i or i in for loops

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Possible Duplicate    Is there a performance difference between i   and   i in C          Is there a reason some programmers write   i in a normal for loop instead of writing i

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when you use postfix it instantiates on more object in memory  Some people say that it is better to use suffix operator in for loop

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As others have already noted  pre-increment is usually faster than post-increment for user-defined types  To understand why this is so  look at the typical code pattern to implement both operators   Foo amp  operator           some member increase        return  this     Foo operator   int dummy parameter indicating postfix        Foo copy  this           this       return copy      As you can see  the prefix version simply modifies the object and returns it by reference   The postfix version  on the other hand  must make a copy before the actual increment is performed  and then that copy is copied back to the caller by value  It is obvious from the source code that the postfix version must do more work  because it includes a call to the prefix version      this    For built-in types  it does not make any difference as long as you discard the value  i e  as long as you do not embed   i or i   in a larger expression such as a     i or b   i

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With integers  it s preference    If the loop variable is a class object  it can make a difference  only profiling can tell you if it s a significant difference   because the post-increment version requires that you create a copy of that object that gets discarded    If creating that copy is an expensive operation  you re paying that expense once for every time you go through the loop  for no reason at all    If you get into the habit of always using   i in for loops  you don t need to stop and think about whether what you re doing in this particular situation makes sense  You just always are

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No compiler worth its weight in salt will run differently between  for int i 0  i lt 10  i      and   for int i 0 i lt 10   i      i and i   have the same cost  The only thing that differs is that the return value of   i is i 1  whereas the return value of i   is i   So for those prefering   i  there s probably no valid justification  just personal preference   EDIT  This is wrong for classes  as said in about every other post  i   will generate a copy if i is a class

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There is a reason for this  performance  i   generates a copy  and that s a waste if you immediately discard it  Granted  the compiler can optimize away this copy if i is a primitive  but it can t if it isn t  See this question

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Personal preference   Usually   Sometimes it matters but  not to seem like a jerk here  but if you have to ask  it probably doesn t

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i is slightly more efficient due to its semantics     i      Fetch i  increment it  and return it i        Fetch i  copy it  increment i  return copy   For int-like indices  the efficiency gain is minimal  if any    For iterators and other heavier-weight objects  avoiding that copy can be a real win  particularly if the loop body doesn t contain much work    As an example  consider the following loop using a theoretical BigInteger class providing arbitrary precision integers  and thus some sort of vector-like internals    std  vector lt BigInteger gt  vec  for  BigInteger i   0  i  lt  99999999L  i        vec push back i       That i   operation includes copy construction  i e  operator new  digit-by-digit copy  and destruction  operator delete  for a loop that won t do anything more than essentially make one more copy of the index object   Essentially you ve doubled the work to be done  and increased memory fragmentation most likely  by simply using the postfix increment where prefix would have been sufficient

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For integers  there is no difference between pre- and post-increment   If i is an object of a non-trivial class  then   i is generally preferred  because the object is modified and then evaluated  whereas i   modifies after evaluation  so requires a copy to be made

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i is a pre-increment  i   is post-increment  The downside of post-increment is that it generates an extra value  it returns a copy of the old value while modifying i   Thus  you should avoid it when possible

[c++] ++i or i++ in for loops ??

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