Inheritance of constructors.

[A n g l e r]

Hi all.

I'm facing the following construction:

class ClOld
{
public ClOld(string a) { ... }
}

class ClNew: ClOld
{
SomeData[] arr;
public ClNew(string a, SomeObject b): base(a)
{
arr = new SomeData[] {b.data1, b.data2, b.data3 };
}
}

Now, I wonder how to pass initialized arr to a base constructor. Why
can't I code something like below:

class ClOld
{
public ClOld(string a, ref SomeData[] arr) { ... }
}

class ClNew: ClOld
{
SomeData[] arr;
public ClNew(string a, SomeObject b):
{
arr = new SomeData[] {b.data1, b.data2, b.data3 };
base(a, ref arr);
}
}
Thanks in advance,
Pete

[Jon Skeet [C# MVP]]

On Jun 30, 12:17*pm, A n g l e r <p|k|o|n|i|u|.. .@h-o-t-m-a-i-l.c_o_m>
wrote:

I'm facing the following construction:

<snip>

Now, I wonder how to pass initialized arr to a base constructor. Why
can't I code something like below:

<snip>

Basically you can't write any actual code in the constructor before
the base constructor call. You *can* make a static method call, so you
can do:

public ClNew(string a) : base(a, CreateArray())
{
// Other stuff
}

That won't help you for a ref parameter, however. Are you sure you
need a ref parameter in the constructor call? That's pretty unusual.

Jon

[A n g l e r]

Basically you can't write any actual code in the constructor before
the base constructor call. You *can* make a static method call, so you
can do:

Brilliant, I thought so. Blooming drawbacks of the garbage collector and
the whole automated memory management by ms, right? :/

public ClNew(string a) : base(a, CreateArray())
{
// Other stuff
}

That will help for now :)

That won't help you for a ref parameter, however. Are you sure you
need a ref parameter in the constructor call? That's pretty unusual.

No, I guess I don't cos I pass just an array of objects which anyway
means (I guess so) it's handled in a similar way to reference. Isn't it?

DataGridView[] arr = new DataGridView[] { grid1, grid2, grid3 };

Cheers,
P.

[Jon Skeet [C# MVP]]

On Jun 30, 1:43*pm, A n g l e r <p|k|o|n|i|u|.. .@h-o-t-m-a-i-l.c_o_m>
wrote:

Basically you can't write any actual code in the constructor before
the base constructor call. You *can* make a static method call, so you
can do:

Brilliant, I thought so. Blooming drawbacks of the garbage collector and
the whole automated memory management by ms, right? :/

Well, it's more that doing things within an object before it's been
initialized in the parent is somewhat questionable.

<snip>

That won't help you for a ref parameter, however. Are you sure you
need a ref parameter in the constructor call? That's pretty unusual.

No, I guess I don't cos I pass just an array of objects which anyway
means (I guess so) it's handled in a similar way to reference. Isn't it?

DataGridView[] arr = new DataGridView[] { grid1, grid2, grid3 };

There's a big difference between "pass by reference" and "pass
reference by value". It confuses quite a lot of people, and it's well
worth being aware of the difference. It's mostly down to confusingly
named terminology, IMO.

See http://pobox.com/~skeet/csharp/parameters.html

Jon

[A n g l e r]

Well, it's more that doing things within an object before it's been

initialized in the parent is somewhat questionable.

Consider an example where a new class (for instance it's constructor) is
in charge of the data preparation which is fed to a parent class
constructor. OK, you can pass it via static variable as suggested,
though what happens if you want to have different data in it for each
instance of a new class? Unless static in C# means it exists over the
whole cycle of a code execution but doesn't limit it to one global data
copy shared amongst all instances like in C++. Is this the case?

There's a big difference between "pass by reference" and "pass
reference by value". It confuses quite a lot of people, and it's well
worth being aware of the difference. It's mostly down to confusingly
named terminology, IMO.

See http://pobox.com/~skeet/csharp/parameters.html

Sure, it's just all happens on different levels. The actual reference is
all about where it is in the memory. The value reference is more like a
container that is being copied while passing.

Thanks,
P.

[A n g l e r]

Consider an example where a new class (for instance it's constructor) is

in charge of the data preparation which is fed to a parent class
constructor. OK, you can pass it via static variable as suggested,
though what happens if you want to have different data in it for each
instance of a new class? Unless static in C# means it exists over the
whole cycle of a code execution but doesn't limit it to one global data
copy shared amongst all instances like in C++. Is this the case?

