Convert iterator to pointer

Question

I have a std  vector with n elements  Now I need to pass a pointer to a vector that has the last n-1 elements to a function   For example  my vector lt int gt  foo contains  5 2 6 87 251   A function takes vector lt int gt   and I want to pass it a pointer to  2 6 87 251    Can I just  safely  take the iterator   foo begin    convert it to a pointer and pass that to the function  Or use  amp foo 1    UPDATE  People suggest that I change my function to take an iterator rather than a pointer  That seems not possible in my situation  since the function I mentioned is the find function of unordered set lt std  vector  gt   So in that case  is copying the n-1 elements from foo into a new vector and calling find with a pointer to that the only option  Very inefficient  It s like Shlemiel the painter  especially since i have to query many subsets  the last n-1  then n-2  etc  elements and see if they are in the unordered set

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Vector is a template class and it is not safe to convert the contents of a class to a pointer   You cannot inherit the vector class to add this new functionality  and changing the function parameter is actually a better idea  Jst create another vector of int vector  temp foo  foo begin X  foo end     and pass this vector to you functions

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For example  my vector lt int gt  foo contains  5 2 6 87 251   A function takes vector lt int gt   and I want to pass it a pointer to  2 6 87 251     A pointer to a vector lt int gt  is not at all the same thing as a pointer to the elements of the vector   In order to do this you will need to create a new vector lt int gt  with just the elements you want in it to pass a pointer to  Something like    vector lt int gt  tempVector  foo begin   1  foo end          now you can pass  amp tempVector to your function   However  if your function takes a pointer to an array of int  then you can pass  amp foo 1

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If your function really takes vector lt int gt     a pointer to vector   then you should pass  amp foo since that will be a pointer to the vector  Obviously that will not simply solve your problem  but you cannot directly convert an iterator to a vector  since the memory at the address of the iterator will not directly address a valid vector   You can construct a new vector by calling the vector constructor   template  lt class InputIterator gt  vector InputIterator  InputIterator    This constructs a new vector by copying the elements between the two iterators  You would use it roughly like this   bar std  vector lt int gt  foo begin   1  foo end

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here it is  obtaining a reference to the coresponding pointer of an iterator use     example   string my str   hello world    string  iterator it my str begin      char  pointer inside buffer  amp   it      lt --    notice operator   returns a reference so doing  amp  on a reference will give you the address

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I haven t tested this but could you use a set of pairs of iterators instead  Each iterator pair would represent the begin and end iterator of the sequence vector  E g    typedef std  vector lt int gt  Seq  typedef std  pair lt Seq  const iterator  Seq  const iterator gt  SeqRange   bool operator lt   const SeqRange amp  lhs  const SeqRange amp  rhs        Seq  const iterator lhsNext   lhs first      Seq  const iterator rhsNext   rhs first       while  lhsNext    lhs second  amp  amp  rhsNext    rhs second          if   lhsNext  lt   rhsNext              return true          else if   lhsNext  gt   rhsNext              return false       return false     typedef std  set lt SeqRange  std  less lt SeqRange gt   gt  SeqSet   Seq sequences   void test  const SeqSet amp  seqSet  const SeqRange amp  seq        bool find   seqSet find  seq     seqSet end         bool find2   seqSet find  SeqRange  seq first   1  seq second      seqSet end         Obviously the vectors have to be held elsewhere as before  Also if a sequence vector is modified then its entry in the set would have to be removed and re-added as the iterators may have changed   Jon

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The direct answer to your question is yes   If foo is a vector  you can do this   amp foo 1    This only works for vectors however  because the standard says that vectors implement storage by using contigious memory   But you still can  and probably should  pass iterators instead of raw pointers because it is more expressive   Passing iterators does not make a copy of the vector

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That seems not possible in my situation  since the function I mentioned is the find function of unordered set lt std  vector  gt      Are you using custom hash predicate function objects  If not  then you must pass unordered set lt std  vector lt int gt   gt   find   the pointer to the exact vector that you want to find  A pointer to another vector with the same contents will not work  This is not very useful for lookups  to say the least    Using unordered set lt std  vector lt int gt   gt  would be better  because then you could perform lookups by value  I think that would also require a custom hash function object because hash does not to my knowledge have a specialization for vector lt int gt    Either way  a pointer into the middle of a vector is not itself a vector  as others have explained  You cannot convert an iterator into a pointer to vector without copying its contents

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A safe version to convert an iterator to a pointer  exactly what that means regardless of the implications  and by safe I mean no worries about having to dereference the iterator and cause possible exceptions   errors due to end     other situations  include  lt iostream gt   include  lt vector gt   include  lt string h gt   int main         std  vector lt int gt  vec       char itPtr 25       long long itPtrDec           std  vector lt int gt   iterator it   vec begin        memset  amp itPtr  0  25       sprintf itPtr   quot  llu quot   it       itPtrDec   atoll itPtr       printf  quot it   0x X n quot   itPtrDec            vec push back 123       it   vec begin        memset  amp itPtr  0  25       sprintf itPtr   quot  llu quot   it       itPtrDec   atoll itPtr       printf  quot it   0x X n quot   itPtrDec      will print something like  it   0x0 it   0x2202E10  It s an incredibly hacky way to do it  but if you need it  it does the job  You will receive some compiler warnings which  if really bothering you  can be removed with  pragma

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Use vector  front  it should be the most portable solution   I ve used this when I m interfacing with a fixed API that wants a char ptr   Example   void funcThatTakesCharPtr char  start  size t size         void myFunc vector lt char gt  amp  myVec           Get a pointer to the front element of my vector      char  myDataPtr    amp  myVec front             Pass that pointer to my external API      funcThatTakesCharPtr myDataPtr  myVec size

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Your function shouldn t take vector lt int gt    it should take vector lt int gt   iterator or vector lt int gt   const iterator as appropriate  Then  just pass in foo begin     1

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A vector is a container with full ownership of it s elements  One vector cannot hold a partial view of another  even a const-view  That s the root cause here    If you need that  make your own container that has views with weak ptr s to the data  or look at ranges  Pair of iterators  even pointers work well as iterators into a vector  or  even better  boost  iterator range that work pretty seamlessly   It depends on the templatability of your code  Use std  pair if you need to hide the code in a cpp

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If you can  a better choice may be to change the function to take either an iterator to an element or a brand new vector  if it does not modify    While you can do this sort of things with arrays since you know how they are stored  it s probably a bad idea to do the same with vectors   amp foo 1  does not have the type vector lt int gt     Also  while the STL implementation is available online  it s usually risky to try and rely on the internal structure of an abstraction

[c++] Convert iterator to pointer?

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