473,320 Members | 2,107 Online
Bytes | Software Development & Data Engineering Community
Post Job

Home Posts Topics Members FAQ

Join Bytes to post your question to a community of 473,320 software developers and data experts.

Nonstatic member example?

fl
Hi,
There is a question about nonstatic member. C++ primer says: A
nonstatic member is restricted to being declared as a pointer or
reference to an object of its class. It only gives an example of
pointer *b.

class Bar {
public:

private:
static Bar a; // OK
Bar *b; // OK
Bar c; // error

My question is how a nonstatic member is declared as a reference to an
object of its class. Because a reference is legal only after the
original variable has been declared, where is the original object? I
feel it is really bizarre. Could you give me an example? Thanks in
advance.


private variable
Dec 31 '07 #1
6 2186
fl wrote:
Hi,
There is a question about nonstatic member. C++ primer says: A
nonstatic member is restricted to being declared as a pointer or
reference to an object of its class. It only gives an example of
pointer *b.

class Bar {
public:

private:
static Bar a; // OK
Bar *b; // OK
Bar c; // error

My question is how a nonstatic member is declared as a reference to an
object of its class. Because a reference is legal only after the
original variable has been declared, where is the original object? I
feel it is really bizarre. Could you give me an example? Thanks in
advance.
Passing it as a parameter to the constructor is one way. this (the instance
pointer) is another. In fact, class member references have to be
initialized in the constructor initialization list (I know of no other way)
and passing as a paramter would be the usuall way. Something like (untested
code)

class Bar {
public:
Bar( Bar& foo ): d( foo ) {}
private:
Bar& d;
};
--
Jim Langston
ta*******@rocketmail.com
Dec 31 '07 #2
On Dec 30, 11:24 pm, fl <rxjw...@gmail.comwrote:
Hi,
There is a question about nonstatic member. C++ primer says: A
nonstatic member is restricted to being declared as a pointer or
reference to an object of its class. It only gives an example of
pointer *b.

class Bar {
public:

private:
static Bar a; // OK
Bar *b; // OK
Bar c; // error

My question is how a nonstatic member is declared as a reference to an
object of its class. Because a reference is legal only after the
original variable has been declared, where is the original object? I
feel it is really bizarre. Could you give me an example? Thanks in
advance.

private variable
the original object, in a special case like this one, would have to
refer to itself, which would then basicly mean that such a class could
not have a static member since the static member has no 'this'.
Its a special case, don't dissmiss references. They solve many, many
problems.

[10.7] Should you use the this pointer in the constructor?
http://www.parashift.com/c++-faq-lit....html#faq-10.7

#include <iostream>

class A
{
const A& r_a;
public:
A() : r_a(*this) { }
A(const A& copy) : r_a(copy) { }
A& operator=(const A& rhv); // disabled
A const& get_r() const { return r_a; }
};

void foo(const A& r)
{
std::cout << "&r = " << &r;
std::cout << "\tr.r_a = " << &r.get_r();
std::cout << std::endl;
}

int main()
{
A a;
foo(a);
A another = a; // is NOT an assignment
foo(another);
}

/*
&r = 0x7fff0f2e1930 r.r_a = 0x7fff0f2e1930
&r = 0x7fff0f2e1920 r.r_a = 0x7fff0f2e1930
*/

Dec 31 '07 #3
fl
On 31 déc, 04:27, Salt_Peter <pj_h...@yahoo.comwrote:
On Dec 30, 11:24 pm, fl <rxjw...@gmail.comwrote:


Hi,
There is a question about nonstatic member. C++ primer says: A
nonstatic member is restricted to being declared as a pointer or
reference to an object of its class. It only gives an example of
pointer *b.
class Bar {
public:
private:
static Bar a; * * *// OK
Bar *b; * * * * * * *// OK
Bar c; * * * * * * * // error
My question is how a nonstatic member is declared as a reference to an
object of its class. Because a reference is legal only after the
original variable has been declared, where is the original object? I
feel it is really bizarre. Could you give me an example? Thanks in
advance.
private variable

the original object, in a special case like this one, would have to
refer to itself, which would then basicly mean that such a class could
not have a static member since the static member has no 'this'.
Its a special case, don't dissmiss references. They solve many, many
problems.

[10.7] Should you use the this pointer in the constructor?http://www.parashift.com/c++-faq-lit....html#faq-10.7

#include <iostream>

class A
{
* const A& r_a;
public:
* A() : r_a(*this) { }
* A(const A& copy) : r_a(copy) { }
* A& operator=(const A& rhv); // disabled
* A const& get_r() const { return r_a; }

};

void foo(const A& r)
{
* std::cout << "&r = " << &r;
* std::cout << "\tr.r_a = " << &r.get_r();
* std::cout << std::endl;

}

int main()
{
* A a;
* foo(a);
* A another = a; // is NOT an assignment
* foo(another);

}

/*
&r = 0x7fff0f2e1930 * * r.r_a = 0x7fff0f2e1930
&r = 0x7fff0f2e1920 * * r.r_a = 0x7fff0f2e1930
*/- Masquer le texte des messages précédents -

- Afficher le texte des messages précédents -
Hi,
I find the modified code,see below, has the same output as yours.
Why the overload:
A another = a; // is NOT an assignment
does not take effect? Thank you very much.

