Choice between vector resize and vector reserve

Question

I am pre-allocating some memory to my a vector member variable  Below code is minimal part   class A     vector lt string gt  t Names  public    A      t Names 1000          Now at some point of time  if the t Names size   equals 1000  I am intending to increase the size by 100  Then if it reaches 1100  again increase by 100 and so on   My question is  what to choose between vector  resize   and vector  reserve    Is there any better choice in this kind of scenario    Edit  I have sort of precise estimate for the t Names  I estimate it to be around 700 to 800  However in certain  seldom  situations  it can grow more than 1000

User · Answer

From your description  it looks like that you want to  reserve  the allocated storage space of vector t Names   Take note that resize initialize the newly allocated vector where reserve just allocates but does not construct  Hence   reserve  is much faster than  resize   You can refer to the documentation regarding the difference of resize and reserve

User · Answer

reserve when you do not want the objects to be initialized when reserved  also  you may prefer to logically differentiate and track its count versus its use count when you resize  so there is a behavioral difference in the interface - the vector will represent the same number of elements when reserved  and will be 100 elements larger when resized in your scenario      Is there any better choice in this kind of scenario    it depends entirely on your aims when fighting the default behavior  some people will favor customized allocators -- but we really need a better idea of what it is you are attempting to solve in your program to advise you well   fwiw  many vector implementations will simply double the allocated element count when they must grow - are you trying to minimize peak allocation sizes or are you trying to reserve enough space for some lock free program or something else

User · Answer

resize   not only allocates memory  it also creates as many instances as the desired size which you pass to resize   as argument  But reserve   only allocates memory  it doesn t create instances  That is   std  vector lt int gt  v1  v1 resize 1000     allocation   instance creation cout  lt  lt  v1 size      1000  lt  lt  endl      prints 1 cout  lt  lt  v1 capacity    1000  lt  lt  endl    prints 1  std  vector lt int gt  v2  v2 reserve 1000     only allocation cout  lt  lt  v2 size      1000  lt  lt  endl      prints 0 cout  lt  lt  v2 capacity    1000  lt  lt  endl    prints 1   Output  online demo    1 1 0 1   So resize   may not be desirable  if you don t want the default-created objects  It will be slow as well  Besides  if you push back   new elements to it  the size   of the vector will further increase by allocating new memory  which also means moving the existing elements to the newly allocated memory space   If you have used reserve   at the start to ensure there is already enough allocated memory  the size   of the vector will increase when you push back   to it  but it will not allocate new memory again until it runs out of the space you reserved for it

User · Answer

The two functions do vastly different things   The resize   method  and passing argument to constructor is equivalent to that  will insert or delete appropriate number of elements to the vector to make it given size  it has optional second argument to specify their value   It will affect the size    iteration will go over all those elements  push back will insert after them and you can directly access them using the operator     The reserve   method only allocates memory  but leaves it uninitialized  It only affects capacity    but size   will be unchanged  There is no value for the objects  because nothing is added to the vector  If you then insert the elements  no reallocation will happen  because it was done in advance  but that s the only effect   So it depends on what you want  If you want an array of 1000 default items  use resize    If you want an array to which you expect to insert 1000 items and want to avoid a couple of allocations  use reserve     EDIT  Blastfurnace s comment made me read the question again and realize  that in your case the correct answer is don t preallocate manually  Just keep inserting the elements at the end as you need  The vector will automatically reallocate as needed and will do it more efficiently than the manual way mentioned  The only case where reserve   makes sense is when you have reasonably precise estimate of the total size you ll need easily available in advance   EDIT2  Ad question edit  If you have initial estimate  then reserve   that estimate  If it turns out to be not enough  just let the vector do it s thing

[c++] Choice between vector::resize() and vector::reserve()

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