ADT/InMemoryStruct: Add an experimental helper class intended for use in

situations where on the common path an API can return a pointer to some direct
memory, but which on an exceptional path may need to return a pointer to a
temporary struct.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@120201 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/InMemoryStruct.h

@@ -0,0 +1,77 @@
+//===- InMemoryStruct.h - Indirect Struct Access Smart Pointer --*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_ADT_INMEMORYSTRUCT_H
+#define LLVM_ADT_INMEMORYSTRUCT_H
+
+#include <cassert>
+
+namespace llvm {
+
+/// \brief Helper object for abstracting access to an in-memory structure which
+/// may require some kind of temporary storage.
+///
+/// This class is designed to be used for accessing file data structures which
+/// in the common case can be accessed from a direct pointer to a memory mapped
+/// object, but which in some cases may require indirect access to a temporary
+/// structure (which, for example, may have undergone endianness translation).
+template<typename T>
+class InMemoryStruct {
+  typedef T value_type;
+  typedef value_type &reference;
+  typedef value_type *pointer;
+  typedef const value_type &const_reference;
+  typedef const value_type *const_pointer;
+
+  /// \brief The smart pointer target.
+  value_type *Target;
+
+  /// \brief A temporary object which can be used as a target of the smart
+  /// pointer.
+  value_type Contents;
+
+private:
+
+public:
+  InMemoryStruct() : Target(0) {}
+  InMemoryStruct(reference Value) : Target(&Contents), Contents(Value) {}
+  InMemoryStruct(pointer Value) : Target(Value) {}
+  InMemoryStruct(const InMemoryStruct<T> &Value) { *this = Value; }
+  
+  void operator=(const InMemoryStruct<T> &Value) {
+    if (Value.Target != &Value.Contents) {
+      Target = Value.Target;
+    } else {
+      Target = &Contents;
+      Contents = Value.Contents;
+    }
+  }
+  
+  const_reference operator*() const {
+    assert(Target && "Cannot dereference null pointer");
+    return *Target;
+  }
+  reference operator*() {
+    assert(Target && "Cannot dereference null pointer");
+    return *Target;
+  }
+
+  const_pointer operator->() const {
+    return Target;
+  }
+  pointer operator->() {
+    return Target;
+  }
+
+  operator bool() const { return Target != 0; }
+};
+
+}
+
+#endif