Add support for getting components by name to struct_exprt

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

@@ -8,6 +8,8 @@ Author: Daniel Kroening, [email protected]
 
 #include "std_expr.h"
 
+#include <util/namespace.h>
+
 #include <cassert>
 
 #include "arith_tools.h"
@@ -167,3 +169,31 @@ address_of_exprt::address_of_exprt(const exprt &_op):
   unary_exprt(ID_address_of, _op, pointer_type(_op.type()))
 {
 }
+
+// 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)
+{
+  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
+{
+  return ::component(*this, name, ns);
+}

src/util/std_expr.h

@@ -1823,6 +1823,9 @@ class struct_exprt:public exprt
     exprt(ID_struct, _type)
   {
   }
+
+  exprt &component(const irep_idt &name, const namespacet &ns);
+  const exprt &component(const irep_idt &name, const namespacet &ns) const;
 };
 
 /*! \brief Cast a generic exprt to a \ref struct_exprt