C++ STL container and in-place construction

Question

Please consider the following:

class CMyClass
{
public:
  CMyClass()
  {
    printf( "Constructor\n" );
  }
  CMyClass( const CMyClass& )
  {
    printf( "Copy constructor\n" );
  }
};

int main()
{
  std::list<CMyClass> listMyClass;

  listMyClass.resize( 1 );

  return 0;
}

It produces the following output:

Constructor

Copy constructor

Now my question is: How do I avoid the copy constructor? Or to put it in another way: How can I create objects inside an STL container without the unnecessary copy operation. Is there some way to do an "in-place" construction using the default constructor?

---

Update - answers so far:

1. It can't be done
2. Use pointers or smart pointers instead.

Smart pointers are an overkill for my application. But I really wonder why this can't be done. It seems like such an obvious thing to want to do. Any other ideas? I will even accept a nasty hack if it works...

---

Solution

I think I just found a solution for my problem from all the comments and answers posed here. The solution is to construct an empty object and to keep it around for the sole purpose of using it later for making clean copies of. Then you can use one of the methods that take a reference (like push_back or insert). This still calls the copy constructor for every new object inserted, but at least it is not both the default constructor AND copy constructor:

int main()
{
  CMyClass Empty;

  std::list<CMyClass> listMyClass;

  for ( int c=0; c<10; ++c )
  {
    listMyClass.push_back( Empty );
  }

  return 0;
}

Answer 1

use pointers

std::list<CMyClass*> listMyClass;

Answer 2

Sorry, not currently with std::list and similar containers. (But you can write your own slightly-different container if you really need this, and still follow the rest of the STL interface.)

You don't have to use the default ctor, however:

std::list<CMyClass> listMyClass;
listMyClass.resize(1, obj_to_copy_from);

Such as:

std::list<CMyClass> listMyClass;
listMyClass.resize(1, CMyClass(use, specific, ctor));

Resize looks like:

void list<T>::resize(size_type new_size, T copy_from = T());

Which creates a new object (using the default ctor) by default.

Answer 3

By design, all the C++ Standard Library containers store copies. Therefore the call to the copy constructor cannot be avoided if you wish to store values in the container - the only way out is to store pointers instead. If you want to mitigate the overhead of copying, investigate the use of reference counting.

Answer 4

use a pointer or smart pointer (boost::shared_ptr and not auto_ptr)

Answer 5

You're not prematurely optimizing, are you? ;)

Why the copy constructor is invoked

When you resize(), the container inserts new elements at the end, as in the following pseudo-code:

void list<CMyClass>::resize(size_type new_size, CMyClass val = CMyClass())
{
   ...

   for each new element
   {
      insert(end(), val);
   }
}

Constructing the default parameter val = CMyClass() is what causes the "Constructor" message. Inside the implementation of insert, the container copy-constructs the data for new nodes using val as the right-hand-side. This is where you are seeing your "Copy constructor" messages.

I think it is done this way so that containers of basic types (int, float, etc.) are able to properly zero-initialize their elements:

// In list<int>::resize pamameter list:
val = int();

// Somewhere in list<int>::insert implementation:
node.data = new int(val); // Initialized to zero

vs

node.data = new int; // Initialized to undefined value