Hi,
Due to my lack of understanding of how vectors work i have a question regarding cleaning vectors. The situation is the following:
Is it better to do this :
orCode:while(){ vector<int>vec; //fill vector }
when i say better i mean faster since my vectors are quite large and they need to be filled and emptied many times over.Code:vector<int>vec; while(){ vector<int>().swap(vec) //fill vector }
thnx
baxy
It is unlikely for the former to be worse than the latter, but if you want to be sure then measure. Also, you might to consider just reusing the same vector object since it is not like your elements have destructors that need to be called.Originally Posted by baxy
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Like laserlight said, there is probably not much difference between your options: both involve a destruction, a construction, and a reallocation (just in different orders).
I'd also suggest considering the use of "vec.resize(0)", with vec defined outside the loop.
The destruct/construct/reallocate cycle is often not particularly efficient if subsequent operations then reallocate large blocks of memory of similar size. It is not specified whether resize(0) actually releases any resources or not but avoiding that cycle can't hurt.
Note I used the word "often". The specific advantages and disdvantages depend on how the particular library implements std::vector and its allocators (so vary between implementations).
If the vector is huge enough that freeing it makes a material difference to the memory pressure on the system as a whole, then that's a different discussion actually. You will not get that sort of deterministic memory behavior with an STL container. Or even with malloc() and free() in some cases.
If you are just "emptying" the vector so you can use its size() for algorithmic purposes, then calling resize(0) or clear() is all you need to do.
Code://try //{ if (a) do { f( b); } while(1); else do { f(!b); } while(1); //}
Agreed brewbuck, the faster option is to declare the vector outside of the loop, and inside the loop, simply use .resize(0)
This then normally avoids reallocating the vector's storage space unless the vector needs to grow.
In fact, I made this exact optimisation today at work!
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