Re: Default constructors, passing argument
From: Peter (peter_at_peter.com)
Date: 07/09/04
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Date: Fri, 9 Jul 2004 00:33:21 +0000 (UTC)
On Fri, 9 Jul 2004 00:00:04 +0000 (UTC), Peter wrote:
> On Thu, 08 Jul 2004 20:15:27 GMT, Chad J McQuinn wrote:
>
>> In article <6fdhqkfba6m8$.143lq7694c4m4$.dlg@40tude.net>,
>> Peter <peter@peter.com> wrote:
>>
>>> 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?
>>
>> Thing(int value) : val(value), hello(value)
>> {}
>>
>> The part after the colon in Thing's constructor is called the
>> initialization list. You can use it to initialize member variables as
>> well as to call base class constructors of class Thing (if there were
>> any).
>>
>> It looks like in this simple example, you might have created the 'val'
>> member variable just construct 'hello'. If so that is nether possible
>> nor necessary.
>>
>> -Chad
>
> thanks for the replies, but there is a problem
>
> I have made changes like this now:
>
>
> class MySubClass : MyClass {
> public:
> int val; //public member of class for a good reason
>
> MySubClass() : MyClass("a string", 25) {
> Func(); //function that needs to use val
> }
> }
>
> class Thing {
> public:
> MySubClass hello; //public because I was told to in this thread
>
> Thing(int val) : hello(val) {
>
> }
> }
>
> As you can see val is no longer an attribute of class Thing, because val is
> in reality an attribute of Thing placed in MySubClass to take advantage of
> important functions there. MySubClass compiles ok. Thing does not
> compile, but if I change MySubClass to take an int argument in its
> constructor then the compiler says it does not have "an appropriate default
> constructor", if I leave it as above there are more error messages.
>
> ANGST FOLLOWS
>
> What is
>
> hello(val)
>
> doing to clarify? Is it calling a constructor of MySubClass that takes an
> int? How do I get a value into val for hello as Thing enters the
> constructor? I do not want hello constructed without its val argument.
>
> Prior to asking this, my workaround was to have a MySubClass* as an
> attribute and construct it after receiving val ie
>
> //val is passed in and construction of hello takes place only when value
> //is there
> hello = new MySubClass(val);
>
> I do not want the pointer solution above because despite creating and
> deleting appropriately somewhere along the line it provoked an access
> violation. I also have learned to hate pointers with class wide scope due
> to some nasty experiences recently with access violations or maybe I am
> blaming them on the wrong thing...it is hard to say but there seems to be
> fewer problems without messing around with pointers.
>
> I need more explanations on this, if people will be so kind, to understand
> more.
>
> Thanks.
Sorry about messing up the follow up...grrr
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