Duplicate constructor code issue

  • Thread starter Thread starter Steven Blair
  • Start date Start date
S

Steven Blair

I have encountered this problem a few times and would like to see how
other people deal with it.

A library has a Constructor that takes in n params and does some basic
validation and populates the class member.
I then get a requirement to add another paramter in at initialisation
time so the options I have (as far as I can see) are:

1. Change the current interface with the new param(s). No duplicate code
but the existing interface is changed and can cause me backwards
compatibilty problems.

2. Create a new constructor and give the user the choice. Problem is the
code in Constructor one must be copied in here as the same
initialisation is required.

3. I don't allow the user to pass in the new params. I simply make a
public member (or a get/set) and hope the user picks this up.

4. Create a base constructor common to all constructors.
This involves another class but might be the best solution.
The base constructor has all the standard validation I need, and the
child classes have the specific params validation etc.

Interested to hear other peoples views on this.

Regards,

Steven
 
use the "this" construct to invoke the other constructor, as in

public Foo(p1, p2, p3) //original constructor
{
//do assignments/logic with p1, p2, p3
}


public Foo (p1, p2, p3, p4):this(p1, p2, p3) //new constructor, with
p4, invokes
//prev
construct. via "this"
{
//assign/logic with p4
}
 
Fred Mellender said:
use the "this" construct to invoke the other constructor, as in

public Foo(p1, p2, p3) //original constructor
{
//do assignments/logic with p1, p2, p3
}


public Foo (p1, p2, p3, p4):this(p1, p2, p3) //new constructor,
with p4, invokes

//prev construct. via "this"
{
//assign/logic with p4
}
Click to expand...

It is generally a better Idea to have the constructor with less
parameters call the one with more parameters, passing in some default
value for the additional parameter

Change this
public Foo(p1, p2, p3) //original constructor
{
//do assignments/logic with p1, p2, p3
}

to this
public Foo(p1, p2, p3) : this (p1, p2, p3, 0) //new 3 param constructor
{ }

public Foo(p1, p2, p3, p4) //new 4 param constructor
{
//do assignments/logic with p1, p2, p3, p4
}

The library code continues to call the 3 param constructor, but it is
now a passthrough to the 4 param constructor.

This allows you to put the initialization in ONE constructor and simply
have the others call that constructor with the appropriate defaults.

Bill
 
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