Trying to use STL without smart pointers - trying to avoid temporary object creation

Question

I particularly like the simplicity of using STL containers in the straightforward way.

I have never really figured out how to get the Boost library working on my dev platforms, in fact I don't think I've even tried. I guess you could say I am just trying to delay the inevitable since Boost is clearly a helpful library that I should be using.

But my question is essentially the same as this topic: http://stackoverflow.com/questions/2580189/how-to-initialise-a-stl-vector-list-with-a-class-without-invoking-the-copy-constr

I have std::list<ExpensiveClass> mylist; and I just want a function that pushes a new instance into the list and calls the default constructor, rather than copying it from a temporary stack instance of it. In the other topic there was mention of move constructors. I looked them up and quite frankly it does nothing but strike fear into my heart. Two ampersands!!

Would it work if I just made an array of ExpensiveClass objects? ExpensiveClass *mylist = new ExpensiveClass[20]; Does this call the constructor 20 times?

Seems to me I should just use boost:ptr_list.

Answer 1

Inserting an object into a container invokes the copy constructor on that object. There's really no way around that (yet), hence why pointer containers are used for large objects: it's dirt cheap to copy a pointer.

If you choose to use a container of smart pointers, you can either use one of the Boost pointer containers or you can use an STL container of shared_ptrs.

To answer your question about:

ExpensiveClass *mylist = new ExpensiveClass[20];

The default constructor for ExpensiveClass is called 20 times: once for each element of the array.

Answer 2

I just want a function that pushes a new instance into the list and calls the default constructor

Since you mentioned "two ampersands", I am going to give you the C++0x solution:

mylist.emplace_back();

If you want to use a different constructor, simply provide arguments:

mylist.emplace_back(whatever, arguments, your, constructor, takes);

This member function constructs the object in-place, without copying (or moving).

Answer 3

Depending on implementation support for C++0x features, have a look at emplace_back and similar member functions - they do precisely what you ask for, i.e. construct an object directly inside the container, "in-place", without any copying.