How to access the contents of a vector from a pointer to the vector in C

Question

I have a pointer to a vector  Now  how can I read the contents of the vector through pointer

User · Answer

vector lt int gt  v  v push back 906   vector lt int gt    p    amp v  cout  lt  lt    p  0   lt  lt  endl

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There are many solutions  here s a few I ve come up with   int main int nArgs  char    vArgs        vector lt int gt   v   new vector lt int gt  10       v- gt at 2     Retrieve using pointer to member     v- gt operator   2     Retrieve using pointer to operator member     v- gt size      Retrieve size     vector lt int gt   amp vr    v    Create a reference     vr 2     Normal access through reference     delete  amp vr    Delete the reference  You could do the same with                   a pointer  but not both

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You can access the iterator methods directly   std  vector lt int gt   intVec  std  vector lt int gt   iterator it   for  it   intVec- gt begin    it    intVec- gt end      it         If you want the array-access operator  you d have to de-reference the pointer  For example   std  vector lt int gt   intVec   int val     intVec  0

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The easiest way use it as array is use vector  data   member

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vector  lt int gt  numbers  10 20 30 40   vector  lt int gt   ptr  nullptr    ptr    amp numbers   for auto num   ptr    cout  lt  lt  num  lt  lt  endl      cout  lt  lt    ptr  at 2   lt  lt  endl     20  cout  lt  lt   quot ------- quot   lt  lt  endl   cout  lt  lt  ptr - gt  at 2   lt  lt  endl     20

User · Answer

Do you have a pointer to a vector because that s how you ve coded it  You may want to reconsider this and use a  possibly const  reference  For example    include  lt iostream gt   include  lt vector gt   using namespace std   void foo vector lt int gt   a        cout  lt  lt  a- gt at 0   lt  lt  a- gt at 1   lt  lt  a- gt at 2   lt  lt  endl         expected result is  123     int main         vector lt int gt  a      a push back 1       a push back 2       a push back 3        foo  amp a       While this is a valid program  the general C   style is to pass a vector by reference rather than by pointer  This will be just as efficient  but then you don t have to deal with possibly null pointers and memory allocation cleanup  etc  Use a const reference if you aren t going to modify the vector  and a non-const reference if you do need to make modifications    Here s the references version of the above program    include  lt iostream gt   include  lt vector gt   using namespace std   void foo const vector lt int gt  amp  a        cout  lt  lt  a 0   lt  lt  a 1   lt  lt  a 2   lt  lt  endl         expected result is  123     int main         vector lt int gt  a      a push back 1       a push back 2       a push back 3        foo a       As you can see  all of the information contained within a will be passed to the function foo  but it will not copy an entirely new value  since it is being passed by reference  It is therefore just as efficient as passing by pointer  and you can use it as a normal value rather than having to figure out how to use it as a pointer or having to dereference it

User · Answer

There are a lot of solutions  For example you can use at   method    I assumed that you a looking for equivalent to    operator

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Access it like any other pointer value   std  vector lt int gt   v   new std  vector lt int gt      v- gt push back 0   v- gt push back 12   v- gt push back 1    int twelve   v- gt at 1   int one     v  2       iterate it for std  vector lt int gt   const iterator cit   v- gt begin    e   v- gt end       cit    e     cit        int value    cit        or  more perversely for int x   0  x  lt  v- gt size      x        int value     v  x         Or -- with C   11 support for auto i    v       int value   i

[c++] How to access the contents of a vector from a pointer to the vector in C++?

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