Solved

a reference to an object ? (C++)

Posted on 2003-10-26
7
276 Views
Last Modified: 2010-04-17
To C++ expert :

   Is it legal to have a reference to an object ? The following code won't compile, and I don't understand
why:
---------------------------
class A{
  int i ;
 public:
   A(int x) : i(x){}
} ;

int main()
{
  A &a_ref = new A(1);
  return 0 ;
}
--------------------------------
 it works when I change "A &a_ref" to
 "const A &a_ref". But I don't understand why can't
  we have a reference to an object .... ?
--------------------------------
similarly,

string a("meow") ;
char *c_ptr = a.c_str() ;

doesn't compile either ...... I need to change
"char *c_ptr" to "const *c_ptr" as well ? why is that ??
-----------------
 Thanks a lot !!!
0
Comment
Question by:meow00
  • 3
  • 2
  • 2
7 Comments
 
LVL 1

Assisted Solution

by:marcin1
marcin1 earned 110 total points
ID: 9624503
First example does not work because new operator returns a pointer
to the object, so it should look like this:

A *a_poiner = new A(1);

Now you can access public methods and fields using ->

a_point->i = 5;
(if "i" is public of course)

Or you can dereference the pointer returned by new, using the star

A &a_ref = *(new A(1));

Dereferencing means getting the underlying object from the pointer.

I have no idea why it works when you change "A &a_ref" to
"const A &a_ref", it doesn't work on Borland C++ Builder 6

As for the second part, I don't know which library you use, but
I think the c_str() method of string class returns "const * char",
which is not the same as "char *".

"c_str()" is desinged for reading purposes only, that is why it returns
"const char*" - to prevent from changing the string, what could be
catastrophic in the "string" object.

0
 
LVL 1

Expert Comment

by:marcin1
ID: 9624506
I meant :

A *a_point = new A(1);

Sorry
0
 
LVL 1

Author Comment

by:meow00
ID: 9624645
Hi,
   I know this is a silly question ... but I really don't understand :
 if "new" operator returns a pointer to an object, why we can
 do :
   A a_obj = new A ;
 does the assignment operator " = " implicit do something there ?
 
 Thanks a lot !!!

meow.....
 
0
IT, Stop Being Called Into Every Meeting

Highfive is so simple that setting up every meeting room takes just minutes and every employee will be able to start or join a call from any room with ease. Never be called into a meeting just to get it started again. This is how video conferencing should work!

 
LVL 1

Assisted Solution

by:jcwlc
jcwlc earned 40 total points
ID: 9624763
References were created to make life easier for people who were confused by pointers!

It allows you to use . (DOT) notation instead of -> (POINTER) notation. However, unlike a pointer in C++, you cannot change it's value. Hence, the compiler forces you declare it as constant. You cannot have a reference to a reference, like you can have a pointer to a pointer...

I suggest you stick to one or the other when learning C++... Generally I only use references when passing/returning pointers to objects...

eg:

//returns a copy string object...
CString & func(CString &s)
{
   CString x = new CString(s);
   return x;
}


0
 
LVL 1

Author Comment

by:meow00
ID: 9624822
Sorry .....
but
---------------------------
 string& func(string &s)
{
   string x = new string(s);
   return x;
}
-----------------------
does not compile for me ......
I got error message :
--------------------------
conversion from `string *' to non-scalar type `basic_string<char,strin
g_char_traits<char>,__default_alloc_template<false,0> >' requested
test.C:6: warning: reference to local variable `x' returned
-----------------------------
what does it mean .......
0
 
LVL 1

Expert Comment

by:jcwlc
ID: 9625008
Sorry,

string(s) should be CString(s)

regards,
0
 
LVL 1

Accepted Solution

by:
jcwlc earned 40 total points
ID: 9625317

const CString& func(CString &s)
{
    static CString x;

    x = s;

   return x;
}

CString y("hhh");
CString x = func(y);
0

Featured Post

Better Security Awareness With Threat Intelligence

See how one of the leading financial services organizations uses Recorded Future as part of a holistic threat intelligence program to promote security awareness and proactively and efficiently identify threats.

Join & Write a Comment

Suggested Solutions

Title # Comments Views Activity
for loop with Set 4 46
count8 challlenge 13 87
countX 22 70
C# code editing and collaboration 3 49
This is an explanation of a simple data model to help parse a JSON feed
Whether you've completed a degree in computer sciences or you're a self-taught programmer, writing your first lines of code in the real world is always a challenge. Here are some of the most common pitfalls for new programmers.
In this fifth video of the Xpdf series, we discuss and demonstrate the PDFdetach utility, which is able to list and, more importantly, extract attachments that are embedded in PDF files. It does this via a command line interface, making it suitable …
In this seventh video of the Xpdf series, we discuss and demonstrate the PDFfonts utility, which lists all the fonts used in a PDF file. It does this via a command line interface, making it suitable for use in programs, scripts, batch files — any pl…

743 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question

Need Help in Real-Time?

Connect with top rated Experts

13 Experts available now in Live!

Get 1:1 Help Now