In order for agg::path_cmd_curve3, agg::path_cmd_curve4 etc to take effect, it is necessary to add an extra step that compute the actual curve path using agg::conv_curve. Then use this curve path to generate the stroke path.

There are a number of advantages with this approach.

One does not have to pay the cost of generating curves when there are no curves in the path.
It is easy to add / combine path generators depending on the need.

agg::conv_curve<CmdVertexPath> curvePath (path);

agg::conv_stroke<agg::conv_curve<CmdVertexPath>> strokePath (curvePath);

Example

tutorial_linedrawing_2.cpp generates an image containing a caret and a dome.

An equivalent SVG is