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| DATE | 2016-11-21 |
| FROM | ruben safir
|
| SUBJECT | Subject: [Learn] Fwd: when is the constructor called for an object
|
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Great review of constructors
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From: Popping mad
Newsgroups: alt.comp.lang.learn.c-c++
Subject: when is the constructor called for an object
Date: Mon, 21 Nov 2016 02:24:54 +0000 (UTC)
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I have a class
class{
public:
Node(Node *){};
Node(){};
Node* left()
{
return _left;
}
left(Node *in)
{
_left=in;
}
private:
Node * _left;
Node * _right;
and in another class I call
Node * tmp;
and later I want to allocate its memory (without using new)
I use
tmp = nullptr;
I can't seem to use
tmp{nullptr};
and when I try
tmp(nullptr){};
if says tmp is not a function
when is the constructor called?
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From: "Alf P. Steinbach"
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 04:04:51 +0100
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On 21.11.2016 03:24, Popping mad wrote:
>
> I have a class
> class{
> public:
> Node(Node *){};
> Node(){};
> Node* left()
> {
> return _left;
> }
> left(Node *in)
> {
> _left=in;
> }
>
> private:
> Node * _left;
> Node * _right;
You forgotten to provide a name for the class.
You have not provided any way to set the left and right pointers of an
instance.
Leading underscore is an ungood convention because that convention is
used to distinguish global namespace names reserved to the
implementation. E.g. the Boost library code generally uses trailing
underscore.
A simple data carrier class like this is best expressed as a POD struct,
maybe with some helper functions:
struct Node
{
Node* left_;
Node* right_;
};
> and in another class I call
>
> Node * tmp;
That's not a call, that's a declaration.
> and later I want to allocate its memory (without using new)
>
> I use
>
> tmp = nullptr;
That does not allocate, that assigns a nullpointer value to `tmp`.
> I can't seem to use
> tmp{nullptr};
That's not valid syntax. It could be valid as part of a declaration.
> and when I try
> tmp(nullptr){};
>
> if says tmp is not a function
Because it guessed that you were trying to express a function call.
This is again not valid syntax.
> when is the constructor called?
It's not.
But if you had created some `Node` instance, then a constructor would
have been called. However, the two constructors you have defined do
absolutely nothing. So regardless of which one you'd asked to be
executed, it wouldn't do anything.
Cheers & hth.,
- Alf
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From: Popping mad
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 03:17:07 +0000 (UTC)
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On Mon, 21 Nov 2016 04:04:51 +0100, Alf P. Steinbach wrote:
> It's not.
>
> But if you had created some `Node` instance, then a constructor would
> have been called. However, the two constructors you have defined do
> absolutely nothing. So regardless of which one you'd asked to be
> executed, it wouldn't do anything.
>
>
> Cheers & hth.,
>
> - Alf
it is sample code Alf .. the whole thing does nothing ;)
class WOODY{
public:
WOODY(WOODY *){};
WOODY(){}; WOODY* left()
{
return _left;
}
left(Node *in) {
_left=in;
}
private:
WOODY * _left; WOODY * _right;
and in another class I call
WOODY * tmp;
and later I want to allocate its memory (without using new)
I use
tmp = nullptr;
I can't seem to use tmp{nullptr}; <<== that is only good in declarations?
and when I try tmp(nullptr){}; <<== that doesn't call the constructor?
if says tmp is not a function
when is the constructor called?
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From: Ben Bacarisse
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 11:15:09 +0000
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I'll just add a couple of things...
Popping mad writes:
> and in another class I call
>
> WOODY * tmp;
That's not a function call (it's a declaration) so the verb "call" is
very confusing here. The best verb to use here is just "write": "in
another class I write..."
> and later I want to allocate its memory (without using new)
>
> I use
>
> tmp = nullptr;
This is an assignment. A lot of confusion has been caused over the
years by the fact that C and C++ both use the same symbol for two very
different things:
Thing x = 1; // initialisation
x = 2; // assignment
> I can't seem to use tmp{nullptr}; <<== that is only good in
> declarations?
Pretty much. The relatively recent {} syntax is only good in
initialisations. Declarations are where most things are initialised,
but you can use it in other contexts (for example where a temporary
object is created).
One reason to use {} is to make the distinction between assignment and
initialisation syntactically distinct.
--
Ben.
