Re: equivalent class to POD

"Victor Bazarov" <>
Mon, 17 Sep 2007 10:50:35 -0400
<fcm47q$fa5$> wrote:

On Sep 17, 3:44 pm, "Victor Bazarov" <> wrote:

I am not certain what the reason would be to provide a way to
*not* initialise an object which will throw an exception when
accessed *instead of* simply requiring for an object to always
be in a valid state by implementing only the constructor with
the argument.

It is a class with lots of parameters (memebers of type double).
These values need to be set by some setting member functions
(like e.g.:
void SetParam1( double value);
When the calculation is requested, I need a way to check if all
relevant parameters have been set.

OK, so you're saying that for some operations only some members are
required, and for other operations other members are required to have
valid values, right? IOW, an object is allowed to have "part of it"
invalid if the operation does not require it.

That seems to require a flag for every operation (IOW, you would need
to either set all flags ahead of time by combining the validity of
the members in some manner, or set the validity flags and then every
operation will do the hard-coded check by combining the flags it knows
about, again in some manner). Too much work, and for what gain?

As some of the parameters might take any value, just setting a default
value is not

In the setting member functions I could of course do some checking,
but for every
newly introduced parameter (extending the program) these functions
would need
to be adapted then (and result in a bool flag per parameter).

As I am probably not the first to think about this basic issue,
is there already an implemtation for the basic POD available?

What for? PODs are a hack, necessary for C compatibility. Try
to see beyond them. Initialise all objects with proper values.
Then you will have no such problem.

The proper values are not known at construction time.

Then you might want to reconsider your model.

Another advantage of custom double replacement (class Double) would be
extensions like range checking.

That's not an extension, that's a limitation.

My example was just as simple as

It was most likely too simple.

I am guessing that even I can come up with a made-up example where not
all members would be "set" at the construction of an object and then
if an operation were to be performed, a check would be done and a green
light would be given if all needed parameters are set, and an exception
would be thrown otherwise.

I can even think of an example from the Standard library: a vector that
has N elements, N < M, can have new values inserted into, but cannot be
asked to supply any element with the index N..M-1, but this is not the
same, unfortunately. All member values _are_ set, they just don't meet
certain conditions. Same with, for example, a standard stream that was
read past the last element - the stream is in EOF condition - you can't
read from it, but you can close it. Again, all elements are set, but
not all operations can be performed (or lead to meaningful result).

Now, I imagine a car on the assembly line is closer. You can't really
sit inside [comfortably] until the seats are installed. You can't turn
it on until the engine is there. You can't move it independently until
the wheels have been attached, etc. But those contributing elements of
a car should then be probably modeled by pointers since nobody really
expects the car to be driven, or sat in, while the car is still on the
assembly line, and the only functionality it is expected to provide is
to be further assembled (completed). Individual tests could be still
performed (like turing on the lights to check connectivity). And the
system that performs those tests would of course (a) check if the system
to be test is present and reports (alleges) to be exercisable and (b)
that all exceptions are caught in the same scope where the test is
performed. IOW, the availability of the members that participate in
a certain operation is exposed to the object manager (and not limited
to the object itself so it could throw the exception when the operation
is attempted).

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