Should my std::vector contain pointers or structs?

Question

I know that holding pointers incurs the overhead of an extra dereference operation but it saves me including the (potentially large) header file that contains the definition of my struct.

However my preference is to be determined by the advantage of having a std::vector<myStruct> *ptr2Vect member. Namely, not having to call delete on each element. How big a performance advantage is this? Can vector really allocate objects on the stack? I am fairly new to template classes and wonder if it could be possible for a dynamic array to expand on the stack and at what price?

_EDIT_

I fail in understanding default copy constructor and operator= members and am trying to keep things as simplistic structs. I have neither implementation defined explicitly so fear that making the vector element an object instead of pointer will create temporary object at assignment time that will be destructed and so ruin its copy.

_EDIT_

Sorry for the delay in delivering pertinent information (I am shy with my code).

I want to call push_back(newObj). Now if I don't use pointers I have a big problem in that I don't want to perform a deep copy but my dtor will free up the memory shared by the LHS and RHS of this invocation of the copy constructor.

Answer 1

As a general rule of thumb I'd say you probably don't want to put pointers in your containers, unless there's a good reason.

Possible reasons to consider pointers:

• You have virtual functions
• You have a class hierarchy
• You don't know the size of the objects where you're using them this. (You can only use pointers or references in that case and you can't have a vector of references)
• Your objects are exceedingly large (probably benchmark this)

The biggest reason not to put pointers in containers would be that it makes it much easier not to make a mistake and accidentally leak memory. This is especially true when you start to consider exceptions.

Not having pointers in your containers makes it much easier to use STL <algorithms>, consider:

#include <vector>
#include <string>
#include <iostream>
#include <iterator>
#include <algorithm>

int main() {
  std::vector<std::string> test;
  test.push_back("hello world");
  std::copy(test.begin(), test.end(), 
            std::ostream_iterator<std::string>(std::cout, "\n"));
}

Versus:

#include <vector>
#include <string>
#include <iostream>
#include <iterator>
#include <algorithm>

int main() {
  std::vector<std::string*> test;
  // if push_back throws then this will leak:
  test.push_back(new std::string("hello world"));
  // Can't do:
  std::copy(test.begin(), test.end(), 
            std::ostream_iterator<std::string>(std::cout, "\n"));
  // Will now leak too
}

(which I would never do)

Or possibly:

#include <vector>
#include <string>
#include <iostream>
#include <iterator>
#include <algorithm>

int main() {
  std::vector<std::string*> test;
  std::string str("hello world");
  test.push_back(&str);
  // Can't do:
  std::copy(test.begin(), test.end(), std::ostream_iterator<std::string>(std::cout, "\n"));
}

But the semantics of this one make me feel uncomfortable - it's not clear at all that delete elsewhere in the code would be a very bad thing and you still can't use STL algorithms very comfortably even if there is no leak issue.