Question

In my code I have a map which holds a large amount of data (~100MB) I need to copy all that data from one map to another. currently I am doing this with swap but to my understanding, swap is a fancy way to do a copy. Is there a way to simply transfer the memory used by the two maps? I think that I can do this with pointers but I was hoping for a more elegant way.

Answer 1

Unless this came up in a profiler run as a bottleneck, you may be optimizing prematurely.

My compiler’s std::map::swap() has the following comment, which indicates that a map swap is likely to be very fast:

  /**
   *  This exchanges the elements between two maps in constant
   *  time.  (It is only swapping a pointer, an integer, and an
   *  instance of the @c Compare type (which itself is often
   *  stateless and empty), so it should be quite fast.)  Note
   *  that the global std::swap() function is specialized such
   *  that std::swap(m1,m2) will feed to this function.
   */

(g++ 4.4.5)