Null task

 * Add a way to have a null-task which is invalid for evaluation

src/plugins/projectexplorer/task.cpp

@@ -32,12 +32,10 @@
 namespace ProjectExplorer
 {
 
-unsigned int Task::s_nextId = 0;
+unsigned int Task::s_nextId = 1;
 
-Task::Task() : taskId(s_nextId), type(Unknown), line(-1)
-{
-    ++s_nextId;
-}
+Task::Task() : taskId(0), type(Unknown), line(-1)
+{ }
 
 Task::Task(TaskType type_, const QString &description_,
            const QString &file_, int line_, const QString &category_) :
@@ -47,6 +45,9 @@ Task::Task(TaskType type_, const QString &description_,
     ++s_nextId;
 }
 
+bool Task::isNull() const
+{ return taskId == 0; }
+
 //
 // functions
 //

src/plugins/projectexplorer/task.h

@@ -52,6 +52,8 @@ public:
     Task(TaskType type_, const QString &description_,
          const QString &file_, int line_, const QString &category_);
 
+    bool isNull() const;
+
     unsigned int taskId;
     TaskType type;
     QString description;