cmConstStack: Factor out of cmListFileBacktrace

This presents value semantics for a stack of constant values.
Internally it shares ownership to avoid copies.  Previously
this was implemented by `cmListFileBacktrace` explicitly,
but the approach can be re-used for other kinds of stacks.

1 files changed, 8 insertions, 26 deletions

diff --git a/Source/cmListFileCache.h b/Source/cmListFileCache.h
index 7c19f24..f7c2509 100644
--- a/Source/cmListFileCache.h
+++ b/Source/cmListFileCache.h
@@ -12,6 +12,7 @@
 
 #include <cm/optional>
 
+#include "cmConstStack.h"
 #include "cmSystemTools.h"
 
 /** \class cmListFileCache
@@ -157,35 +158,16 @@ bool operator<(const cmListFileContext& lhs, const cmListFileContext& rhs);
 bool operator==(cmListFileContext const& lhs, cmListFileContext const& rhs);
 bool operator!=(cmListFileContext const& lhs, cmListFileContext const& rhs);
 
-// Represent a backtrace (call stack).  Provide value semantics
-// but use efficient reference-counting underneath to avoid copies.
+// Represent a backtrace (call stack) with efficient value semantics.
 class cmListFileBacktrace
+  : public cmConstStack<cmListFileContext, cmListFileBacktrace>
 {
-public:
-  // Default-constructed backtrace is empty.
-  cmListFileBacktrace() = default;
-
-  // Get a backtrace with the given call context added to the top.
-  cmListFileBacktrace Push(cmListFileContext const& lfc) const;
-
-  // Get a backtrace with the top level removed.
-  // May not be called until after a matching Push.
-  cmListFileBacktrace Pop() const;
-
-  // Get the context at the top of the backtrace.
-  // This may be called only if Empty() would return false.
-  cmListFileContext const& Top() const;
-
-  // Return true if this backtrace is empty.
-  bool Empty() const;
-
-private:
-  struct Entry;
-  std::shared_ptr<Entry const> TopEntry;
-  cmListFileBacktrace(std::shared_ptr<Entry const> parent,
-                      cmListFileContext const& lfc);
-  cmListFileBacktrace(std::shared_ptr<Entry const> top);
+  using cmConstStack::cmConstStack;
+  friend class cmConstStack<cmListFileContext, cmListFileBacktrace>;
 };
+#ifndef cmListFileCache_cxx
+extern template class cmConstStack<cmListFileContext, cmListFileBacktrace>;
+#endif
 
 // Wrap type T as a value with a backtrace.  For purposes of
 // ordering and equality comparison, only the original value is