Returning a pointer to a vector element in c

Question

I have a vector of myObjects in global scope  I have a method which uses a std  vector lt myObject gt   const iterator to traverse the vector  and doing some comparisons to find a specific element  Once I have found the required element  I want to be able to return a pointer to it  the vector exists in global scope    If I return  amp iterator  am I returning the address of the iterator or the address of what the iterator is pointing to   Do I need to cast the const iterator back to a myObject  then return the address of that

User · Answer

As long as your vector remains in global scope you can return:

&(*iterator)

I'll caution you that this is pretty dangerous in general. If your vector is ever moved out of global scope and is destructed, any pointers to myObject become invalid. If you're writing these functions as part of a larger project, returning a non-const pointer could lead someone to delete the return value. This will have undefined, and catastrophic, effects on the application.

I'd rewrite this as:

myObject myFunction(const vector<myObject>& objects)
{
    // find the object in question and return a copy
    return *iterator;
}

If you need to modify the returned myObject, store your values as pointers and allocate them on the heap:

myObject* myFunction(const vector<myObject*>& objects)
{
    return *iterator;
}

That way you have control over when they're destructed.

Something like this will break your app:

g_vector<tmpClass> myVector;

    tmpClass t;
    t.i = 30;
    myVector.push_back(t);

    // my function returns a pointer to a value in myVector
    std::auto_ptr<tmpClass> t2(myFunction());

User · Answer

Return the address of the thing pointed to by the iterator    amp   iterator    Edit  To clear up some confusion   vector  lt int gt  vec              a global vector of ints  void f        vec push back  1          add to the global vector    vector  lt int gt   iterator it   vec begin         it   2                  change what was 1 to 2    int   p    amp   it           get pointer to first element      p   3                   change what was 2 to 3     No need for vectors of pointers or dynamic allocation

User · Answer

Returning  amp iterator will return the address of the iterator  If you want to return a way of referring to the element return the iterator itself   Beware that you do not need the vector to be a global in order to return the iterator pointer  but that operations in the vector can invalidate the iterator  Adding elements to the vector  for example  can move the vector elements to a different position if the new size   is greater than the reserved memory  Deletion of an element before the given item from the vector will make the iterator refer to a different element   In both cases  depending on the STL implementation it can be hard to debug with just random errors happening each so often   EDIT after comment   yes  I didn t want to return the iterator a  because its const  and b  surely it is only a local  temporary iterator      Krakkos   Iterators are not more or less local or temporary than any other variable and they are copyable  You can return it and the compiler will make the copy for you as it will with the pointer   Now with the const-ness  If the caller wants to perform modifications through the returned element  whether pointer or iterator  then you should use a non-const iterator   Just remove the  const   from the definition of the iterator

User · Answer

You are storing the copies of the myObject in the vector  So I believe the copying the instance of myObject is not a costly operation  Then I think the safest would be return a copy of the myObject from your function

User · Answer

It is not a good idea to return iterators  Iterators become invalid when modifications to the vector  inversion deletion   happens  Also  the iterator is a local object created on stack and hence returning the address of the same is not at all safe  I d suggest you to work with myObject rather than vector iterators   EDIT  If the object is lightweight then its better you return the object itself  Otheriwise return pointers to myObject stored in the vector

User · Answer

Say  you have the following   std  vector lt myObject gt   const iterator first   vObj begin      Then the first object in the vector is   first  To get the address  use   amp   first    However  in keeping with the STL design  I d suggest return an iterator instead if you plan to pass it around later on to STL algorithms

User · Answer

You can use the data function of the vector      Returns a pointer to the first element in the vector    If don t want the pointer to the first element  but by index  then you can try  for example     the index to the element that you want to receive its pointer  int i   n     n is whatever integer you want   std  vector lt myObject gt  vec  myObject  ptr to first   vec data       or  std  vector lt myObject gt   vec  myObject  ptr to first   vec- gt data       then  myObject element   ptr to first i     element at index i myObject  ptr to element    amp element

User · Answer

I m not sure if returning the address of the thing pointed by the iterator is needed  All you need is the pointer itself  You will see STL s iterator class itself implementing the use of  Ptr for this purpose  So  just do   return iterator  Ptr

User · Answer

Refer to dirkgently s and anon s answers  you can call the front function instead of begin function  so you do not have to write the    but only the  amp    Code Example   vector lt myObject gt  vec    You have a vector of your objects myObject first   vec front      returns reference  not iterator  to the first object in the vector so you had only to write the data type in the generic of your vector  i e  myObject  and not all the iterator stuff and the vector again and    of course myObject  pointer to first object    amp first      between  amp  and first is not there anymore  first is already the first object  not iterator to it

[c++] Returning a pointer to a vector element in c++

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