How to set initial size of std vector

Question

I have a vector lt CustomClass  gt  and I put a lot of items in the vector and I need fast access  so I don t use list  How to set initial size of vector  for example to be 20 000 places  so to avoid copy when I insert new

User · Accepted Answer

std  vector lt CustomClass   gt  whatever 20000     or   std  vector lt CustomClass   gt  whatever  whatever reserve 20000     The former sets the actual size of the array -- i e   makes it a vector of 20000 pointers  The latter leaves the vector empty  but reserves space for 20000 pointers  so you can insert  up to  that many without it having to reallocate   At least in my experience  it s fairly unusual for either of these to make a huge difference in performance--but either can affect correctness under some circumstances  In particular  as long as no reallocation takes place  iterators into the vector are guaranteed to remain valid  and once you ve set the size reserved space  you re guaranteed there won t be any reallocations as long as you don t increase the size beyond that

User · Answer

You need to use the reserve function to set an initial allocated size or do it in the initial constructor   vector lt CustomClass   gt  content 20000     or  vector lt CustomClass   gt  content      content reserve 20000     When you reserve   elements  the vector will allocate enough space for  at least   that many elements  The elements do not exist in the vector  but the memory is ready to be used  This will then possibly speed up push back   because the memory is already allocated

[c++] How to set initial size of std::vector?

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