Adding new add_vertex function and corresponding tests.

include/graaflib/graph.h

@@ -179,6 +179,16 @@ class graph {
    */
   [[nodiscard]] vertex_id_t add_vertex(auto&& vertex);
 
+  /**
+   * Add a vertex to the graph with a specific ID
+   *
+   * @param  vertex The vertex to be added
+   * @param  id The requested ID for the new vertex
+   * @return vertices_id_t - The ID of the new vertex
+   * @throws id_taken exception - If the relevant ID is already in use
+   */
+  [[nodiscard]] vertex_id_t add_vertex(auto&& vertex, vertex_id_t id);
+
   /**
    * Remove a vertex from the graph and update all its neighbors
    *

include/graaflib/graph.tpp

@@ -135,6 +135,18 @@ vertex_id_t graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::add_vertex(auto&& vertex) {
   return vertex_id;
 }
 
+template <typename VERTEX_T, typename EDGE_T, graph_type GRAPH_TYPE_V>
+vertex_id_t graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::add_vertex(auto&& vertex,
+                                                              vertex_id_t id) {
+  if (has_vertex(id)) {
+    throw std::invalid_argument{"Vertex already exists at ID [" +
+                                std::to_string(id) + "]"};
+  }
+
+  vertices_.emplace(id, std::forward<VERTEX_T>(vertex));
+  return id;
+}
+
 template <typename VERTEX_T, typename EDGE_T, graph_type GRAPH_TYPE_V>
 void graph<VERTEX_T, EDGE_T, GRAPH_TYPE_V>::remove_vertex(
     vertex_id_t vertex_id) {

test/graaflib/graph_test.cpp

@@ -38,6 +38,13 @@ TYPED_TEST(GraphTest, VertexCount) {
   ASSERT_EQ(graph.vertex_count(), 2);
   ASSERT_TRUE(graph.has_vertex(vertex_id_2));
   ASSERT_EQ(graph.get_vertex(vertex_id_2), 20);
+
+  // WHEN - THEN
+  constexpr int specific_id = 4;
+  const auto vertex_specific_id{graph.add_vertex(30, specific_id)};
+  ASSERT_EQ(graph.vertex_count(), 3);
+  ASSERT_TRUE(graph.has_vertex(specific_id));
+  ASSERT_EQ(graph.get_vertex(specific_id), 30);
 }
 
 TYPED_TEST(GraphTest, RemoveVertex) {
@@ -173,6 +180,24 @@ TYPED_TEST(GraphTest, VertexTests) {
   EXPECT_FALSE(graph.has_vertex(nonExistingVertexId));
   EXPECT_TRUE(graph.has_vertex(vertex_id_2));
   EXPECT_EQ(graph.get_vertex(vertex_id_2), 20);
+
+  ASSERT_THROW(
+      {
+        try {
+          // Add vertex to ID that already exists
+          [[maybe_unused]] const auto duplicate_id{
+              graph.add_vertex(50, vertex_id_1)};
+          FAIL()
+              << "Expected std::invalid_argument exception, but no exception "
+                 "was thrown.";
+        } catch (const std::invalid_argument &ex) {
+          EXPECT_EQ(ex.what(), fmt::format("Vertex already exists at ID [{}]",
+                                           vertex_id_1));
+          throw;
+        }
+      },
+      std::invalid_argument);
+  EXPECT_EQ(graph.get_vertex(vertex_id_1), 1);
 }
 
 TYPED_TEST(GraphTest, GetEdgeNonExistingEdge) {