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Elementary question on const

P: n/a
I encountered the following code in Bruce Eckel's online book. Can you
please clarify what "const char* const data;" means? Thanks
//: C01:MyError.cpp {RunByHand}

class MyError {
const char* const data;
public:
MyError(const char* const msg = 0) : data(msg) {}
};

void f() {
// Here we "throw" an exception object:
throw MyError("something bad happened");
}

int main() {
// As you'll see shortly, we'll want a "try block" here:
f();
} ///:~

Jul 23 '05 #1
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P: n/a

"Last Timer" <da********@yahoo.com> wrote in message
news:11*********************@c13g2000cwb.googlegro ups.com...
I encountered the following code in Bruce Eckel's online book. Can you
please clarify what "const char* const data;" means?
Doesn't the book explain it?
//: C01:MyError.cpp {RunByHand}

class MyError {
const char* const data;


This means that the data member 'data' is a const
pointer to a const type 'char' object. I.e. neither
the pointer nor what it points to may be modified.
(If you try, your compiler should complain).

-Mike
Jul 23 '05 #2

P: n/a
On 30 Jan 2005 11:31:14 -0800 in comp.lang.c++, "Last Timer"
<da********@yahoo.com> wrote,
please clarify what "const char* const data;" means? Thanks


The first "const" means the characters pointed to cannot be changed
via this pointer. The second one means the pointer value cannot be
changed.

Jul 23 '05 #3

P: n/a
Last Timer wrote:
I encountered the following code in Bruce Eckel's online book. Can you
please clarify what "const char* const data;" means? Thanks


Pointers have two attributes that can be constant:
the content of the pointer and the object {date} it
points to.

Many people say that reading from right to left
is easier.
const char * const data;
^ ^-- The pointer is constant.
|-- The data is constant.

When the pointer is constant, it cannot point
to other objects. When the data is constant,
the data cannot be modified by dereferencing
the pointer.

--
Thomas Matthews

C++ newsgroup welcome message:
http://www.slack.net/~shiva/welcome.txt
C++ Faq: http://www.parashift.com/c++-faq-lite
C Faq: http://www.eskimo.com/~scs/c-faq/top.html
alt.comp.lang.learn.c-c++ faq:
http://www.comeaucomputing.com/learn/faq/
Other sites:
http://www.josuttis.com -- C++ STL Library book
http://www.sgi.com/tech/stl -- Standard Template Library

Jul 23 '05 #4

P: n/a
Thanks for your kind replies. Do you read it Left to Right or vice
versa to determine "first" and "second"?

The C++ for Dummies saz on pg 63

const char * pcc="This is a constant string";
char * const cpc="tjhis is also a string";
*pcc='a'; //illegal
*cpc ='b';//legal
pcc="another string" ; //legal
cpc="another string"; illegal

I'm trying to develop a mnemonic to remember these. Instead of rote
learning may be someone can help me figure this out logically.
Thanks again.

Jul 23 '05 #5

P: n/a
"Last Timer" <da********@yahoo.com> wrote in message
news:11**********************@f14g2000cwb.googlegr oups.com...
Thanks for your kind replies. Do you read it Left to Right or vice
versa to determine "first" and "second"?

The C++ for Dummies saz on pg 63

const char * pcc="This is a constant string";
char * const cpc="tjhis is also a string";
*pcc='a'; //illegal
*cpc ='b';//legal
pcc="another string" ; //legal
cpc="another string"; illegal

I'm trying to develop a mnemonic to remember these. Instead of rote
learning may be someone can help me figure this out logically.


http://www.ericgiguere.com/articles/...larations.html
Yes, it's about C, but applies to C++ as well. (But of course it
won't cover the 'C++-only' stuff such as class members, etc.
I don't know of a web link for that, but this one should give you
a good start.)

Also, google for a utility called 'cdecl', which can take
a C declaration and translate it to English. It's available
for most platforms, and is supplied with some.

-Mike
Jul 23 '05 #6

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