virtual wxSize GetSize() const;
 
     // Implementation only, don't use nor override
     virtual bool WXActivateCell(const wxRect& cell,
@@ -136,8 +136,8 @@ public:
                                 wxDataViewCellMode mode = wxDATAVIEW_CELL_INERT,
                                 int align = wxDVR_DEFAULT_ALIGNMENT );
 
-    bool SetValue( const wxVariant &value );
-    bool GetValue( wxVariant& value ) const;
+    virtual bool SetValue( const wxVariant &value );
+    virtual bool GetValue( wxVariant& value ) const;
 
     virtual bool Render(wxRect cell, wxDC *dc, int state);
     virtual wxSize GetSize() const;
@@ -161,8 +161,8 @@ public:
                                 wxDataViewCellMode mode = wxDATAVIEW_CELL_INERT,
                                 int align = wxDVR_DEFAULT_ALIGNMENT );
 
-    bool SetValue( const wxVariant &value );
-    bool GetValue( wxVariant &value ) const;
+    virtual bool SetValue( const wxVariant &value );
+    virtual bool GetValue( wxVariant &value ) const;
 
     virtual bool Render(wxRect cell, wxDC *dc, int state);
     virtual wxSize GetSize() const;
diff --git a/include/wx/stream.h b/include/wx/stream.h
index 9af5bac9ad..b8432607bd 100644
--- a/include/wx/stream.h
+++ b/include/wx/stream.h
@@ -308,9 +308,9 @@ public:
     wxFilterInputStream(wxInputStream *stream);
     virtual ~wxFilterInputStream();
 
-    char Peek() { return m_parent_i_stream->Peek(); }
+    virtual char Peek() { return m_parent_i_stream->Peek(); }
 
-    wxFileOffset GetLength() const { return m_parent_i_stream->GetLength(); }
+    virtual wxFileOffset GetLength() const { return m_parent_i_stream->GetLength(); }
 
     wxInputStream *GetFilterInputStream() const { return m_parent_i_stream; }
 
@@ -330,7 +330,7 @@ public:
     wxFilterOutputStream(wxOutputStream *stream);
     virtual ~wxFilterOutputStream();
 
-    wxFileOffset GetLength() const { return m_parent_o_stream->GetLength(); }
+    virtual wxFileOffset GetLength() const { return m_parent_o_stream->GetLength(); }
 
     wxOutputStream *GetFilterOutputStream() const { return m_parent_o_stream; }
 
@@ -561,13 +561,13 @@ public:
 
     virtual ~wxBufferedInputStream();
 
-    char Peek();
-    wxInputStream& Read(void *buffer, size_t size);
+    virtual char Peek();
+    virtual wxInputStream& Read(void *buffer, size_t size);
 
     // Position functions
-    wxFileOffset SeekI(wxFileOffset pos, wxSeekMode mode = wxFromStart);
-    wxFileOffset TellI() const;
-    bool IsSeekable() const { return m_parent_i_stream->IsSeekable(); }
+    virtual wxFileOffset SeekI(wxFileOffset pos, wxSeekMode mode = wxFromStart);
+    virtual wxFileOffset TellI() const;
+    virtual bool IsSeekable() const { return m_parent_i_stream->IsSeekable(); }
 
     // the buffer given to the stream will be deleted by it
     void SetInputStreamBuffer(wxStreamBuffer *buffer);
@@ -604,17 +604,17 @@ public:
 
     virtual ~wxBufferedOutputStream();
 
-    wxOutputStream& Write(const void *buffer, size_t size);
+    virtual wxOutputStream& Write(const void *buffer, size_t size);
 
     // Position functions
-    wxFileOffset SeekO(wxFileOffset pos, wxSeekMode mode = wxFromStart);
-    wxFileOffset TellO() const;
-    bool IsSeekable() const { return m_parent_o_stream->IsSeekable(); }
+    virtual wxFileOffset SeekO(wxFileOffset pos, wxSeekMode mode = wxFromStart);
+    virtual wxFileOffset TellO() const;
+    virtual bool IsSeekable() const { return m_parent_o_stream->IsSeekable(); }
 
     void Sync();
     bool Close();
 
-    wxFileOffset GetLength() const;
+    virtual wxFileOffset GetLength() const;
 
     // the buffer given to the stream will be deleted by it
     void SetOutputStreamBuffer(wxStreamBuffer *buffer);
diff --git a/include/wx/wfstream.h b/include/wx/wfstream.h
index 613b4c273b..488d964d62 100644
--- a/include/wx/wfstream.h
+++ b/include/wx/wfstream.h
@@ -35,20 +35,20 @@ public:
     wxFileInputStream(int fd);
     virtual ~wxFileInputStream();
 
