--- a/src/FreetypeRenderer.h
+++ b/src/FreetypeRenderer.h
@@ -124,7 +124,7 @@
         hb_glyph_position_t *glyph_pos;
     };
 
-    struct done_glyph : public std::unary_function<const GlyphData *, void> {
+    struct done_glyph {
         void operator() (const GlyphData *glyph_data) {
             FT_Done_Glyph(glyph_data->get_glyph());
             delete glyph_data;