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From: Barry Schwarz
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Sun, 20 Nov 2016 20:39:15 -0800
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On Mon, 21 Nov 2016 02:24:54 +0000 (UTC), Popping mad
wrote:
>
>
>I have a class
>class{
>public:
> Node(Node *){};
> Node(){};
Where is Node defined? Based on the two constructors you have
defined, is the first line of the class declaration actually
class Node{
and not an unnamed class as your message text implies?
> Node* left()
> {
> return _left;
> }
> left(Node *in)
> {
> _left=in;
> }
>
>private:
> Node * _left;
> Node * _right;
>
>
>and in another class I call
>
>Node * tmp;
tmp is a pointer to a class.
>and later I want to allocate its memory (without using new)
What do you actually mean by allocate if you won't use new? There are
only three possible values for tmp:
It can be indeterminate (the result of the above definition that
does not contain any initialization).
It can be a null value (the result of the assignment you coded
below).
It can point to an object of type Node. This will only happen if
you assign it the address of an existing object or the address of an
object you allocate using new or an equivalent process.
>I use
>
>tmp = nullptr;
>
>I can't seem to use
>tmp{nullptr};
What syntax do you think the above statement is an example of? It is
not initialization since it is not the definition of an object
>and when I try
>tmp(nullptr){};
What syntax do you think this is an example of? It cannot be a
function definition since there is no return type, tmp is not a
function name, and nullptr is not a type.
>if says tmp is not a function
Quite so. tmp is a pointer to an object of type Node. Currently
there is no object of that type and tmp contains a null value which
documents that it does not currently point to an object.
>when is the constructor called?
The constructor is called when an object of that type is created. This
normally happens when an object is defined or when new allocates space
for an object of that type. It is not called when a pointer to an
object is defined unless initialization causes allocation.
--
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From: Popping mad
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 05:59:16 +0000 (UTC)
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On Sun, 20 Nov 2016 20:39:15 -0800, Barry Schwarz wrote:
> What syntax do you think this is an example of? It cannot be a function
> definition since there is no return type, tmp is not a function name,
> and nullptr is not a type.
it would be an attempt to use the constructor of the object.
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From: Barry Schwarz
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 08:36:32 -0800
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On Mon, 21 Nov 2016 05:59:16 +0000 (UTC), Popping mad
wrote:
>On Sun, 20 Nov 2016 20:39:15 -0800, Barry Schwarz wrote:
>
>> What syntax do you think this is an example of? It cannot be a function
>> definition since there is no return type, tmp is not a function name,
>> and nullptr is not a type.
>
>it would be an attempt to use the constructor of the object.
You don't call a constructor of an object. The appropriate
constructor is called for you automatically when the object is being
created by one of the methods described in my previous message.
For you to call a member function of a class, you normally need to
have an object of that class. You don't have any objects of your
class.
--
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From: Ike Naar
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 08:31:22 -0000 (UTC)
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On 2016-11-21, Popping mad wrote:
> I have a class
> class{
You probably meant
class Node {
> public:
> Node(Node *){};
> Node(){};
> Node* left()
> {
> return _left;
> }
> left(Node *in)
You probably meant
void left(Node *in)
> {
> _left=in;
> }
>
> private:
> Node * _left;
> Node * _right;
};
> and in another class I call
>
> Node * tmp;
>
> and later I want to allocate its memory (without using new)
Why can't you use new?
> I use
>
> tmp = nullptr;
>
> I can't seem to use
> tmp{nullptr};
>
> and when I try
> tmp(nullptr){};
>
> if says tmp is not a function
>
> when is the constructor called?
The constructur is called when an instance of class Node is created.
E.g. like this:
Node *px = new Node; /* constructor Node() is called */
Node *py = new Node(px); /* constructor Node(Node*) is called */
Node z; /* constructor Node() is called */
Node *pz = &z;
Node w(z); /* constructor Node(Node*) is called */
Node *pw = &w;
Node *
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From: ruben safir
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 03:37:52 -0500
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On 11/21/2016 03:31 AM, Ike Naar wrote:
> On 2016-11-21, Popping mad wrote:
>> I have a class
>
>> class{
>
> You probably meant
>
> class Node {
>
>> public:
>> Node(Node *){};
>> Node(){};
>> Node* left()
>> {
>> return _left;
>> }
>> left(Node *in)
>
> You probably meant
>
> void left(Node *in)
>
>> {
>> _left=in;
>> }
>>
>> private:
>> Node * _left;
>> Node * _right;
>
> };
>
>> and in another class I call
>>
>> Node * tmp;
>>
>> and later I want to allocate its memory (without using new)
>
> Why can't you use new?
