diff --git a/threads/barrier.hpp b/threads/barrier.hpp
index dccf29b8..87786643 100644
--- a/threads/barrier.hpp
+++ b/threads/barrier.hpp
@@ -22,7 +22,7 @@
 namespace util { namespace threads {
     class barrier {
     public:
-        barrier (unsigned count);
+        explicit barrier (unsigned count);
         ~barrier ();
 
         void wait (void);
diff --git a/threads/semaphore.hpp b/threads/semaphore.hpp
index 1f3745ce..37510292 100644
--- a/threads/semaphore.hpp
+++ b/threads/semaphore.hpp
@@ -22,7 +22,7 @@
 namespace util { namespace threads {
     class semaphore {
     public:
-        semaphore (unsigned value = 0);
+        explicit semaphore (unsigned value = 0);
         ~semaphore ();
 
         void wait (void);