Re: Classes and Default Constructors

"Daniel T." <>
Sun, 11 Jan 2009 08:57:57 -0500
<> wrote:

I have a few questions regarding classes in C++. First of all I have
two classes. I have a default constructor that assigns default values:

//class D.h
class D
    D(int id = 0, int sn =0);


I want to associate Class D with Class E:

//class E.h
class E
   D instance_;

//class E.cpp
   void E::SendData()
      D instance_(50.12);

However, the D instance_ delcaration calls the default constructor
with default values. So as well as calling the constructor in a
function in class E, the default constructor is called first with
default values.

With the above, 'instance_' is a member-variable of E. When an object of
type E is constructed, a member of type D must also be constructed. The
sequence is, the E object starts constructing, it fully constructs the D
object, then it finishes constructing.

Since the E object has the compiler defined default constructor, the D
object is also constructed with its default constructor. That's why the
default constructor of D is called.

When E::SendData is called, you are declaring a new object named
'instance_' and constructing it with the values provided (I'm assuming
here that you meant to type "50,12" not "50.12".) This new object is not
associated with the member-variable E::instance_ in any way.

What is the correct way of associating an instance
with the other object without invoking the default constructor? The
isue is I can't really set those parameters in the declaration, and
I'm not sure I really should anyway, it doesn't seem correct.

Since a D object must be constructed when the E object gets constructed,
then the most obvious solution is to provide the parameters in the E
object's constructor, like this:

class E {
   D instance_;
   E():instance_(50, 12) { }
   void SendData() {
      // use 'instance_' here.

If you don't know what the values will be until runtime (i.e., if you
are using variables for the values and the variables aren't set until
after the programs been running for a bit,) then you need to use new to
avoid the default construction, like this:

class E {
   D* instance_;
   E(): instance_(0) { }
   ~E() { delete instance_ }
   E(const E& o): instance_(0) {
      if (o.instance_)
         instance_ = new D(*o.instance_);
   void operator=(const E& o) {
      E tmp(o);
      swap(tmp.instance_, instance_);
   void SendData(int x, int y) {
      delete instance_;
      instance_ = new D(x, y);
      // use 'instance_' here.

Yes, all that extra code is necessary if you want copy construction and
assignment to work properly.

Unless you have a very powerful reason to avoid the default
construction, it would be easier to go ahead and let the default
construction happen. Then the code would look like this:

class E {
   D instance_;
   void SendData(int x, int y) {
      instance_ = D(x, y);

Note in the above code how I assigned to instance_ rather than declaring
a variable named instance_ as you did in your code.

And in a related question, I'm not entirely clear when you would call
D instance_(50,12) and D *instance_ = new D(50,12). I was doing this
initially as I thought using pointers would be more efficient over
references, but I'm not sure this is true.

Your example doesn't compare references to pointers. 'D instance_(50,
12)' defines an object, not a reference to an object. I suggest you
avoid using new whenever you can. It makes the code much easer to
understand and write in most cases.

As far as I can gather I
should use references as much as possible and only use pointers when I
need to change what I'm pointing to, since I can't "move" a reference.

References were invented to provide more options when passing parameters
to functions and returning values from functions. I suggest you don't
use references in other contexts.

And also I would guess when I want to create an indeterminate number
of objects at runtime dynamically, which would presumably force me to
use pointers as this is what new returns in C++ when creating objects
of classes.

I suggest you use one of the standard containers when you have to do the
above, usually vector.

Perfection is achieved, not when there is nothing more to add,
but when there is nothing left to take away.
    -- Antoine de Saint-Exupery

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