revert and make sure verts are def. constructible

include/reticula/network_concepts.hpp

@@ -21,6 +21,7 @@ namespace reticula {
 
   template <typename T>
   concept network_vertex =
+    std::default_initializable<T> &&
     std::totally_ordered<T> &&
     hashable_with<T, hash>;
 

include/reticula/static_edges.hpp

@@ -72,7 +72,7 @@ namespace reticula {
       or the relation.
       @param v2 Head end of the edge, often the receiving end of an effect.
      */
-    directed_edge(VertexType v1, VertexType v2);
+    directed_edge(const VertexType& v1, const VertexType& v2);
 
 
     /**
@@ -213,11 +213,9 @@ namespace reticula {
 
     undirected_edge() = default;
     /**
-      Create an undirected edge. Order of the parameters are arbitrary unless
-      dealing with IO functions. IO functions try to preserve the same order as
-      of vertices from input string in the output string.
+      Create an undirected edge. Order of the parameters are arbitrary.
      */
-    undirected_edge(VertexType v1, VertexType v2);
+    undirected_edge(const VertexType& v1, const VertexType& v2);
 
     /**
       An undirected edge is incident to vertex `v` iff `v` is either of its

include/reticula/temporal_edges.hpp

@@ -107,7 +107,7 @@ namespace reticula {
       @param time Timestamp at which the event "happened".
      */
     directed_temporal_edge(
-        VertexType tail, VertexType head, TimeType time);
+        const VertexType& tail, const VertexType& head, TimeType time);
     /**
       Create a directed temporal edge from a static projection and a time. The
       resulting edge would have the same endpoints as projection and the time
@@ -322,7 +322,7 @@ namespace reticula {
       @param effect_time The timestamp at which the event was "received".
      */
     directed_delayed_temporal_edge(
-        VertexType tail, VertexType head,
+        const VertexType& tail, const VertexType& head,
         TimeType cause_time, TimeType effect_time);
     /**
       Create a directed delayed temporal edge from a static projection, a cause
@@ -514,7 +514,7 @@ namespace reticula {
       @param time Timestamp at which the event "happened".
     */
     undirected_temporal_edge(
-        VertexType v1, VertexType v2, TimeType time);
+        const VertexType& v1, const VertexType& v2, TimeType time);
     /**
       Create an undirected temporal edge from a static projection and a time.
       The resulting edge would have the same endpoints as projection and the

src/static_edges.tpp

@@ -53,7 +53,8 @@ namespace reticula {
   // properties of directed edge:
 
   template <network_vertex VertexType>
-  directed_edge<VertexType>::directed_edge(VertexType tail, VertexType head)
+  directed_edge<VertexType>::directed_edge(
+      const VertexType& tail, const VertexType& head)
       : _tail(tail), _head(head) {}
 
   template <network_vertex VertexType>
@@ -146,10 +147,9 @@ namespace reticula {
   // properties of undirected edge:
 
   template <network_vertex VertexType>
-  undirected_edge<VertexType>::undirected_edge(VertexType v1, VertexType v2) {
-    auto [i, j] = std::minmax(v1, v2);
-    _v1 = i;
-    _v2 = j;
+  undirected_edge<VertexType>::undirected_edge(
+      const VertexType& v1, const VertexType& v2) {
+    std::tie(_v1, _v2) = std::minmax(v1, v2);
   }
 
   template <network_vertex VertexType>

src/temporal_edges.tpp

@@ -91,7 +91,7 @@ namespace reticula {
 
   template <network_vertex VertexType, typename TimeType>
   directed_temporal_edge<VertexType, TimeType>::directed_temporal_edge(
-    VertexType tail, VertexType head, TimeType time)
+    const VertexType& tail, const VertexType& head, TimeType time)
     : _time(time), _tail(tail), _head(head) {}
 
   template <network_vertex VertexType, typename TimeType>
@@ -209,7 +209,7 @@ namespace reticula {
   template <network_vertex VertexType, typename TimeType>
   directed_delayed_temporal_edge<VertexType, TimeType>::
     directed_delayed_temporal_edge(
-      VertexType tail, VertexType head,
+      const VertexType& tail, const VertexType& head,
       TimeType cause_time, TimeType effect_time) :
       _cause_time(cause_time), _effect_time(effect_time),
       _tail(tail), _head(head) {
@@ -344,10 +344,9 @@ namespace reticula {
 
   template <network_vertex VertexType, typename TimeType>
   undirected_temporal_edge<VertexType, TimeType>::undirected_temporal_edge(
-      VertexType v1, VertexType v2, TimeType time) : _time(time) {
-    auto [i, j] = std::minmax(v1, v2);
-    _v1 = i;
-    _v2 = j;
+      const VertexType& v1, const VertexType& v2,
+      TimeType time) : _time(time) {
+    std::tie(_v1, _v2) = std::minmax(v1, v2);
   }
 
   template <network_vertex VertexType, typename TimeType>