Mail Archives: djgpp/1998/01/18/18:30:23
| From: | Erik Max Francis <max AT alcyone DOT com>
|
| Newsgroups: | comp.os.msdos.djgpp,gnu.gcc.help,comp.os.os2.programmer.misc
|
| Subject: | Re: C++ constructor problem solved!! Read how, it might come in useful in your future programs!
|
| Date: | Sun, 18 Jan 1998 14:26:02 -0800
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| Organization: | Alcyone Systems
|
| Lines: | 56
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| Message-ID: | <34C2817A.5547BE86@alcyone.com>
|
| References: | <bWLoegW7sFse-pn2-bxyqvlUyVUyY AT localhost>
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| NNTP-Posting-Host: | newton.alcyone.com
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| Mime-Version: | 1.0
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| To: | djgpp AT delorie DOT com
|
| DJ-Gateway: | from newsgroup comp.os.msdos.djgpp
|
Gili wrote:
> However, we wanted to be able to call the base constructor
> from the
> body of the File() constructor. This would allow more flexibility,
> mainly allowing you to use exception-handling for this call. The
> solution took me 2 days to find, but here it is:
>
> File::File(<parameters>)
> {
> ((fstream*)this)->fstream(<parameters>);
> <body goes here>
> }
Explicitly calling constructors is dangerous. Here what's _really_
happening is that the fstream is being constructed with the default
constructor, and _then_ you're explicitly calling a nondefault
constructor. That is, it's functionally precisely the same thing as:
File::File(...):
fstream()
{
((fstream *) this)->fstream(...);
...
}
This is unwise and can have unwanted side effects, particularly if
memory is being allocated in the default constructor (because then it
will get allocated twice).
The proper (meaning, standard and safe) way to do what you want is to
have a pointer to an fstream contained within the class, and then create
that dynamically when needed:
class File
{
public:
fstream *fs;
File(...);
...
};
File::File(...)
{
fs = new fstream(...);
...
}
--
Erik Max Francis, &tSftDotIotE / mailto:max AT alcyone DOT com
Alcyone Systems / http://www.alcyone.com/max/
San Jose, California, United States / icbm://+37.20.07/-121.53.38
\
"Life may be / the product of imperfections."
/ (Marclo Gleiser)
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