R – Passing Parameters to New Sub of a User Control

Let's get pedantic, because there are differences that can actually affect your code's behavior. Much of the following is taken from comments made to an "Old New Thing" article.

Sometimes the memory returned by the new operator will be initialized, and sometimes it won't depending on whether the type you're newing up is a POD (plain old data), or if it's a class that contains POD members and is using a compiler-generated default constructor.

• In C++1998 there are 2 types of initialization: zero and default
• In C++2003 a 3rd type of initialization, value initialization was added.

Assume:

struct A { int m; }; // POD
struct B { ~B(); int m; }; // non-POD, compiler generated default ctor
struct C { C() : m() {}; ~C(); int m; }; // non-POD, default-initialising m

In a C++98 compiler, the following should occur:

• new A - indeterminate value

• new A() - zero-initialize

• new B - default construct (B::m is uninitialized)

• new B() - default construct (B::m is uninitialized)

• new C - default construct (C::m is zero-initialized)

• new C() - default construct (C::m is zero-initialized)

In a C++03 conformant compiler, things should work like so:

• new A - indeterminate value

• new A() - value-initialize A, which is zero-initialization since it's a POD.

• new B - default-initializes (leaves B::m uninitialized)

• new B() - value-initializes B which zero-initializes all fields since its default ctor is compiler generated as opposed to user-defined.

• new C - default-initializes C, which calls the default ctor.

• new C() - value-initializes C, which calls the default ctor.

So in all versions of C++ there's a difference between new A and new A() because A is a POD.

And there's a difference in behavior between C++98 and C++03 for the case new B().

This is one of the dusty corners of C++ that can drive you crazy. When constructing an object, sometimes you want/need the parens, sometimes you absolutely cannot have them, and sometimes it doesn't matter.

Design decisions made regarding the way Windows Forms works more or less preclude parameterized .ctors for windows forms components. You can use them, but when you do you're stepping outside the generally approved mechanisms. Rather, Windows Forms prefers initialization of values via properties. This is a valid design technique, if not widely used.

This has some benefits, though.

1. Ease of use for clients. Client code doesn't need to track down a bunch of data, it can immediately create something and just see it with sensible (if uninteresting) results.
2. Ease of use for the designer. Designer code is clearer and easier to parse in general.
3. Discourages unusual data dependencies within a single component. (Though even microsoft blew this one with the SplitContainer)

There's a lot of support in forms for working properly with the designer in this technique also. Things like DefaultValueAttribute, DesignerSerializationVisibilityAttribute, and BrowsableAttribute give you the opportunity to provide a rich client experience with minimal effort.

(This isn't the only compromise that was made for client experience in windows forms. Abstract base class components can get hairy too.)

I'd suggest sticking with a parameterless constructor and working within the windows forms design principles. If there are real preconditions that your UserControl must enforce, encapsulate them in another class and then assign an instance of that class to your control via a property. This will give a bit better separation of concern as well.