1 files changed, 48 insertions, 0 deletions
diff --git a/src/libutil/lazy.hh b/src/libutil/lazy.hh
new file mode 100644
index 000000000..d073e486c
--- /dev/null
+++ b/src/libutil/lazy.hh
@@ -0,0 +1,48 @@
+#include <exception>
+#include <functional>
+#include <mutex>
+
+namespace nix {
+
+/* A helper class for lazily-initialized variables.
+
+     Lazy<T> var([]() { return value; });
+
+   declares a variable of type T that is initialized to 'value' (in a
+   thread-safe way) on first use, that is, when var() is first
+   called. If the initialiser code throws an exception, then all
+   subsequent calls to var() will rethrow that exception. */
+template<typename T>
+class Lazy
+{
+
+    typedef std::function<T()> Init;
+
+    Init init;
+
+    std::once_flag done;
+
+    T value;
+
+    std::exception_ptr ex;
+
+public:
+
+    Lazy(Init init) : init(init)
+    { }
+
+    const T & operator () ()
+    {
+        std::call_once(done, [&]() {
+            try {
+                value = init();
+            } catch (...) {
+                ex = std::current_exception();
+            }
+        });
+        if (ex) std::rethrow_exception(ex);
+        return value;
+    }
+};
+
+}