C vector s insert push back difference

Question

I want to know what are difference s  between vector s push back and insert functions   Is there a structural difference s    Is there a really big performance difference s

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The functions have different purposes  vector  insert allows you to insert an object at a specified position in the vector  whereas vector  push back will just stick the object on the end  See the following example   using namespace std  vector lt int gt  v    1  3  4   v insert next begin v    2   v push back 5      v now contains  1  2  3  4  5    You can use insert to perform the same job as push back with v insert v end    value

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The biggest difference is their functionality  push back always puts a new element at the end of the vector and insert allows you to select new element s position  This impacts the performance  vector elements are moved in the memory only when it s necessary to increase it s length because too little memory was allocated for it  On the other hand insert forces to move all elements after the selected position of a new element  You simply have to make a place for it  This is why insert might often be less efficient than push back

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Beside the fact  that push back x  does the same as insert x  end     maybe with slightly better performance   there are several important thing to know about these functions    push back exists only on BackInsertionSequence containers - so  for example  it doesn t exist on set  It couldn t because push back   grants you that it will always add at the end  Some containers can also satisfy FrontInsertionSequence and they have push front  This is satisfied by deque  but not by vector  The insert x  ITERATOR  is from InsertionSequence  which is common for set and vector  This way you can use either set or vector as a target for multiple insertions  However  set has additionally insert x   which does practically the same thing  this first insert in set means only to speed up searching for appropriate place by starting from a different iterator - a feature not used in this case     Note about the last case that if you are going to add elements in the loop  then doing container push back x  and container insert x  container end    will do effectively the same thing  However this won t be true if you get this container end   first and then use it in the whole loop   For example  you could risk the following code   auto pe   v end    for  auto amp  s  a      v insert pe  v     This will effectively copy whole a into v vector  in reverse order  and only if you are lucky enough to not get the vector reallocated for extension  you can prevent this by calling reserve   first   if you are not so lucky  you ll get so-called UndefinedBehavior tm   Theoretically this isn t allowed because vector s iterators are considered invalidated every time a new element is added   If you do it this way   copy a begin    a end    back inserter v     it will copy a at the end of v in the original order  and this doesn t carry a risk of iterator invalidation    EDIT  I made previously this code look this way  and it was a mistake because inserter actually maintains the validity and advancement of the iterator   copy a begin    a end    inserter v  v end       So this code will also add all elements in the original order without any risk

[c++] C++ vector's insert & push_back difference

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