looping through a std::deque and remove entries

Question

I am trying to iterate through a std::deque and remove all of its content. I can do it along the way:

for(auto & x: myDeque)
{
    // do something
    myDeque.pop_front();
}

Or I can do myDeque.clear() at the end after the loop. I am wondering which way I should be using? Thanks!

well you see I want to do something inside the loop, not just erasing the content — Ryan, Jan 28 '21 at 20:47

Jerry Coffin · Accepted Answer · 2021-01-28T21:02:05.537

It's almost certainly preferable to do the loop to process the data first, then separately do a myDeque.clear(); to empty it.

From a purely theoretical viewpoint, it's O(n) either way, but from a practical viewpoint, clear is almost always going to be at least as fast, and usually faster. The possible exception would be if you're dealing with a deque so large that it won't fit in the cache. In this case, doing everything you need to (including deleting) with a particular piece of data at once can avoid having to re-load that data into the cache twice: once for processing, and again to destroy it.

In particular, a deque is normally implemented as a two-level structure: something pretty much like a vector of pointers to blocks, where each block holds a fixed number of data items. When you do pop_front, it has to look at the first block, figure out whether this pop_front has emptied the first block or not. If it has, it deletes that block. If not, it just updates an index to tell it which location in that block is currently the front.

But when you do clear it can just walk through the data, and destroy all the contents.

as a follow-up: what would be the best way to iterate through and remove entry until certain condition is met? — Ryan, Jan 28 '21 at 20:50
@Ryan: Typically you want to use the find/erase idiiom. That is: find the first element that satisfies the condition, then erase everything from the beginning up to that element. Not as important with deque as with vector, but still typically better than deleting one at a time (and experienced programmers will immediately recognize what the code is doing as well). — Jerry Coffin, Jan 28 '21 at 20:55

score 0 · Answer 2 · answered Jan 28 '21 at 20:45

myDeque.clear() will be much more easier as it'll destroy all of its nodes on itself. Use the features which they provide because most of the time, it does it much efficiently and takes the burden off of you.

Another plus for it is that it'll be easier for someone else who reads your code to understand what's going on.

looping through a std::deque and remove entries

2 Answers2