Functional: Add ScopeGuard implementation

modules/juce_core/misc/juce_Functional.h

@@ -86,4 +86,31 @@ Object withMember (Object copy, Member OtherObject::* member, Other&& value)
     return copy;
 }
 
+/** An easy way to ensure that a function is called at the end of the current
+    scope.
+
+    Usage:
+    @code
+    {
+        if (flag == true)
+            return;
+
+        // While this code executes, flag is true e.g. to prevent reentrancy
+        flag = true;
+        // When we exit this scope, flag must be false
+        const ScopeGuard scope { [&] { flag = false; } };
+
+        if (checkInitialCondition())
+            return; // Scope's lambda will fire here...
+
+        if (checkCriticalCondition())
+            throw std::runtime_error{}; // ...or here...
+
+        doWorkHavingEstablishedPreconditions();
+    } // ...or here!
+    @endcode
+*/
+template <typename Fn> struct ScopeGuard : Fn { ~ScopeGuard() { Fn::operator()(); } };
+template <typename Fn> ScopeGuard (Fn) -> ScopeGuard<Fn>;
+
 } // namespace juce