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From: David Webber on 4 May 2008 19:32

"Jeov� Almeida" <jeovaalmeida(a)yahoo.com> wrote in message
news:%2305F%23qirIHA.5872(a)TK2MSFTNGP04.phx.gbl...

> How can I make a CString parameter optional?
>
> I tried
>
> [in the .h file:]
> virtual CString Get(CString strURL, CString& strErrorMsg = NULL);
>
> It gives me the error
> Error 4 error C2440: 'default argument' : cannot convert from 'int' to
> 'CString &
>
> What I want to do: make the strErrorMsg parameter optional so I could call
> CString result = obj.Get("www.server.com");
> CString errMsg;
> CString result2 = obj.Get("www.server.com", errMsg);

One obvious way is to overload the function:

CString Get( const CString &strURL );
CString Get( const CString &strURL, CString &errMsg );

Dave
--
David Webber
Author of 'Mozart the Music Processor'
http://www.mozart.co.uk
For discussion/support see
http://www.mozart.co.uk/mozartists/mailinglist.htm

From: Jeov� Almeida on 4 May 2008 19:46
Feedback:

Analysing all the sugestions, and considering strErr is an output parameter:

virtual CString Get(LPCTSTR strURL, CString& strErrorMsg=CString() );

which solved my problem.

Thanks all for the tips and lessons.

Jeov�

"Jeov� Almeida" <jeovaalmeida(a)yahoo.com> escreveu na mensagem
news:%2305F%23qirIHA.5872(a)TK2MSFTNGP04.phx.gbl...
> Hello,
>
> How can I make a CString parameter optional?
>
> I tried
>
> [in the .h file:]
> virtual CString Get(CString strURL, CString& strErrorMsg = NULL);
>
> It gives me the error
> Error 4 error C2440: 'default argument' : cannot convert from 'int' to
> 'CString &
>
> What I want to do: make the strErrorMsg parameter optional so I could call
> CString result = obj.Get("www.server.com");
> CString errMsg;
> CString result2 = obj.Get("www.server.com", errMsg);
>
>


From: Joseph M. Newcomer on 5 May 2008 02:26
Well, the message is correct; you can't default a reference parameter.

virtual CString Get(const CString & strURL, CString * strErrorMsg = NULL)

or just do

virtual CString Get(const CString & strULR, CString & strErrorMsg)

and then have an overload

virtual CString Get(const CString & strULR)
{
CString dummy;
return Get(strULR, dummy);
}

joe

On Sun, 4 May 2008 17:06:35 -0400, "Jeov� Almeida" <jeovaalmeida(a)yahoo.com> wrote:

>Hello,
>
>How can I make a CString parameter optional?
>
>I tried
>
>[in the .h file:]
>virtual CString Get(CString strURL, CString& strErrorMsg = NULL);
>
>It gives me the error
>Error 4 error C2440: 'default argument' : cannot convert from 'int' to
>'CString &
>
>What I want to do: make the strErrorMsg parameter optional so I could call
>CString result = obj.Get("www.server.com");
>CString errMsg;
>CString result2 = obj.Get("www.server.com", errMsg);
>
Joseph M. Newcomer [MVP]
email: newcomer(a)flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm
From: Serge Wautier on 5 May 2008 02:30
> I don't want to be picky

Neither do I ;-)

> I also tend to pass LPCTSTR as input string parameters

Why lose the benefits of CString?
In addition, if your function contains

CString myString(pszInputStr);

-> duplicates the string buffer. Expensive & useless!

My 2 cents,

Serge.
http://www.apptranslator.com - Localization tool for your MFC applications



"Giovanni Dicanio" <giovanni.dicanio(a)invalid.com> wrote in message
news:OuPQUBjrIHA.3780(a)TK2MSFTNGP03.phx.gbl...
>
> "Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp> ha scritto nel
> messaggio news:ewoK9zirIHA.4884(a)TK2MSFTNGP06.phx.gbl...
>
>> virtual CString Get(CString strURL, LPCTSTR strErrorMsg = NULL);
>
> I don't want to be picky, but I'd like to add to correct Scott's answer
> that you should aksi consider passing CString input parameters by *const
> reference* (so no copy constructor is called):
>
> // Note const CString &
> CString Get( /* input */ const CString & strURL ... )
>
> I also tend to pass LPCTSTR as input string parameters, and use CString
> for output parameters or return values.
>
> However, I read that the OP's method is virtual, so I'm not sure if using
> default parameters is good for virtual methods...
>
> Maybe for virtual methods, using overload is better than using default
> parameters (at least when there are few default parameters), e.g.
>
> // Two overloads for virtual methods (to "simulate" default parameters)
> virtual CString Get( const CString & strURL, const CString &
> strErrorMsg );
> virtual CString Get( const CString & strURL ); // default strErrorMsg
>
> Giovanni
>
>

From: Giovanni Dicanio on 5 May 2008 03:37

"Serge Wautier" <serge(a)wautier.nospam.net> ha scritto nel messaggio
news:eTyYdnnrIHA.5872(a)TK2MSFTNGP04.phx.gbl...
>> I don't want to be picky
>
> Neither do I ;-)

:-)


>> I also tend to pass LPCTSTR as input string parameters
>
> Why lose the benefits of CString?

Doug answered this point in his detailed post.
In his words:

<cite>

Finally, if the parameters don't /need/ to
be CStrings, use LPCTSTR parameters. That way, when you say...

CString result = obj.Get("www.server.com");

.... you don't needlessly construct a temporary CString to represent the
parameter.

</cite>

I used to pass 'const CString &' references for input strings, but then
after reading some source code (maybe it was ATL Server code... or some
article on MSDN, I'm not sure), I noted that technique of using LPCTSTR (or
explicit 'const wchar_t *') just for input string parameters (and CString
for output parameters or return values).
Then I realized that that would save the creation of a temporary CString
instance in cases like shown in Doug's post.

It is one of the good coding practices, like *pre*incrementing an STL
iterator (++i instead of i++), that does not cost much to us (the
programmers), and contributes to build more quality code, IMHO.

Giovanni



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