lockTry should not be callable on temporary lock

utilite/include/rtabmap/utilite/UMutex.h

@@ -177,6 +177,17 @@ public:
  * }
  * @endcode
  *
+ * The object must be named: a temporary would be destroyed, and the mutex unlocked,
+ * at the end of the expression. lock(), lockTry() and unlock() can only be called on
+ * a named object, so that `if(UScopeMutex(m, false).lockTry() == 0)` doesn't compile.
+ * In C++17, the object can be scoped to an if statement instead:
+ * @code
+ * if(UScopeMutex sm(m, false); sm.lockTry() == 0)
+ * {
+ *    // locked here
+ * } // unlocked here, only if lockTry() succeeded
+ * @endcode
+ *
  * @see UMutex
  */
 class UScopeMutex
@@ -215,7 +226,7 @@ public:
 	 * Lock the mutex, if this object doesn't hold it already.
 	 * @return 0 on success, an error code otherwise.
 	 */
-	int lock()
+	int lock() &
 	{
 		if(locked_)
 		{
@@ -232,7 +243,7 @@ public:
 	 * @return 0 if the mutex is held by this object, EBUSY (or another
 	 *         error code) otherwise.
 	 */
-	int lockTry()
+	int lockTry() &
 	{
 		if(locked_)
 		{
@@ -248,7 +259,7 @@ public:
 	 * Unlock the mutex before this object goes out of scope, only if this object locked it.
 	 * @return 0 on success (or if this object didn't hold the mutex), an error code otherwise.
 	 */
-	int unlock()
+	int unlock() &
 	{
 		if(!locked_)
 		{

utilite/test/test_umutex.cpp

@@ -3,6 +3,8 @@
 #include <thread>
 #include <chrono>
 #include <atomic>
+#include <type_traits>
+#include <utility>
 #include <vector>
 
 TEST(UMutexTest, Constructor)
@@ -149,6 +151,15 @@ TEST(UMutexTest, UScopeMutexWithPointer)
     t.join();
 }
 
+// lock(), lockTry() and unlock() can only be called on a named UScopeMutex: a temporary
+// would unlock the mutex at the end of the expression, before the code it should protect.
+template<typename T, typename = void>
+struct CanLockTry : std::false_type {};
+template<typename T>
+struct CanLockTry<T, decltype(void(std::declval<T>().lockTry()))> : std::true_type {};
+static_assert(CanLockTry<UScopeMutex &>::value, "lockTry() must be callable on a named UScopeMutex");
+static_assert(!CanLockTry<UScopeMutex>::value, "lockTry() must not be callable on a temporary UScopeMutex");
+
 TEST(UMutexTest, UScopeMutexDeferredIsNotLocked)
 {
     UMutex mutex;