---------------------
#include <iostream>
class A
{
const A& r_a;
public:
A() : r_a(*this) { }
// A(const A& copy) : r_a(copy) { }
// A& operator=(const A& rhv); // disabled
A const& get_r() const { return r_a; }

};
void foo(const A& r)
{
std::cout << "&r = " << &r;
std::cout << "\tr.r_a = " << &r.get_r();
std::cout << std::endl;
}
int main()
{
A a;
foo(a);
A another; // = a; // is NOT an assignment
foo(another);
}
Dec 31 '07 #4
fl
On 31 déc, 04:27, Salt_Peter <pj_h...@yahoo.comwrote:
On Dec 30, 11:24 pm, fl <rxjw...@gmail.comwrote:


Hi,
There is a question about nonstatic member. C++ primer says: A
nonstatic member is restricted to being declared as a pointer or
reference to an object of its class. It only gives an example of
pointer *b.
class Bar {
public:
private:
static Bar a; * * *// OK
Bar *b; * * * * * * *// OK
Bar c; * * * * * * * // error
My question is how a nonstatic member is declared as a reference to an
object of its class. Because a reference is legal only after the
original variable has been declared, where is the original object? I
feel it is really bizarre. Could you give me an example? Thanks in
advance.
private variable

the original object, in a special case like this one, would have to
refer to itself, which would then basicly mean that such a class could
not have a static member since the static member has no 'this'.
Its a special case, don't dissmiss references. They solve many, many
problems.

[10.7] Should you use the this pointer in the constructor?http://www.parashift.com/c++-faq-lit....html#faq-10.7

#include <iostream>

class A
{
* const A& r_a;
public:
* A() : r_a(*this) { }
* A(const A& copy) : r_a(copy) { }
* A& operator=(const A& rhv); // disabled
* A const& get_r() const { return r_a; }

};

void foo(const A& r)
{
* std::cout << "&r = " << &r;
* std::cout << "\tr.r_a = " << &r.get_r();
* std::cout << std::endl;

}

int main()
{
* A a;
* foo(a);
* A another = a; // is NOT an assignment
* foo(another);

}

/*
&r = 0x7fff0f2e1930 * * r.r_a = 0x7fff0f2e1930
&r = 0x7fff0f2e1920 * * r.r_a = 0x7fff0f2e1930
*/- Masquer le texte des messages précédents -

- Afficher le texte des messages précédents -
Sorry, my previous response is not right, i.e. Your's

A another = a; // is NOT an assignment

is right.
My question now is: why there is no effect when I comment out:
// A(const A& copy) : r_a(copy) { }
// A& operator=(const A& rhv); // disabled

Maybe the system does the copy in a special initializer fashion?
I know now the object of "A" class is an address, which points to
itself. That is from the following two lines. Right?
const A& r_a;
public:
A() : r_a(*this) { }

---------------
Then, what's the meaning of "another" after "another=a"?
I am not even clear:

A(const A& copy) : r_a(copy) { }
Thanks.
Dec 31 '07 #5
fl <rx*****@gmail.comwrote:
There is a question about nonstatic member. C++ primer says: A
nonstatic member is restricted to being declared as a pointer or
reference to an object of its class. It only gives an example of
pointer *b.

class Bar {
public:

private:
static Bar a; // OK
Bar *b; // OK
Bar c; // error

My question is how a nonstatic member is declared as a reference to an
object of its class. Because a reference is legal only after the
original variable has been declared, where is the original object? I
feel it is really bizarre. Could you give me an example? Thanks in
advance.
I have yet to see why one should have a non-static reference in a class
in any case, no matter what it refers to.
Dec 31 '07 #6
On Dec 31, 10:35 am, fl <rxjw...@gmail.comwrote:
On 31 déc, 04:27, Salt_Peter <pj_h...@yahoo.comwrote:
On Dec 30, 11:24 pm, fl <rxjw...@gmail.comwrote:
Hi,
There is a question about nonstatic member. C++ primer says: A
nonstatic member is restricted to being declared as a pointer or
reference to an object of its class. It only gives an example of
pointer *b.
class Bar {
public:
private:
static Bar a; // OK
Bar *b; // OK
Bar c; // error
My question is how a nonstatic member is declared as a reference to an
object of its class. Because a reference is legal only after the
original variable has been declared, where is the original object? I
feel it is really bizarre. Could you give me an example? Thanks in
advance.
private variable
the original object, in a special case like this one, would have to
refer to itself, which would then basicly mean that such a class could
not have a static member since the static member has no 'this'.
Its a special case, don't dissmiss references. They solve many, many
problems.
[10.7] Should you use the this pointer in the constructor?http://www.parashift.com/c++-faq-lit....html#faq-10.7
#include <iostream>
class A
{
const A& r_a;
public:
A() : r_a(*this) { }
A(const A& copy) : r_a(copy) { }
A& operator=(const A& rhv); // disabled
A const& get_r() const { return r_a; }
};
void foo(const A& r)
{
std::cout << "&r = " << &r;
std::cout << "\tr.r_a = " << &r.get_r();
std::cout << std::endl;
}
int main()
{
A a;
foo(a);
A another = a; // is NOT an assignment
foo(another);
}
/*
&r = 0x7fff0f2e1930 r.r_a = 0x7fff0f2e1930
&r = 0x7fff0f2e1920 r.r_a = 0x7fff0f2e1930
*/- Masquer le texte des messages précédents -
- Afficher le texte des messages précédents -