Erm, OK, I see what happens in this case. It's passed via static method,
the value reference is stored locally in a base class. No harm done
unless I would prefer it would be capable of following every change of
the value reference in outer classes ... what then?

[Jon Skeet [C# MVP]]

On Jun 30, 2:35*pm, A n g l e r <p|k|o|n|i|u|.. .@h-o-t-m-a-i-l.c_o_m>
wrote:

Well, it's more that doing things within an object before it's been
initialized in the parent is somewhat questionable.

Consider an example where a new class (for instance it's constructor) is
in charge of the data preparation which is fed to a parent class
constructor. OK, you can pass it via static variable as suggested,
though what happens if you want to have different data in it for each
instance of a new class? Unless static in C# means it exists over the
whole cycle of a code execution but doesn't limit it to one global data
copy shared amongst all instances like in C++. Is this the case?

I didn't suggest a static *variable*. I suggested a static *method*.
The method should be able to construct the information required for
the base class's constructor which no information about the current
(uninitialized) instance.

There's a big difference between "pass by reference" and "pass
reference by value". It confuses quite a lot of people, and it's well
worth being aware of the difference. It's mostly down to confusingly
named terminology, IMO.

Seehttp://pobox.com/~skeet/csharp/parameters.html

Sure, it's just all happens on different levels. The actual reference is
all about where it is in the memory. The value reference is more like a
container that is being copied while passing.

Not sure what you mean by "value reference" but the point is that
there's a big difference between
void Foo(ref object[] x)
and
void Foo(object[] x)
even though object[] is a reference type. I rarely see a genuine need
for the former signature.

Jon

[Jon Skeet [C# MVP]]

On Jun 30, 2:48*pm, A n g l e r <p|k|o|n|i|u|.. .@h-o-t-m-a-i-l.c_o_m>
wrote:

Consider an example where a new class (for instance it's constructor) is
in charge of the data preparation which is fed to a parent class
constructor. OK, you can pass it via static variable as suggested,
though what happens if you want to have different data in it for each
instance of a new class? Unless static in C# means it exists over the
whole cycle of a code execution but doesn't limit it to one global data
copy shared amongst all instances like in C++. Is this the case?

Erm, OK, I see what happens in this case. It's passed via static method,
the value reference is stored locally in a base class. No harm done
unless I would prefer it would be capable of following every change of
the value reference in outer classes ... what then?

I don't really understand what you mean by the last sentence. Could
you give a concrete example in code of what you'd like to be able to
do?

Jon

[A n g l e r]

I don't really understand what you mean by the last sentence. Could

you give a concrete example in code of what you'd like to be able to
do?

Ok, here it goes:

class ClOld
{
public SomeData[] arr;
public ClOld(string a, SomeData[] b) { arr=b; }
}

class ClNew: ClOld
{
static SomeData[] arr;
static SomeData[] StaticPassing(S omeClass src)
{
arr = new SomeData[] { src.d1, src.d2, src.d3 };
return arr;
}

public void UpdateArr(src)
{
arr = new SomeData[] { src.d1, src.d2, src.d3 };
// the arr is now assigned a new reference by value
// though, this doesn't mean that base.arr is also updated
// with a new reference value unless
// I'll update it on myself, for instance base.arr=arr; or more
//likely by set, get that act on arr while it remains the
// private member of ClOld

//also, if I made the arr public in thr ClNew and modified its
//reference by value from outside of the ClNew class, I'd have
//to handle some further updating actions which is inconvenient
}

public ClNew(string a, SomeObject b): base(a, StaticCopier())
{
}
}

ClNew aa= new ClNew(src);
aa.arr=new SomeData[] { data }

// aa.base.arr stays unchanged unless I'll update it. In case of memory
// references it wouldn't be any concern as long as all this happens in
// one thread

[A n g l e r]

Not sure what you mean by "value reference" but the point is that

Sorry, I mean reference by value which indicates that the reference is
copied and isn't maintained automatically by another instances of the
actual "reference by value" container :d

class1 aa=new class1("d1");
bb=aa;
aa= new class1("d2");

bb still contains d1
aa contains d2

while in case of memory-wise C++ reference
bb would contain d2
aa would contain also d2

Therefore I said that reference by value is like a container of
reference. It helps you avoid copying of the whole objects, but you have
to maintain on your own the actual copies of the references ...