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authorVincent Ambo <tazjin@google.com>2020-05-17T14·52+0100
committerVincent Ambo <tazjin@google.com>2020-05-17T14·52+0100
commit7994fd1d545cc5c876d6f21db7ddf9185d23dad6 (patch)
tree32dd695785378c5b9c8be97fc583e9dfc62cb105 /third_party/nix/src/libutil/lazy.hh
parentcf8cd640c1adf74a3706efbcb0ea4625da106fb2 (diff)
parent90b3b31dc27f31e9b11653a636025d29ddb087a3 (diff)
Add 'third_party/nix/' from commit 'be66c7a6b24e3c3c6157fd37b86c7203d14acf10' r/724
git-subtree-dir: third_party/nix
git-subtree-mainline: cf8cd640c1adf74a3706efbcb0ea4625da106fb2
git-subtree-split: be66c7a6b24e3c3c6157fd37b86c7203d14acf10
Diffstat (limited to 'third_party/nix/src/libutil/lazy.hh')
-rw-r--r--third_party/nix/src/libutil/lazy.hh48
1 files changed, 48 insertions, 0 deletions
diff --git a/third_party/nix/src/libutil/lazy.hh b/third_party/nix/src/libutil/lazy.hh
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+++ b/third_party/nix/src/libutil/lazy.hh
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+#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;
+    }
+};
+
+}