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STL vector of obects ( NOT ptr to object!)

Posted on 2009-05-05
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Last Modified: 2013-12-14
Hi Guys,

I'm writing a program which stores objects (not pointers to objects) in a vector, I have a few different iterators using the vector, and in some cases I need a pointer to the object which is in the vector.

In the example below I use the *funky* syntax

SType *s = &(*it);

The following are my objectives, I want the vector to manage the objects themselves, not ptr-to-objects, and I need to get a pointer to one of those objects for use outside of the vector itself.

Is there a more elegant syntax for the above?

Note that the following does not work!
SType *s = it;

Any ideas on tidying this up?
#include <vector.h>
 

using namespace std;
 

struct SType

{

  double d;

  // Some other funky stuff here, ok to perform copy operations

  // but prefer to avoid them where possible

};
 

vector<SType> VType;
 

void DoStuff(VType &V)

{

  for (int i = 0; i < 20; i++)

  {

    SType s;

    s.d = i * 100.0;

    V.push_back(s);

  }

}
 

// This function NEEDS to accept a ptr-to-object

// which modifies the content of the element in the vector

// because the function works in many other contexts of the program

// it can't accept an iterator etc, must be ptr-to-object

void DoubleMe(SType *S)

{

  S->d *= 2;

}
 
 

void Func()

{

  VType v;
 

  DoStuff(v);
 

  VType::iterator it;

  for (it = v.begin(); it != v.end(); ++it)

  {

    // Funky syntax to get a pointer to the underlying object

    SType *s = &(*it);

    DoubleMe(s);

  }

}

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Accepted Solution

by:
Gregdo earned 250 total points
ID: 24307703
SType *s = &(*it); is pretty much the only way to do it.  The main problem with this method is that if you add more data to the vector, then your pointer could become invalid.  Vectors use contiguous memory, so when you add items, it they do not fit in the currently available space, new memory will be allocated and the vector will be copied to the new memory and the existing memory will be freed which would then make your pointer invalid.

As long as you are certain that no new data is being added to the vector while you are using the pointer, then this will work.

AFAIK, your only other option would be to store pointers to your structure in the vector instead of the objects themselves.  This would then remove the possibility of memory moving when new elements are added to the vector.  It would then put the responsibility back on you to free all the pointers before the vector is deleted. You could write a class to contain your structure that handle all the memory allocation and deletion.
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Author Closing Comment

by:A_Ridiculously_Long_Unique_Name
ID: 31577929
Great response! I hadn't considered the possibility of the pointer becoming invalid!!!
Due to the nature of the particular problem I am working with, no new data is being added so I think I can continue with this technique.
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