NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/lang/c++-part1/7440

Text File | 1996-08-05 | 4.8 KB | 171 lines

Path: news.u-net.com!news
From: [e-mail] (Thomas Christensen)
Newsgroups: comp.lang.c++
Subject: Re: Call order of constructors
Date: Fri, 23 Feb 1996 09:55:43 GMT
Organization: U-NET limited
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[e-mail] (John Antypas) wrote:

>Hello C++ wizards...

>I'm trying to create a library access some private netowrk resources.
>I thought I'd try something like this:

>class TRANSPORT {
>    ... magic stuff...
>public:
>    TRANSPORT(char *name)    // Called to activate and bind transport(name)
>    ~TRANSPORT()        // Called when we're done with transport(name)

>}

>class SESSION(
>    TRANSPORT transport&;
>    ....
>    SESSION(char *transport_name,
>        char *transport_address);
>    ~SESSION();
>    ....
>}

>I was hoping that main code such as :

>    SESSION    my_session("iee488", "unit-4B");

>would create a session on the IEEE488 controller, to unit Unit-4B, specifically,    - open a trasnport to the IEEE-488 system by calling the IEEE488
>      constructor
>    - open a session by calling the session constructor and have it
>      finish the job.

>The destructor sequence runs in the exact reverse.

>First off, the compiler complains about the TRANSPORT constructors, second,
>I find myself asking what to do if a constructor or destructor fails?
>How do I catch the error condition to notify outer levels of the program?

>Help as always is appreciated.

>John Antypas    [e-mail]

Hi John

1. The constructor situation
I would change the SESSION class to something like this:

class SESSION(
    TRANSPORT &transport;
    ....
    SESSION(char *transport_name, char *transport_address)
    {
        transport = NULL;
        transport = new TRANSPORT(transport_address);
    }
    ~SESSION()
    {
        if (transport)
            delete transport;
    }
    ....
}

In this way you control the construction of the TRANSPORT object.

2. The error handling situation
You said the word: catch!
I would use exception handling to catch the errors so if the TRANSPORT
couldn't connect then I throw an exception, which you'd have to catch
in the main program like this:
main()
{
    try
    {
        SESSION    my_session("iee488", "unit-4B");
    }
    catch(char *text)
    {
        cout << "Exception thrown: " << text << endl;
        // Do some more magic...
    }
    ...
}

The exception need to be thrown from anyone of the TRANSPORT or
SESSION classes, and the syntax is like that of a return statement:
    throw "Transport object couldn't connect!";
You can even throw a c++ class containing further description of the
exception ect.
The real power of C++ exception handling lies not only in its ability
to deal with exceptions of varying types, but also in its ability to
automatically call destructor functions during stack unwinding for all
local objects constructed before the exception was thrown.

3. The alternative solution situation
As always there's  numerous ways to do this. Here's how Microsoft
would do it:

Initializing Member Objects
Classes can contain member objects of class type, but to ensure that
initialization requirements for the member objects are met, one of the
following conditions must be met:
╖    The contained objectÆs class requires no constructor.
╖    The contained objectÆs class has an accessible default
constructor.
╖    The containing classÆs constructors all explicitly initialize the
contained object.

The following example shows how to perform such an initialization:

// Declare a class Point.
class Point
{
public:
    Point( int x, int y ) { _x = x; _y = y; }
private:
    int _x, _y;
};

// Declare a rectangle class that contains objects of type Point.
class Rect
{
public:
    Rect( int x1, int y1, int x2, int y2 );
private:
    Point _topleft, _bottomright;
};

//  Define the constructor for class Rect. This constructor
//   explicitly initializes the objects of type Point.
Rect::Rect( int x1, int y1, int x2, int y2 ) :
_topleft( x1, y1 ), _bottomright( x2, y2 )
{
}

The Rect class, shown in the preceding example, contains two member
objects of class Point. Its constructor explicitly initializes the
objects _topleft and _bottomright. Note that a colon follows the
closing parenthesis of the constructor (in the definition). The colon
is followed by the member names and arguments with which to initialize
the objects of type Point.

Warning   The order in which the member initializers are specified in
the constructor does not affect the order in which the members are
constructed; the members are constructed in the order in which they
are declared in the class.

Reference and const member objects must be initialized using the
member initialization syntax shown in Syntax in Initializing Bases and
Members. There is no other way to initialize these objects.

Hope you can use some of this...

Thomas Christensen
E-me-at: [e-mail]