-    wxFileOffset GetLength() const;
+    virtual wxFileOffset GetLength() const;
 
     bool Ok() const { return IsOk(); }
     virtual bool IsOk() const;
-    bool IsSeekable() const { return m_file->GetKind() == wxFILE_KIND_DISK; }
+    virtual bool IsSeekable() const { return m_file->GetKind() == wxFILE_KIND_DISK; }
 
     wxFile* GetFile() const { return m_file; }
 
 protected:
     wxFileInputStream();
 
-    size_t OnSysRead(void *buffer, size_t size);
-    wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode);
-    wxFileOffset OnSysTell() const;
+    virtual size_t OnSysRead(void *buffer, size_t size);
+    virtual wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode);
+    virtual wxFileOffset OnSysTell() const;
 
 protected:
     wxFile *m_file;
@@ -67,20 +67,20 @@ public:
 
     void Sync();
     bool Close() { return m_file_destroy ? m_file->Close() : true; }
-    wxFileOffset GetLength() const;
+    virtual wxFileOffset GetLength() const;
 
     bool Ok() const { return IsOk(); }
     virtual bool IsOk() const;
-    bool IsSeekable() const { return m_file->GetKind() == wxFILE_KIND_DISK; }
+    virtual bool IsSeekable() const { return m_file->GetKind() == wxFILE_KIND_DISK; }
 
     wxFile* GetFile() const { return m_file; }
 
 protected:
     wxFileOutputStream();
 
-    size_t OnSysWrite(const void *buffer, size_t size);
-    wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode);
-    wxFileOffset OnSysTell() const;
+    virtual size_t OnSysWrite(const void *buffer, size_t size);
+    virtual wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode);
+    virtual wxFileOffset OnSysTell() const;
 
 protected:
     wxFile *m_file;
@@ -99,14 +99,14 @@ public:
     WXDLLIMPEXP_INLINE_BASE virtual bool Commit() { return m_file->Commit(); }
     WXDLLIMPEXP_INLINE_BASE virtual void Discard() { m_file->Discard(); }
 
-    wxFileOffset GetLength() const { return m_file->Length(); }
-    bool IsSeekable() const { return true; }
+    virtual wxFileOffset GetLength() const { return m_file->Length(); }
+    virtual bool IsSeekable() const { return true; }
 
 protected:
-    size_t OnSysWrite(const void *buffer, size_t size);
-    wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode)
+    virtual size_t OnSysWrite(const void *buffer, size_t size);
+    virtual wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode)
         { return m_file->Seek(pos, mode); }
-    wxFileOffset OnSysTell() const { return m_file->Tell(); }
+    virtual wxFileOffset OnSysTell() const { return m_file->Tell(); }
 
 private:
     wxTempFile *m_file;
@@ -166,20 +166,20 @@ public:
     wxFFileInputStream(FILE *file);
     virtual ~wxFFileInputStream();
 
-    wxFileOffset GetLength() const;
+    virtual wxFileOffset GetLength() const;
 
     bool Ok() const { return IsOk(); }
     virtual bool IsOk() const;
-    bool IsSeekable() const { return m_file->GetKind() == wxFILE_KIND_DISK; }
+    virtual bool IsSeekable() const { return m_file->GetKind() == wxFILE_KIND_DISK; }
 
     wxFFile* GetFile() const { return m_file; }
 
 protected:
     wxFFileInputStream();
 
-    size_t OnSysRead(void *buffer, size_t size);
-    wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode);
-    wxFileOffset OnSysTell() const;
+    virtual size_t OnSysRead(void *buffer, size_t size);
+    virtual wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode);
+    virtual wxFileOffset OnSysTell() const;
 
 protected:
     wxFFile *m_file;
@@ -198,20 +198,20 @@ public:
 
     void Sync();
     bool Close() { return m_file_destroy ? m_file->Close() : true; }
-    wxFileOffset GetLength() const;
+    virtual wxFileOffset GetLength() const;
 
     bool Ok() const { return IsOk(); }
     virtual bool IsOk() const;
-    bool IsSeekable() const { return m_file->GetKind() == wxFILE_KIND_DISK; }
+    virtual bool IsSeekable() const { return m_file->GetKind() == wxFILE_KIND_DISK; }
 
     wxFFile* GetFile() const { return m_file; }
 
 protected:
     wxFFileOutputStream();
 
-    size_t OnSysWrite(const void *buffer, size_t size);
-    wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode);
-    wxFileOffset OnSysTell() const;
+    virtual size_t OnSysWrite(const void *buffer, size_t size);
+    virtual wxFileOffset OnSysSeek(wxFileOffset pos, wxSeekMode mode);
+    virtual wxFileOffset OnSysTell() const;
 
 protected:
     wxFFile *m_file;