If you invert the walls and the input map then you might be able to use the dijkstra map to find interesting paths between rooms.

map.cpp

@@ -38,7 +38,7 @@ inline void add_neighbors(PairList &neighbors, Matrix &closed, size_t j, size_t
 }
 
 Map::Map(size_t width, size_t height) : m_limit(1000) {
-  m_walls = Matrix(height, MatrixRow(width, 1));
+  m_walls = Matrix(height, MatrixRow(width, 0));
   m_input_map = Matrix(height, MatrixRow(width, 1));
 }
 
@@ -100,7 +100,10 @@ void Map::make_room(size_t origin_x, size_t origin_y, size_t w, size_t h) {
     dbc::check(y < m_walls.size(), "y is out of bounds");
     for(size_t x = origin_x; x < origin_x + w; ++x) {
       dbc::check(x < m_walls[y].size(), "x is out of bounds");
-      m_walls[y][x] = 0;
+      if(x == origin_x && y == origin_y) {
+        m_input_map[y-1][x] = 0;
+      }
+      m_walls[y][x] = 1;
     }
   }
 }