Question

I want to implement a timer queuing system using the C++ STL priority_queue container adapter.

My problem is that I want to occasionally cancel a timer, however there are no interfaces that enable me to easily delete an item in the priority_queue that is not the top item.

Any suggestions?.

Thank you for your help.

Answer 1

I had the exact same scenario once and did the following:

• the structure I kept in std::priority_queue contained only the time to sort by and an index to a std::vector<Handler> (in my case Handler was boost::function, but could as well be pointer to interface or function)
• when adding a timer, I’d find a free index in the vector of handlers¹ and store the handler at that index. Store the index and the time in the priority_queue. Return the index to the client as token to cancel
• to cancel a timer, pass the index received when adding it. Clear the handler at that index (for boost::function call clear(), if using pointers, set it to zero)
• when it’s time to callback a timer, get its handler index from the priority queue and check the handlers vector – if the handler at that position is empty()/NULL, the timer has been canceled. Mark that handler index as free².

¹ To make finding a free index fast, I used a separate std::stack of indices. When adding a timer and that stack is empty, add at the end of vector; otherwise pop the top index and use it.

² Here’s the point when you push the index to the free indices stack

The whole thing is somewhat tricky and error-prone, especially if your timer callbacks need to add or cancel timers. Here’s a link to my canceling timer class described above, this code is public domain