C delete vector objects free memory

Question

I am totally confused with regards to deleting things in C    If I declare an array of objects and if I use the clear   member function  Can I be sure that the memory was released   For example     tempObject obj1  tempObject obj2  vector lt tempObject gt  tempVector   tempVector pushback obj1   tempVector pushback obj2     Can I safely call clear to free up all the memory  Or do I need to iterate through to delete one by one   tempVector clear      If this scenario is changed to a pointer of objects  will the answer be the same as above   vector lt tempObject gt   tempVector    push objects     tempVector- gt clear

User · Answer

Move semantics allows for a straightforward way to release memory  by simply applying the assignment     operator from an empty rvalue  std  vector lt uint32 t gt  vec 100  0   std  cout  lt  lt  vec capacity       100  vec   vector lt uint32 t gt           Same as  quot vector lt uint32 t gt    swap vec  quot       std  cout  lt  lt  vec capacity       0  It is as much efficient as the  quot swap   quot -based method described in other answers  indeed  both are conceptually doing the same thing   When it comes to readability  however  the assignment version makes a better job at expressing the programmer s intention while being more concise

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if I use the clear   member function  Can I be sure that the memory was released    No  the clear   member function destroys every object contained in the vector  but it leaves the capacity of the vector unchanged  It affects the vector s size  but not the capacity   If you want to change the capacity of a vector  you can use the clear-and-minimize idiom  i e   create a  temporary  empty vector and then swap both vectors     You can easily see how each approach affects capacity  Consider the following function template that calls the clear   member function on the passed vector   template lt typename T gt  auto clear std  vector lt T gt  amp  vec       vec clear       return vec capacity        Now  consider the function template empty swap   that swaps the passed vector with an empty one   template lt typename T gt  auto empty swap std  vector lt T gt  amp  vec       std  vector lt T gt    swap vec      return vec capacity        Both function templates return the capacity of the vector at the moment of returning  then   std  vector lt double gt  v 1000   u 1000   std  cout  lt  lt  clear v   lt  lt    n   std  cout  lt  lt  empty swap u   lt  lt    n     outputs   1000 0

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You can free memory used by vector by this way    Removes all elements in vector v clear      Frees the memory which is not used by the vector v shrink to fit

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You can call clear  and that will destroy all the objects  but that will not free the memory   Looping through the individual elements will not help either  what action would you even propose to take on the objects    What you can do is this   vector lt tempObject gt    swap tempVector     That will create an empty vector with no memory allocated and swap it with tempVector  effectively deallocating the memory   C  11 also has the function shrink to fit  which you could call after the call to clear    and it would theoretically shrink the capacity to fit the size  which is now 0    This is however  a non-binding request  and your implementation is free to ignore it

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There are two separate things here   object lifetime storage duration  For example        vector lt MyObject gt  v         do some stuff  push some objects onto v     v clear       1        maybe do some more stuff      2  At 1  you clear v  this destroys all the objects it was storing  Each gets its destructor called  if your wrote one  and anything owned by that MyObject is now released  However  vector v has the right to keep the raw storage around in case you want it later  If you decide to push some more things into it between 1 and 2  this saves time as it can reuse the old memory  At 2  the vector v goes out of scope  any objects you pushed into it since 1 will be destroyed  as if you d explicitly called clear again   but now the underlying storage is also released  v won t be around to reuse it any more    If I change the example so v becomes a pointer to a dynamically-allocated vector  you need to explicitly delete it  as the pointer going out of scope at 2 doesn t do that for you  It s better to use something like std  unique ptr in that case  but if you don t and v is leaked  the storage it allocated will be leaked as well  As above  you need to make sure v is deleted  and calling clear isn t sufficient

User · Answer

vector  clear   does not free memory allocated by the vector to store objects  it calls destructors for the objects it holds     For example  if the vector uses an array as a backing store and currently contains 10 elements  then calling clear   will call the destructor of each object in the array  but the backing array will not be deallocated  so there is still sizeof T    10 bytes allocated to the vector  at least    size   will be 0  but size   returns the number of elements in the vector  not necessarily the size of the backing store   As for your second question  anything you allocate with new you must deallocate with delete   You typically do not maintain a pointer to a vector for this reason   There is rarely  if ever  a good reason to do this and you prevent the vector from being cleaned up when it leaves scope   However  calling clear   will still act the same way regardless of how it was allocated

User · Answer

If you need to use the vector over and over again and your current code declares it repeatedly within your loop or on every function call  it is likely that you will run out of memory  I suggest that you declare it outside  pass them as pointers in your functions and use   my arr resize     This way  you keep using the same memory sequence for your vectors instead of requesting for new sequences every time  Hope this helped  Note  resizing it to different sizes may add random values  Pass an integer such as 0 to initialise them  if required

[c++] C++ delete vector, objects, free memory

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