std vector versus std array in C

Question

What are the difference between a std  vector and an std  array in C    When should one be preferred over another  What are the pros and cons of each  All my textbook does is list how they are the same

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To emphasize a point made by  MatteoItalia  the efficiency difference is where the data is stored   Heap memory  required with vector  requires a call to the system to allocate memory and this can be expensive if you are counting cycles   Stack memory  possible for array  is virtually  zero-overhead  in terms of time  because the memory is allocated by just adjusting the stack pointer and it is done just once on entry to a function   The stack also avoids memory fragmentation   To be sure  std  array won t always be on the stack  it depends on where you allocate it  but it will still involve one less memory allocation from the heap compared to vector   If you have a    small  array   under 100 elements say  -  a typical stack is about 8MB  so don t allocate more than a few KB on the stack or less if your code is recursive  the size will be fixed the lifetime is in the function scope  or is a member value with the same lifetime as the parent class  you are counting cycles    definitely use a std  array over a vector  If any of those requirements is not true  then use a std  vector

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Using the std  vector lt T gt  class       is just as fast as using built-in arrays  assuming you are doing only the things built-in arrays allow you to do  read and write to existing elements      automatically resizes when new elements are inserted     allows you to insert new elements at the beginning or in the middle of the vector  automatically  shifting  the rest of the elements  up   does that make sense    It allows you to remove elements anywhere in the std  vector  too  automatically shifting the rest of the elements down     allows you to perform a range-checked read with the at   method  you can always use the indexers    if you don t want this check to be performed     There are two three main caveats to using std  vector lt T gt     You don t have reliable access to the underlying pointer  which may be an issue if you are dealing with third-party functions that demand the address of an array  The std  vector lt bool gt  class is silly  It s implemented as a condensed bitfield  not as an array  Avoid it if you want an array of bools  During usage  std  vector lt T gt s are going to be a bit larger than a C   array with the same number of elements  This is because they need to keep track of a small amount of other information  such as their current size  and because whenever std  vector lt T gt s resize  they reserve more space then they need  This is to prevent them from having to resize every time a new element is inserted  This behavior can be changed by providing a custom allocator  but I never felt the need to do that      Edit  After reading Zud s reply to the question  I felt I should add this   The std  array lt T gt  class is not the same as a C   array  std  array lt T gt  is a very thin wrapper around C   arrays  with the primary purpose of hiding the pointer from the user of the class  in C    arrays are implicitly cast as pointers  often to dismaying effect   The std  array lt T gt  class also stores its size  length   which can be very useful

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std  vector is a template class that encapsulate a dynamic array1  stored in the heap  that grows and shrinks automatically if elements are added or removed  It provides all the hooks  begin    end    iterators  etc  that make it work fine with the rest of the STL  It also has several useful methods that let you perform operations that on a normal array would be cumbersome  like e g  inserting elements in the middle of a vector  it handles all the work of moving the following elements behind the scenes    Since it stores the elements in memory allocated on the heap  it has some overhead in respect to static arrays   std  array is a template class that encapsulate a statically-sized array  stored inside the object itself  which means that  if you instantiate the class on the stack  the array itself will be on the stack  Its size has to be known at compile time  it s passed as a template parameter   and it cannot grow or shrink   It s more limited than std  vector  but it s often more efficient  especially for small sizes  because in practice it s mostly a lightweight wrapper around a C-style array  However  it s more secure  since the implicit conversion to pointer is disabled  and it provides much of the STL-related functionality of std  vector and of the other containers  so you can use it easily with STL algorithms  amp  co  Anyhow  for the very limitation of fixed size it s much less flexible than std  vector   For an introduction to std  array  have a look at this article  for a quick introduction to std  vector and to the the operations that are possible on it  you may want to look at its documentation      Actually  I think that in the standard they are described in terms of maximum complexity of the different operations  e g  random access in constant time  iteration over all the elements in linear time  add and removal of elements at the end in constant amortized time  etc   but AFAIK there s no other method of fulfilling such requirements other than using a dynamic array  As stated by  Lucretiel  the standard actually requires that the elements are stored contiguously  so it is a dynamic array  stored where the associated allocator puts it

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A vector is a container class while an array is an allocated memory

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If you are considering using multidimensional arrays  then there is one additional difference between std  array and std  vector   A multidimensional std  array will have the elements packed in memory in all dimensions  just as a c style array is   A multidimensional std  vector will not be packed in all dimensions   Given the following declarations   int cConc 3  5   std  array lt std  array lt int  5 gt   3 gt  aConc  int   ptrConc          initialized to  3  5  via new and destructed via delete std  vector lt std  vector lt int gt  gt  vConc        initialized to  3  5    A pointer to the first element in the c-style array  cConc  or the std  array  aConc  can be iterated through the entire array by adding 1 to each preceding element   They are tightly packed   A pointer to the first element in the vector array  vConc  or the pointer array  ptrConc  can only be iterated through the first 5  in this case  elements  and then there are 12 bytes  on my system  of overhead for the next vector   This means that a std  vector  array initialized as a  3  1000  array will be much smaller in memory than one initialized as a  1000  3  array  and both will be larger in memory than a std array allocated either way   This also means that you can t simply pass a multidimensional vector  or pointer  array to  say  openGL without accounting for the memory overhead  but you can naively pass a multidimensional std  array to openGL and have it work out

[c++] std::vector versus std::array in C++

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