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qoy (i do not have that many points :-)

Posted on 2002-06-16
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Last Modified: 2010-04-04
Could a private constructor be useful for a Delphi object? Describe a constallation.
What is the problem of Delphi 5 (maybe also 4 and 6) with a private constructor?

100 points for an answer to both parts. 50 for one part.
No sample code required. A good explanation is enough.
Maybe more points for a good description.
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Question by:robert_marquardt
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by:kretzschmar
ID: 7083126
qoy->question of the year?

never used a private constructor,
no idea whats thebenefit of of ths

listening . . .

meikl ;-)
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by:robert_marquardt
ID: 7083177
To precise the question: ONLY a private constructor!
Yes, it IS useful.
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Accepted Solution

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geobul earned 200 total points
ID: 7083193
Hi,
Nice question, Robert.

I've never used it before but made some tests. And the tests show that:
1. if the object is created in the same unit then the private constructor is executed.
2. if the object is created outside its unit then the inherited constructor is used instead.

So, the conclusion:
A private constructor could be useful for an object which is used by another object in the same unit to perform some special operations (initialization) which are specific to that second (master) object and are not mandatory when the first object is used by itself.

Is that clear enough (understandable)? Does that make sense?

Regards, Geo
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by:geobul
ID: 7083217
About your second question: I can't find out any problems using private constructors in Delphi 5. What did you have in mind?
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by:robert_marquardt
ID: 7083268
geobul, almost there.
1. not bad, but i have a more useful task implemented already
2. you mentioned the problem, but failed to recognize it.
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by:robert_marquardt
ID: 7088984
I will solve the question tomorrow.
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by:kretzschmar
ID: 7089016
yep, do it :-)
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Author Comment

by:robert_marquardt
ID: 7091834
I use a private constructor as geobul mentioned. A controller object (component) creates them. The objects itself represent USB devices. No user interference is needed because the devices are plugged and as reaction a device object is created. A user/program created object is not advisable here because a 1 : 1 representation is always needed. For that i wanted to make it impossible to create a device object.

The problem is mentioned by geobul as 2. The TObject.Create is not hidden because it is a static method not a virtual method.
This i consider as a language problem. Visibility clashes with inheritance.
Locally the private constructor hides the TObject.Create. Externally TObject.Create is still visible.
I also get no visibility warning like for virtual methods. So the most difficult to understand language part is completely unvocered by hints or warnings.

I will award geobul the 100 points. Maybe more. It was a really tricky question.
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by:kretzschmar
ID: 7092386
aplause ;-)
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by:geobul
ID: 7092527
Wow, so many many thanks, Robert :-)))

I see what you had in mind now. On the one hand that is some kind of flexibility but on the other hand the lack of warning messages is really bad. This may lead to very-hard-to-find errors.

Thank you again and best regards, Geo
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