i usually find it best to keep all orders info with the unit, IE what to build and where. it sounds like your map holds the what and where info (and perhaps by whom?), or am i just reading it wrong?.

in your example, orders to build would be replaced with orders to return resources to the storage depot, or to do nothing ( or some other default behavior), if the unit wasn't carrying any resources.

Well my current solution is that i am using struct task which have pointers on units. So when i cancel some task i can clearly see if the task is assignment. Now i have to figure out how to make AI to effectively pick and accomplish tasks.

so the player queues up tasks, and the AI assigns appropriate units? and a task struct has all the "orders info": what to build, where to build, unit assigned etc. all that sounds fine.

tasks can be processed first come first serve, or on some priority basis (gather before build, etc). you'll probably want to use first come first served, so the player defines the order.

for assigning units, you'd probably use a best match search for closest available unit of the correct type.