Deduplicate overloads of struct_exprt::component

This commit uses a template to implement both the const and non-const
overloads of `struct_exprt::component`. This reduces the overhead of
maintaining 2 identical versions and means that both overloads must be
implemented in the same way.

Author: thomasspriggs

src/util/std_expr.cpp

@@ -170,21 +170,30 @@ address_of_exprt::address_of_exprt(const exprt &_op):
 {
 }
 
+// Implementation of struct_exprt::component for const / non const overloads.
+template <typename T>
+auto component(T &struct_expr, const irep_idt &name, const namespacet &ns)
+  -> decltype(struct_expr.op0())
+{
+  static_assert(
+    std::is_base_of<struct_exprt, T>::value, "T must be a struct_exprt.");
+  const auto index =
+    to_struct_type(ns.follow(struct_expr.type())).component_number(name);
+  DATA_INVARIANT(
+    index < struct_expr.operands().size(),
+    "component matching index should exist");
+  return struct_expr.operands()[index];
+}
+
 /// \return The expression for a named component of this struct.
 exprt &struct_exprt::component(const irep_idt &name, const namespacet &ns)
 {
-  const auto index = to_struct_type(ns.follow(type())).component_number(name);
-  DATA_INVARIANT(
-    index < operands().size(), "component matching index should exist");
-  return operands()[index];
+  return ::component(*this, name, ns);
 }
 
 /// \return The expression for a named component of this struct.
 const exprt &
 struct_exprt::component(const irep_idt &name, const namespacet &ns) const
 {
-  const auto index = to_struct_type(ns.follow(type())).component_number(name);
-  DATA_INVARIANT(
-    index < operands().size(), "component matching index should exist");
-  return operands()[index];
+  return ::component(*this, name, ns);
 }