MESSAGE
| DATE | 2011-05-26 |
| FROM | Ruben Safir
|
| SUBJECT | Subject: [ruben@mrbrklyn.com: Re: [NYLXS - HANGOUT] C++ Workshop - New
|
----- Forwarded message from Ruben Safir -----
Date: Wed, 25 May 2011 17:10:40 -0400
From: Ruben Safir
To: Ruben Safir
Cc: nylug-talk-at-nylug.org, mrbrklyn-at-panix.com
Subject: Re: [NYLXS - HANGOUT] C++ Workshop - New Webservices with OKWS -
Tuesday Night
User-Agent: Mutt/1.5.20 (2009-06-14)
n Monday 23 May 2011 16:37:36 Ruben Safir wrote:
> I'm having trouble understanding why my program is making a change in
> the argument type of my template class function
>
> template < class T >
> void List::find_value(T val)
...
> I then pass int search_val to method
> chainlist::List::find_value(T)
> where T = stats::Distribution (where T is an int) on this line:
>
> dump->find_value(search_val);
>
> and that's where the weirdness happens. I have a type mismatch
> since
> I'm sending by value an integer instead of a stats::Distribution
< Out of curiosity, is this typcastable? e.g.:
dump->find_value((stats::Distribution)search_val);>
That is exactly the type that the function takes in the context of the
calling, class. T is an int. The issue winds up being that of
implicit conversion which happens if there is a stats::Distirbution
single argument constructor. For this to happen, the Template argument
needs to be specifically defined, which it is in this case since it was
defined in the class.
<< [function for reference: void List::find_value(T val) ]
I think this is a case where the compiler needs help, because
find_value() is
looking for a templated generic value, so the compiler doesn't
know that it
can't send in a simple integer. The whole purpose of using
templates is to
keep the type abstract, so I'm not sure how you'd solve this
within the
function header. >>
As it turns out, this is an explicit rule. If T is not defined, the
implicit conversion can not happen a`nd it is considered, "Can not be
deduced".
o
<<
I've been trying to read your C++ code, but I've been finding
it difficult;
the stream of templated objects... wow. However in the middle
of trying to
read things that I can parse, I noticed the following:
>>
Well, it's an excersize in templates. Your familiar with the rules for
vector<> and list<> because they are part of the standard template
libray. My chainlist::List seems to be pretty decent at this point, but
the syntax is unknown unless you pour over the code *List<>->cursor()
returns a chainlist::Node class object. It contains a pointer to a
value of type T and a pointer to the Next Node
Things get exciting when T is its own class.
<< float mean_list( )
...
while(tallies->cursor() != tallies->endd() ){
...
if(dump->cursor() != NULL)
...
}
...
if(dump->cursor() != 0)
If dump->cursor() is a pointer, then compare it with NULL, not 0.
>>
There is a lot of debate on this issue. Strausoup strongly urges 0,
but frankly, dealing with nulls in C++ is a PIA, and even moreso now
with the new standard which blows a warning at you if you assign NULL
to an int type variable.
<< Even if
this works, I'd choose one test and try to stick to it for
readability
reasons.
>>
Ruben
-- Chris
--
Chris Knadle
Chris.Knadle at coredump.us
--
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----- End forwarded message -----
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