Use emplace_back in extensions

With `emplace_back`, we don't need to create an object ourselves, to be
copied into the container, plus it returns a reference to the new object
for us to use.

This used clang-tidy's `modernize-use-emplace` check, plus some
followups to use the return values.

Author: QuLogic

src/_backend_agg_basic_types.h

@@ -48,7 +48,7 @@ class Dashes
     }
     void add_dash_pair(double length, double skip)
     {
-        dashes.push_back(std::make_pair(length, skip));
+        dashes.emplace_back(length, skip);
     }
     size_t size() const
     {

src/_path.h

@@ -617,11 +617,11 @@ inline void clip_to_rect_one_step(const Polygon &polygon, Polygon &result, const
         if (sinside ^ pinside) {
             double bx, by;
             filter.bisect(sx, sy, px, py, &bx, &by);
-            result.push_back(XY(bx, by));
+            result.emplace_back(bx, by);
         }
 
         if (pinside) {
-            result.push_back(XY(px, py));
+            result.emplace_back(px, py);
         }
 
         sx = px;
@@ -668,7 +668,7 @@ clip_path_to_rect(PathIterator &path, agg::rect_d &rect, bool inside, std::vecto
         polygon1.clear();
         do {
             if (code == agg::path_cmd_move_to) {
-                polygon1.push_back(XY(x, y));
+                polygon1.emplace_back(x, y);
             }
 
             code = curve.vertex(&x, &y);
@@ -678,7 +678,7 @@ clip_path_to_rect(PathIterator &path, agg::rect_d &rect, bool inside, std::vecto
             }
 
             if (code != agg::path_cmd_move_to) {
-                polygon1.push_back(XY(x, y));
+                polygon1.emplace_back(x, y);
             }
         } while ((code & agg::path_cmd_end_poly) != agg::path_cmd_end_poly);
 
@@ -974,23 +974,20 @@ void convert_path_to_polygons(PathIterator &path,
     simplify_t simplified(clipped, simplify, path.simplify_threshold());
     curve_t curve(simplified);
 
-    result.push_back(Polygon());
-    Polygon *polygon = &result.back();
+    Polygon *polygon = &result.emplace_back();
     double x, y;
     unsigned code;
 
     while ((code = curve.vertex(&x, &y)) != agg::path_cmd_stop) {
         if ((code & agg::path_cmd_end_poly) == agg::path_cmd_end_poly) {
             _finalize_polygon(result, 1);
-            result.push_back(Polygon());
-            polygon = &result.back();
+            polygon = &result.emplace_back();
         } else {
             if (code == agg::path_cmd_move_to) {
                 _finalize_polygon(result, closed_only);
-                result.push_back(Polygon());
-                polygon = &result.back();
+                polygon = &result.emplace_back();
             }
-            polygon->push_back(XY(x, y));
+            polygon->emplace_back(x, y);
         }
     }
 

src/tri/_tri.cpp

@@ -269,11 +269,10 @@ void Triangulation::calculate_boundaries()
         auto it = boundary_edges.cbegin();
         int tri = it->tri;
         int edge = it->edge;
-        _boundaries.push_back(Boundary());
-        Boundary& boundary = _boundaries.back();
+        Boundary& boundary = _boundaries.emplace_back();
 
         while (true) {
-            boundary.push_back(TriEdge(tri, edge));
+            boundary.emplace_back(tri, edge);
             boundary_edges.erase(it);
             _tri_edge_to_boundary_map[TriEdge(tri, edge)] =
                 BoundaryEdge(_boundaries.size()-1, boundary.size()-1);
@@ -624,7 +623,7 @@ void TriContourGenerator::clear_visited_flags(bool include_boundaries)
             // Initialise _boundaries_visited.
             _boundaries_visited.reserve(boundaries.size());
             for (const auto & boundary : boundaries) {
-                _boundaries_visited.push_back(BoundaryVisited(boundary.size()));
+                _boundaries_visited.emplace_back(boundary.size());
             }
 
