- wxFilterInputStream* NewStream(wxInputStream& stream) const;
- wxFilterOutputStream* NewStream(wxOutputStream& stream) const;
- wxFilterInputStream* NewStream(wxInputStream* stream) const;
- wxFilterOutputStream* NewStream(wxOutputStream* stream) const;
+ virtual wxFilterInputStream* NewStream(wxInputStream& stream) const = 0;
+ virtual wxFilterOutputStream* NewStream(wxOutputStream& stream) const = 0;
+ virtual wxFilterInputStream* NewStream(wxInputStream* stream) const = 0;
+ virtual wxFilterOutputStream* NewStream(wxOutputStream* stream) const = 0;