C++ array 'matrix' swap (memory leak)

Question

I'm creating a block based engine and I was working on infinite loading. I was testing some code, but soon I noticed some troubles. At first I was using an std::unordered_map, storing xz as key and ChunkContainer* as value. I store this as pointers (with new) because it's such a big object it can't be all stored on the stack. It's size is: CHUNK_SIZE(32)^3 * WORLD_HEIGHT(8 amount of chunks in height) * 4(block bytes) = 1048576 bytes. (And that * 225 makes up my world.)

Then I swapped to using a big array instead of the std::unordered_map so I could achieve faster read speed. So I needed to swap the loading code. I came up with this code,

but I'm having some issues with a memory leak created when using this code:

for(int z = 0; z < size; z++){
    temp = loadedChunkContainers[(size-1)*size + z]; //Store the last container in a temp var

    for(int x = size-1; x > 0; x--){
        loadedChunkContainers[x * size + z] = loadedChunkContainers[(x-1) * size + z]; //Move all containers 1 to the right
    }
    int cx = temp.getX() - size;
    int cz = temp.getZ();

    temp.move(cx, cz);//Move the container internally
    loadedChunkContainers[z] = temp; //put the container back into the array, but this time on the first row
    buildQueue.push_back(&loadedChunkContainers[z]);
}

temp is a global variable because I can't store it locally because it will overflow the stack. I also can't use swap as it will also overflow the stack.

Should I even use this code? It works, but It's quite a slow way in the first place. Would there be another way to have the fastest read access while still being able to swap values (without memory leaks)?

score 0 · Accepted Answer · answered Nov 19 '14 at 15:28

0

An std::unordered_map will never store its elements "on the stack", so this is not a good reason to store pointers. You can basically forget all of your stack-related worries, here. With that goes away all your pointer troubles. Just store the elements directly and be done with it!

If you have measured the std::unordered_map version of your code and found that the (very fast) hash lookup is prohibitively slow in your application, by all means stick with your sparse array, but make it a std::vector so that elements are dynamically allocated [for you]. Then everything I said in the first paragraph still applies. :)

answered Nov 19 '14 at 15:28

Lightness Races in Orbit

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Yeah, but then I still have the read access speed issue. As hashmaps are slower then arrays – Duckdoom5 Nov 19 '14 at 15:30
@Duckdoom5: That sounds like [FUD](http://en.wikipedia.org/wiki/Fear,_uncertainty_and_doubt). Did you measure it and find out that the (very fast) hash lookup was prohibitive in your project? Even if you stick with the sparse array, make it a `std::vector` and then I stand by the advice in my answer. – Lightness Races in Orbit Nov 19 '14 at 15:31
Well, actually, now that I think about it. The reason it was slow, was because I used strings as key. (It has been a while since I used the map so I can't remember exactly what made me switch.) I might try again with the `std::unordered_map`. Can you change the key of a value without replacing it? cus that would fix all of the issues immediately – Duckdoom5 Nov 19 '14 at 15:38
@Duckdoom5: [Heh, no. :)](http://stackoverflow.com/a/6773748/560648) Though Boost.MultiIndex can do it... – Lightness Races in Orbit Nov 19 '14 at 15:43
Hmm, okay. And just removing and re-adding the element, would that cost a lot or would the Boost.MultiIndex be much faster? – Duckdoom5 Nov 19 '14 at 15:49
@Duckdoom5: I don't see why it would be very expensive. Measure it. – Lightness Races in Orbit Nov 19 '14 at 15:57
Could you help me with the stack overflow? I've edited the post. – Duckdoom5 Nov 19 '14 at 16:15
@Duckdoom5: That's a new question. Please submit it as such. – Lightness Races in Orbit Nov 19 '14 at 16:19
As you wish, I thought I'd ask here because I have to wait 90 mins to create a new post – Duckdoom5 Nov 19 '14 at 16:32
@Duckdoom5: There is a reason for that: it is so that you _think_ about your new problem first. It is obviously not there to encourage you to post your question in the wrong place instead. – Lightness Races in Orbit Nov 19 '14 at 16:36

C++ array 'matrix' swap (memory leak)

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