When a non-default constructor provided in a class, the default constructor
is not available anymore.
In what cases shall a default constructor be defined explicitly?
Specifically, in the following cases, shall a default constructor be
provided?
1. Inheritance. Class A is derived from class B. Shall a default constructor
be defined for A and B?
2. Virtual inheritance.
3. When class A contains class B, Shall a default constructor be defined for
A and B?
4. Abstract base class.
Thanks in advance!
17  2501 
* highli:
in the following cases, shall a default constructor be provided?
1. Inheritance. Class A is derived from class B. Shall a default constructor
be defined for A and B?
2. Virtual inheritance.
3. When class A contains class B, Shall a default constructor be defined for
A and B?
4. Abstract base class.
Those possible reasons are _unrelated_ to whether a default constructor
should be defined.
Define a default constructor when there is natural default state.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
"highli" <hi****@cls.com > wrote in message
news:lD******** ************@bg tnsc05-news.ops.worldn et.att.net... When a non-default constructor provided in a class, the default
constructor is not available anymore.
In what cases shall a default constructor be defined explicitly?
Specifically, in the following cases, shall a default constructor be
provided?
1. Inheritance. Class A is derived from class B. Shall a default
constructor be defined for A and B?
2. Virtual inheritance.
3. When class A contains class B, Shall a default constructor be defined
for A and B?
4. Abstract base class.
Thanks in advance!
None of these absolutely require a default constructor assuming that you
have defined what a default constructor is. You can provide an alternate
constructor to satisfy all the above mentioned conditions just like you
might mix default constructors and alternate cstors to generate the end
product.
Whats important is that your derived class indicate which cstor is
appropriate to create base/members in its cstor's initialization list. Which
is critical since the base class(es) and member class(es) need to be
generated before the cstor of the derived class is invoked. You can't build
the house if you haven't poured the foundation.
If what you are asking is whether defining cstors are a requirement, the
answer is no, but since C++ gives the power to define creation, why not take
charge and control the process since that may help in the case of
tracing/debugging? Not to mention the fact that you only get what you want
with explicit invocations.
Incidentally, what you might want to spend more time on, is whether and what
copy cstor, as well as assignment operator you might need. Look up the
subject of "deep copy".
On Sat, 24 Jul 2004 22:44:33 GMT, highli <hi****@cls.com > wrote: When a non-default constructor provided in a class, the default
constructor
is not available anymore.
In what cases shall a default constructor be defined explicitly?
When you want the default constructor to do something different from the
compiler generated default constructor or when you want a default
constructor and the compiler won't generate one for you for some reason.
Specifically, in the following cases, shall a default constructor be
provided?
1. Inheritance. Class A is derived from class B. Shall a default
constructor
be defined for A and B?
2. Virtual inheritance.
3. When class A contains class B, Shall a default constructor be defined
for
A and B?
4. Abstract base class.
None of these issues are relevant in any way that I can see.
john
"highli" <hi****@cls.com > wrote in message
news:lD******** ************@bg tnsc05-news.ops.worldn et.att.net... When a non-default constructor provided in a class, the default
constructor is not available anymore.
In what cases shall a default constructor be defined explicitly?
When you want to create an array of objects of your class. You can't do so
in the absence of a default constructor. Another reason to switch over to
std::vector which doesn't impose such a restriction.
-Sharad
* Sharad Kala:
"highli" <hi****@cls.com > wrote in message
news:lD******** ************@bg tnsc05-news.ops.worldn et.att.net... When a non-default constructor provided in a class, the default constructor is not available anymore.
In what cases shall a default constructor be defined explicitly?
When you want to create an array of objects of your class.
That's most often a Very Bad reason, because it most often adds an
unusable state. Use smart-pointers instead. E.g. boost::shared_p tr.
You can't do so in the absence of a default constructor.
I'm sure that "highli" can write e.g.
class Point
{
int myX, mY;
public:
Point( int x, u ): myX( x ), myY( y ) {}
int x() const { return x; }
int y() const { return y; }
};
Point const points[] =
{ Point( 1, 2 ), Point( 3, 4 ), Point( 5, 6 ) };
or some such, without thinking twice (or at least he or she will be able
to do this after a while).
Another reason to switch over to
std::vector which doesn't impose such a restriction.
It's not very clear what you mean here.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
"Alf P. Steinbach" <al***@start.no > wrote in message
news:41******** ********@news.i ndividual.net.. . * Sharad Kala:
"highli" <hi****@cls.com > wrote in message
news:lD******** ************@bg tnsc05-news.ops.worldn et.att.net... When a non-default constructor provided in a class, the default
constructor is not available anymore.
In what cases shall a default constructor be defined explicitly?
When you want to create an array of objects of your class.
