Re: Default constructors, passing argument

From: Peter (peter_at_peter.com)
Date: 07/09/04 

Date: Fri, 9 Jul 2004 01:17:53 +0000 (UTC)

On Thu, 8 Jul 2004 21:07:58 +0000 (UTC), Chris Newton wrote:

> Peter wrote...
>> I have a subclass derived from another class as a member of my class. It
>> has a default constructor with the initialiser list used to pass values
>> common to each object.
>
>> The way the class works, it needs values from its owner class supplied to
>> it on initialisation. For example,
>>
>> class Thing {
>> private:
>> int val;
>> MySubClassObject hello; //constructed but needs val
>>
>> public:
>> Thing(int value) {
>> val = value; //value got too late
>> }
>> };
>>
>> What are the options?
>
> Do you mean that you want to supply the value of val to the constructor
> for hello?

Yes, at the same time as Thing is constructed I need MySubClass to get the
same argument.

>If so, you can't use the default constructor for
> MySubClassObject, because you need to pass the extra information in.
> hello being a member of your Thing class doesn't give it any special
> abilities to access the value of the val member of the Thing object to
> which it belongs.
>
> Does that answer your question?

Does that mean I cannot have it as a member var unless I make it a pointer
and new it after receiving value? I hate this solution.

>
> Chris

Sounds like "No, you cannot do it" but Chad J McQuinn said:
> The "hello(val)" will call whatever constructor of MySubClass is capable
> of taking an integer; here there is none. Add one and your example
> should be fine.
>
> MySobClass(int myVal) : MyClass(...whatever...) , val(myVal)
> {
> Func();
> }
>
> If, as you say in your comment, Func needs to use val, then you should
> remove the call to Func from the default constructor entirely. Or, if
> you never want MySubClass constructed without passing an int to the
> constructor, eliminate the default constructor entirely. If doing so
> gives you an error, then that means you are creating a MySubClass
> somewhere and not passing in an int.

Relevant Pages

  • Re: Shared classes or modules?
    ... initialization code that is executed the very first time you use any member ... I could have the Sub New above open the log file the first time I went ... Other posts say that a module have no default constructor but it can ... >> Also you should make the constructor private, to prevent instantiating ...
    (microsoft.public.dotnet.languages.vb)
  • Re: C++ 101 dumb question
    ... Its action is to copy the data members of the class, ... If one of those members is a pointer, ... have it's own copy constructor would the default copy constructor call ... as a copy of the class being returned is placed on the stack for the ...
    (microsoft.public.vc.language)
  • Re: Error using ==> class
    ... The argv in there is obviously a holdover. ... varargin because varargin apparently does not actually allow nargin == ... the change, no changes to code are made, and the constructor (as seen ... All instances of an object must have the same member names in the same ...
    (comp.soft-sys.matlab)
  • Re: ECMAScript standard:the code(s) for figure on page 3 ?
    ... You need to assign that object to the public prototype property of the ... assigned a reference to the empty anonymous function on the right hand ... Instance have property from constructor. ... member create from scope of constructor function. ...
    (comp.lang.javascript)
  • Re: Default constructors, passing argument
    ... MySubClass compiles ok. ... but if I change MySubClass to take an int argument in its ... > constructor", if I leave it as above there are more error messages. ... If, as you say in your comment, Func needs to use val, then you should ...
    (microsoft.public.vc.mfc)