>
>> I use
>>
>> tmp = nullptr;
>>
>> I can't seem to use
>> tmp{nullptr};
>>
>> and when I try
>> tmp(nullptr){};
>>
>> if says tmp is not a function
>>
>> when is the constructor called?
>
> The constructur is called when an instance of class Node is created.
> E.g. like this:
>
> Node *px = new Node; /* constructor Node() is called */
> Node *py = new Node(px); /* constructor Node(Node*) is called */
> Node z; /* constructor Node() is called */
> Node *pz = &z;
> Node w(z); /* constructor Node(Node*) is called */
> Node *pw = &w;
> Node *
>
constructor confusion
The real problem here is that then new Strousrup text seems to say that
use of {} can be used in lew of "= new" nomenclature, but it doesn't
seem to work like he suggests, which means I'm not understanding what he
wrote, or what he wrote wasn't clear ;)
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From: Ike Naar
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 09:14:36 -0000 (UTC)
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Xref: panix alt.comp.lang.learn.c-c++:246551
On 2016-11-21, ruben safir wrote:
> On 11/21/2016 03:31 AM, Ike Naar wrote:
>> The constructur is called when an instance of class Node is created.
>> E.g. like this:
>>
>> Node *px = new Node; /* constructor Node() is called */
>> Node *py = new Node(px); /* constructor Node(Node*) is called */
>> Node z; /* constructor Node() is called */
>> Node *pz = &z;
>> Node w(z); /* constructor Node(Node*) is called */
>> Node *pw = &w;
>> Node *
>>
> constructor confusion
>
> The real problem here is that then new Strousrup text seems to say that
> use of {} can be used in lew of "= new" nomenclature, but it doesn't
> seem to work like he suggests, which means I'm not understanding what he
> wrote, or what he wrote wasn't clear ;)
Not sure what you mean.
Of course one could rerwite the above as
Node *x(new Node); /* constructor Node() is called */
Node *y(new Node(x)); /* constructor Node(Node*) is called */
Node z; /* constructor Node() is called */
Node w(&z); /* constructor Node(Node*) is called */
Node *tmp(&w);
which uses () instead of = for the initializations.
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From: ruben safir
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 04:46:18 -0500
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Xref: panix alt.comp.lang.learn.c-c++:246552
On 11/21/2016 04:14 AM, Ike Naar wrote:
> Not sure what you mean.
> Of course one could rerwite the above as
>
> Node *x(new Node); /* constructor Node() is called */
> Node *y(new Node(x)); /* constructor Node(Node*) is called */
> Node z; /* constructor Node() is called */
> Node w(&z); /* constructor Node(Node*) is called */
> Node *tmp(&w);
>
> which uses () instead of = for the initializations.
for one thing I think he is saying that
Node *x{new Node}; works
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From: Ike Naar
Newsgroups: alt.comp.lang.learn.c-c++
Subject: Re: when is the constructor called for an object
Date: Mon, 21 Nov 2016 10:09:51 -0000 (UTC)
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Xref: panix alt.comp.lang.learn.c-c++:246553
On 2016-11-21, ruben safir wrote:
> On 11/21/2016 04:14 AM, Ike Naar wrote:
>> Not sure what you mean.
>> Of course one could rerwite the above as
>>
>> Node *x(new Node); /* constructor Node() is called */
>> Node *y(new Node(x)); /* constructor Node(Node*) is called */
>> Node z; /* constructor Node() is called */
>> Node w(&z); /* constructor Node(Node*) is called */
>> Node *tmp(&w);
>>
>> which uses () instead of = for the initializations.
>
> for one thing I think he is saying that
>
> Node *x{new Node}; works
Yes, that works, too.
One could rewrite the above as:
Node *x{new Node}; /* constructor Node() is called */
Node *y{new Node{x}}; /* constructor Node(Node*) is called */
Node z; /* constructor Node() is called */
Node w{&z}; /* constructor Node(Node*) is called */
Node *tmp{&w};
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