Is it more efficient to copy a vector by reserving and copying or by creating and swapping

Question

I am trying to efficiently make a copy of a vector  I see two possible approaches  std  vector lt int gt  copyVecFast1 const std  vector lt int gt  amp  original      std  vector lt int gt  newVec    newVec reserve original size       std  copy original begin    original end    std  back inserter newVec      return newVec     std  vector lt int gt  copyVecFast2 std  vector lt int gt  amp  original      std  vector lt int gt  newVec    newVec swap original     return newVec     Which of these is preferred  and why  I am looking for the most efficient solution that will avoid unnecessary copying

User · Accepted Answer

Your second example does not work if you send the argument by reference. Did you mean

void copyVecFast(vec<int> original) // no reference
{

  vector<int> new_;
  new_.swap(original); 
}

That would work, but an easier way is

vector<int> new_(original);

User · Answer

new vector assign old vector begin   old vector end        Method 1 new vector   old vector     Method 2

User · Answer

This is another valid way to make a copy of a vector  just use its constructor   std  vector lt int gt  newvector oldvector     This is even simpler than using std  copy to walk the entire vector from start to finish to std  back insert them into the new vector   That being said  your  swap   one is not a copy  instead it swaps the two vectors  You would modify the original to not contain anything anymore  Which is not a copy

User · Answer

They aren t the same though  are they  One is a copy  the other is a swap  Hence the function names   My favourite is   a   b    Where a and b are vectors

User · Answer

Direct answer    Use a   operator   We can use the public member function std  vector  operator  of the container std  vector for assigning values from a vector to another    Use a constructor function   Besides  a constructor function also makes sense  A constructor function with another vector as parameter e g  x  constructs a container with a copy of each of the elements in x   in the same order   Caution    Do not use std  vector  swap    std  vector  swap is not copying a vector to another  it is actually swapping elements of two vectors  just as its name suggests  In other words  the source vector to copy from is modified after std  vector  swap is called  which is probably not what you are expected    Deep or shallow copy    If the elements in the source vector are pointers to other data  then a deep copy is wanted sometimes   According to wikipedia      A deep copy  meaning that fields are dereferenced  rather than references to objects being copied  new copy objects are created for any referenced objects  and references to these placed in B    Actually  there is no currently a built-in way in C   to do a deep copy  All of the ways mentioned above are shallow  If a deep copy is necessary  you can traverse a vector and make copy of the references manually  Alternatively  an iterator can be considered for traversing  Discussion on iterator is beyond this question    References  The page of std  vector on cplusplus com

User · Answer

you should not use swap to copy vectors  it would change the  original  vector   pass the original as a parameter to the new instead

[c++] Is it more efficient to copy a vector by reserving and copying, or by creating and swapping?

Examples related to c++

Examples related to algorithm

Examples related to vector

Examples related to stl