[comp.lang.c++] Constructors and Error Conditions

jungblut@gmdzi.gmd.de (Achim Jungblut) (06/29/91)

Hi out there,

this is my first posting this group and I'm in the beginner state of learnig
C++.

Since Constructors do not return any value on completion how can I react
on error conditions while executing the Constructor i.e what to do if I want
to create an Integer in the range 34..59 via Construktor parameter and that
parameter is out of that range ?

One solution may be to introduce an additional out parameter reflecting
the end state of the execution; but this seems not very smart to me. 

Hope someone can help me ...

Gerd

steve@taumet.com (Stephen Clamage) (06/29/91)

>Since Constructors do not return any value on completion how can I react
>on error conditions while executing the Constructor i.e what to do if I want
>to create an Integer in the range 34..59 via Construktor parameter and that
>parameter is out of that range ?

>One solution may be to introduce an additional out parameter reflecting
>the end state of the execution; but this seems not very smart to me. 

The usual solution is to introduce a 'status' function which is tested
before or after a use of an object, or after constructing it.  A similar
solution, where there are only two possible status cases, is to
define a type conversion to, say, void*, which can be tested in a
boolean expression.  Examples:

enum state { GOOD, OK, FAIL };
class c1 {
    state stat;
public:
    c1(int);
    state status() { return stat; }
};
f1(...)
{
    c1 c(1);
    if( c.status() != FAIL ) ...
    else ...
}

class c2 {
public:
    c2(int);
    operator void*() { return (all is ok) ? this : 0; } 
};
f2(...)
{
    c2 c(1);
    if( c ) ...		// c is automatically coverted to void*
    else ...
}

Once exceptions are widely available in implementations, they are the
way to handle the problem.  You define an exception for failure of
a constructor, and raise the exception inside the constructor.  This
will transfer control to the exception handler, which might simply
abort the program.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com