             // Initialise _boundaries_used.
@@ -796,8 +795,7 @@ void TriContourGenerator::find_boundary_lines(Contour& contour,
             if (startAbove && !endAbove) {
                 // This boundary edge is the start point for a contour line,
                 // so follow the line.
-                contour.push_back(ContourLine());
-                ContourLine& contour_line = contour.back();
+                ContourLine& contour_line = contour.emplace_back();
                 TriEdge tri_edge = *itb;
                 follow_interior(contour_line, tri_edge, true, level, false);
             }
@@ -830,8 +828,7 @@ void TriContourGenerator::find_boundary_lines_filled(Contour& contour,
 
                 if (decr_lower || incr_upper) {
                     // Start point for contour line, so follow it.
-                    contour.push_back(ContourLine());
-                    ContourLine& contour_line = contour.back();
+                    ContourLine& contour_line = contour.emplace_back();
                     TriEdge start_tri_edge = boundary[j];
                     TriEdge tri_edge = start_tri_edge;
 
@@ -859,8 +856,7 @@ void TriContourGenerator::find_boundary_lines_filled(Contour& contour,
             const Boundary& boundary = boundaries[i];
             double z = get_z(triang.get_triangle_point(boundary[0]));
             if (z >= lower_level && z < upper_level) {
-                contour.push_back(ContourLine());
-                ContourLine& contour_line = contour.back();
+                ContourLine& contour_line = contour.emplace_back();
                 for (auto edge : boundary) {
                     contour_line.push_back(triang.get_point_coords(
                                       triang.get_triangle_point(edge)));
@@ -894,8 +890,7 @@ void TriContourGenerator::find_interior_lines(Contour& contour,
             continue;  // Contour does not pass through this triangle.
 
         // Found start of new contour line loop.
-        contour.push_back(ContourLine());
-        ContourLine& contour_line = contour.back();
+        ContourLine& contour_line = contour.emplace_back();
         TriEdge tri_edge = triang.get_neighbor_edge(tri, edge);
         follow_interior(contour_line, tri_edge, false, level, on_upper);
 
@@ -1434,10 +1429,10 @@ TrapezoidMapTriFinder::initialize()
 
     // Set up edges array.
     // First the bottom and top edges of the enclosing rectangle.
-    _edges.push_back(Edge(&_points[npoints],   &_points[npoints+1], -1, -1,
-                          nullptr, nullptr));
-    _edges.push_back(Edge(&_points[npoints+2], &_points[npoints+3], -1, -1,
-                          nullptr, nullptr));
+    _edges.emplace_back(&_points[npoints],   &_points[npoints+1], -1, -1,
+                        nullptr, nullptr);
+    _edges.emplace_back(&_points[npoints+2], &_points[npoints+3], -1, -1,
+                        nullptr, nullptr);
 
     // Add all edges in the triangulation that point to the right.  Do not
     // explicitly include edges that point to the left as the neighboring
@@ -1456,11 +1451,12 @@ TrapezoidMapTriFinder::initialize()
                     const Point* neighbor_point_below = (neighbor.tri == -1) ?
                         nullptr : _points + triang.get_triangle_point(
                                           neighbor.tri, (neighbor.edge+2)%3);
-                    _edges.push_back(Edge(start, end, neighbor.tri, tri,
-                                          neighbor_point_below, other));
+                    _edges.emplace_back(start, end, neighbor.tri, tri,
+                                        neighbor_point_below, other);
+                }
+                else if (neighbor.tri == -1) {
+                    _edges.emplace_back(end, start, tri, -1, other, nullptr);
                 }
-                else if (neighbor.tri == -1)
-                    _edges.push_back(Edge(end, start, tri, -1, other, nullptr));
 
                 // Set triangle associated with start point if not already set.
                 if (start->tri == -1)