Sorry, my previous response is not right, i.e. Your's

A another = a; // is NOT an assignment

is right.
My question now is: why there is no effect when I comment out:
// A(const A& copy) : r_a(copy) { }
// A& operator=(const A& rhv); // disabled
the compiler generates the copy ctor if you don't. it probably does
the exact same as the one commented out.
I prefer declaring it in cases i want to diagnose problems and assert
theories.

A(const A& copy) : r_a(copy) { std::cout << "copy A"; }

is nice to have when observing and troubleshooting.

>
Maybe the system does the copy in a special initializer fashion?
not at all. Its a member-wise copy, nothing special.

Its the same with something like
struct K
{
int n;
double d;
char c;
};

You can initialize an instance of K and you can copy it because the
compiler generates a copy ctor for you. It also generates a defult
ctor and an assignment operator (if needed).
I know now the object of "A" class is an address, which points to
itself. That is from the following two lines. Right?
const A& r_a;
public:
A() : r_a(*this) { }

---------------
Then, what's the meaning of "another" after "another=a"?
I am not even clear:

A(const A& copy) : r_a(copy) { }

Thanks.
instance 'another' is a psuedo-copy of a. The difference is that
another's member reference doesn't refer to another.
This isn't something you need not worry about, you'll not find such a
strategy in code out there.
You do need to understand ctor, copy ctor and init list.
Dec 31 '07 #7

This thread has been closed and replies have been disabled. Please start a new discussion.

Similar topics

8
by: Jinesh | last post by:
I illustrate the compiler error I get using the following example. --------------------------------------------------------------- Class ClassName { private: static const int constVarName = 100;...
3
by: Clay_Culver | last post by:
I have this code: typedef BraidedNode* (BraidedNode::*NodeGet)() const; NodeGet test() { return BraidedNode::getNextID; } This code compiles under MSVC 7.1, but g++ (GCC 3.4.4) will not...
7
by: Chris Clement | last post by:
I have been handed a project that someone else started and most of it was developed in the VS.NET design mode. For whatever reasons, when I try to make changes to the controls in VS.NET design...
7
by: The|Godfather | last post by:
Hi everybody, I read Scotte Meyer's "Effective C++" book twice and I know that he mentioned something specific about constructors and destructors that was related to the following...
10
by: Muffin | last post by:
I am a little new to C# and an have a hard time understanding why I get a nonstatic error. I create an object in my main form that has member properties by using a control. From another form/dialog...
10
by: Jeffrey | last post by:
My understanding is that if you write class X { int y; static int z; }; then you've defined (and declared) X and y, but you have only declared (and not defined) z. If you'd like to...
4
by: nielsp | last post by:
Hi! Why does'nt the following work? #include <iostream> class X { public:
0
by: DolphinDB | last post by:
Tired of spending countless mintues downsampling your data? Look no further! In this article, you’ll learn how to efficiently downsample 6.48 billion high-frequency records to 61 million...
1
isladogs
by: isladogs | last post by:
The next Access Europe meeting will be on Wednesday 6 Mar 2024 starting at 18:00 UK time (6PM UTC) and finishing at about 19:15 (7.15PM). In this month's session, we are pleased to welcome back...
0
by: Vimpel783 | last post by:
Hello! Guys, I found this code on the Internet, but I need to modify it a little. It works well, the problem is this: Data is sent from only one cell, in this case B5, but it is necessary that data...
0
by: ArrayDB | last post by:
The error message I've encountered is; ERROR:root:Error generating model response: exception: access violation writing 0x0000000000005140, which seems to be indicative of an access violation...
1
by: PapaRatzi | last post by:
Hello, I am teaching myself MS Access forms design and Visual Basic. I've created a table to capture a list of Top 30 singles and forms to capture new entries. The final step is a form (unbound)...
0
by: CloudSolutions | last post by:
Introduction: For many beginners and individual users, requiring a credit card and email registration may pose a barrier when starting to use cloud servers. However, some cloud server providers now...
0
by: af34tf | last post by:
Hi Guys, I have a domain whose name is BytesLimited.com, and I want to sell it. Does anyone know about platforms that allow me to list my domain in auction for free. Thank you
0
by: Faith0G | last post by:
I am starting a new it consulting business and it's been a while since I setup a new website. Is wordpress still the best web based software for hosting a 5 page website? The webpages will be...
0
isladogs
by: isladogs | last post by:
The next Access Europe User Group meeting will be on Wednesday 3 Apr 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM). In this session, we are pleased to welcome former...

By using Bytes.com and it's services, you agree to our Privacy Policy and Terms of Use.

To disable or enable advertisements and analytics tracking please visit the manage ads & tracking page.