That's most often a Very Bad reason, because it most often adds an
unusable state. Use smart-pointers instead. E.g. boost::shared_p tr.
I was just mentioning one case when things can not be done without a default
constructor since it was not mentioned in any of the other replies. Not
recommending any way to do things. You can't do so in the absence of a default constructor.
[snip] Another reason to switch over to
std::vector which doesn't impose such a restriction.
It's not very clear what you mean here.
Read the FAQ - http://www.parashift.com/c++-faq-lit....html#faq-10.5
-Sharad
* Sharad Kala:
"Alf P. Steinbach" <al***@start.no > wrote in message
news:41******** ********@news.i ndividual.net.. . * Sharad Kala:
"highli" <hi****@cls.com > wrote in message
news:lD******** ************@bg tnsc05-news.ops.worldn et.att.net...
> When a non-default constructor provided in a class, the default
constructor
> is not available anymore.
>
> In what cases shall a default constructor be defined explicitly?
When you want to create an array of objects of your class.
That's most often a Very Bad reason, because it most often adds an
unusable state. Use smart-pointers instead. E.g. boost::shared_p tr.
I was just mentioning one case when things can not be done without a default
constructor since it was not mentioned in any of the other replies. Not
recommending any way to do things.
You can't do so in the absence of a default constructor.
[snip] Another reason to switch over to
std::vector which doesn't impose such a restriction.
It's not very clear what you mean here.
Read the FAQ - http://www.parashift.com/c++-faq-lit....html#faq-10.5
Thank you.
I'm CC'ing this to Marshall Cline; this FAQ item wasn't meant to create
the impression you got (note that at the very start it mentions "except
as discussed below"). In short, the correct version is that you can't
create an array of objects of a class type that has no default
constructor unless (a) an array initializer is given (the FAQ item gives
an example of that at the end, essentially the same example that I gave
and you snipped but without the typos... ;-) ), or (b) placement new is
used, as it is internally in std::vector. Point (b) should perhaps be
mentioned directly in this FAQ item, and both points placed at the top.
Now study the example at the _end_ of that FAQ item... ;-)
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
"Alf P. Steinbach" <al***@start.no > wrote in message
news:41******** ********@news.i ndividual.net.. . * Sharad Kala:
[snip] Read the FAQ - http://www.parashift.com/c++-faq-lit....html#faq-10.5
Thank you.
Most welcome ;-)
I'm CC'ing this to Marshall Cline; this FAQ item wasn't meant to create
the impression you got (note that at the very start it mentions "except
I don't think I got a wrong impression. The very reason I snipped your
example was because you were correct in saying that I could still create an
array if I gave an initializer and _no_ default constructor. There was
nothing to nitpick there ;-) But still one needs to take the trouble to
initialize each member, isn't that a pain ?
as discussed below"). In short, the correct version is that you can't
create an array of objects of a class type that has no default
constructor unless (a) an array initializer is given (the FAQ item gives
an example of that at the end, essentially the same example that I gave
I think FAQ is correct here. It does say "except as discussed below" at the
start as you also point out.
and you snipped but without the typos... ;-) ), or (b) placement new is
used, as it is internally in std::vector. Point (b) should perhaps be
mentioned directly in this FAQ item, and both points placed at the top.
Do you mind giving one example of point b ?
-Sharad
"Alf P. Steinbach" <al***@start.no > wrote in message
news:41******** ********@news.i ndividual.net.. . * Sharad Kala:
[snip] It's a bit of work to do correctly. Modulo alignment issues and
exception issues and whatnot (omitted here... ;-) ) it goes like
Quite good but not quite ;-)
#include <new>
class Point{ ... };
int main()
{
size_t const nObjects = 54;
char memory[nObjects*sizeof ( Point )];
Point* pObjects = 0;
// Initialize array.
char *pMem = memory;
for( size_t i = 0; i < nObjects; ++i )
{
Point* pNew = ::new( pMem ) Point( i, -i ); // Whatever.
if( pObjects == 0 ) { pObjects = pNew; }
pMem += sizeof( Point );
}
// Use it.
// Whatever, e.g.
for( size_t i = 0; i < nObjects; ++i )
{
// std::cout << pObjects[i].x() << std::endl;
}
The following is not the way arrays would destruct.
// Destroy.
Point* pObj = pObjects;
for( size_t i = 0; i < nObjects; ++i, ++pObj )
{
pObject->~Point(); // Modulo syntax, check it out.
}
Instead something like -
// Destroy.
Point* pObj = pObjects + sizeof(nObjects *sizeof( Point ));
for( size_t i = 0; i < nObjects; ++i, --pObj )
{
pObjects->~Point(); // Modulo syntax, check it out.
}
Right, Alf ?
-Sharad This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics | |
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