473,320 Members | 1,920 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.

Using const qualifier

Hi,
My question is about how to use const properly.
I have two examples describing my problem.

First, let's say I have a linked list and from it I want to
find some special node. I write the function, and then figure
that the function will not be modifying the list at all, so a
const qualifier seems appropriate in the parameter.
So essentially I have:

struct node {
...
struct node *next;
};

struct node *find_special(const struct node *n)
{
while (...) {
...
n = n->next;
....
}
return n;
}

But this will give me a warning since I am returning n,
which is declared const, and my function is not returning
a const. How do I fix this? If I change my function to
return a const, I would be stuck with a variable which I can
not change when I use my function, right?

Also, let's say I have a function that assigns a pointer to a
member of a struct. Here I also think a const would do:

void assign_pointer(struct mystruct *s, const int *p)
{
s->pointer = p;
}

struct mystruct {
int *pointer;
};

Same problem here, I'm assigning a const to a non const.
What to do? I can't add const to my struct member, since
I want to do things with it later.

So how do I use const properly? I do understand the reason
why these examples fail, just not how to handle it.

Thanks!
Dec 30 '07 #1
5 1953
amvoi...@hushmail.com wrote:
Hi,
My question is about how to use const properly.
I have two examples describing my problem.

First, let's say I have a linked list and from it I want to
find some special node. I write the function, and then figure
that the function will not be modifying the list at all, so a
const qualifier seems appropriate in the parameter.
So essentially I have:

struct node {
* * ...
* * struct node *next;
};

struct node *find_special(const struct node *n)
{
* * while (...) {
* * * * ...
* * * * n = n->next;
* * * * ....
* * }
* * return n;
}

But this will give me a warning since I am returning n,
which is declared const, and my function is not returning
a const. How do I fix this?
Cast the return value to non-const.
If I change my function to
return a const, I would be stuck with a variable which I can
not change when I use my function, right?
Yes.
Also, let's say I have a function that assigns a pointer to a
member of a struct. Here I also think a const would do:

void assign_pointer(struct mystruct *s, const int *p)
This would only be a valid design if 's->pointer' is also const.
If it isn't (and isn't meant to be) remove the const
qualifier against *p.
{
* * s->pointer = p;
}

struct mystruct {
* * int *pointer;
};
--
Peter
Dec 30 '07 #2
On Dec 30, 2:11 am, Peter Nilsson <ai...@acay.com.auwrote:
amvoi...@hushmail.com wrote:
Hi,
My question is about how to useconstproperly.
I have two examples describing my problem.
First, let's say I have a linked list and from it I want to
find some special node. I write the function, and then figure
that the function will not be modifying the list at all, so a
constqualifierseems appropriate in the parameter.
So essentially I have:
struct node {
...
struct node *next;
};
struct node *find_special(conststruct node *n)
{
while (...) {
...
n = n->next;
....
}
return n;
}
But this will give me a warning since I am returning n,
which is declaredconst, and my function is not returning
aconst. How do I fix this?

Cast the return value to non-const.
I knew that was one possible way to silence the warning, but
having read this groups for some time, I had the impression that
casts are almost never necessary. Is this a case where a cast
is perfectly okay?
If I change my function to
return aconst, I would be stuck with a variable which I can
not change when I use my function, right?

Yes.
Also, let's say I have a function that assigns a pointer to a
member of a struct. Here I also think aconstwould do:
void assign_pointer(struct mystruct *s,constint *p)

This would only be a valid design if 's->pointer' is alsoconst.
If it isn't (and isn't meant to be) remove theconstqualifieragainst *p.
What makes the first example a valid design, but not this one if 's-
>pointer'
is not const? I mean, in respective function the pointers are not
altered
at all, but after the function ends they can be altered. I don't see
the
difference between them. Does a function need to take into account
that a
pointer may be changed after returning from a function?
{
s->pointer = p;
}
struct mystruct {
int *pointer;
};

--
Peter
Thanks!
Dec 30 '07 #3
On 30 Dec, 00:42, amvoi...@hushmail.com wrote:
Hi,
My question is about how to use const properly.
I have two examples describing my problem.
What Peter and Eric have said elsethread is correct, but I think your
conception may be wrong.
First, let's say I have a linked list and from it I want to
find some special node. I write the function, and then figure
that the function will not be modifying the list at all, so a
const qualifier seems appropriate in the parameter.
So essentially I have:

struct node {
* * ...
* * struct node *next;

};

struct node *find_special(const struct node *n)
{
* * while (...) {
* * * * ...
* * * * n = n->next;
* * * * ....
* * }
* * return n;

}
This function returns a non-const pointer to a node of the list. The
function itself does not modify the list. But it returns a pointer
through which you could modify the list. So your function, having
promised not to modify the list, can't then guarantee that the promise
is kept. This is why you are having problems.
Also, let's say I have a function that assigns a pointer to a
member of a struct. Here I also think a const would do:

void assign_pointer(struct mystruct *s, const int *p)
{
* * s->pointer = p;

}

struct mystruct {
* * int *pointer;

};
Similar problem here. Your function promises that it won't alter the
int that p points to. But it lets "pointer" point to it, and "pointer"
is free to alter the value pointed at.

I think you need to ask yourself the question "Do I want the pointed-
at thing to be alterable, or not?" and arrange your consts
accordingly.

Hope that helps.
Paul.
Dec 30 '07 #4
In article <73**********************************@e25g2000prg. googlegroups.com>
<gw****@aol.comwrote:
>[The node-finding] function returns a non-const pointer to a
node of the list. The function itself does not modify the list.
But it returns a pointer through which you could modify the list.
So your function, having promised not to modify the list, can't
then guarantee that the promise is kept. This is why you are
having problems.
Indeed: given a function that does not modify the object or objects
to which its argument pointer(s) point, but *does* return a pointer
to one of those objects, you have a problem.

Do you use "const" to imply "I will not change this, and thus you
can pass to me a pointer to an actually-const, really-in-ROM object",
and then have the return value "de-const-ified"? In which case,
you can do bad things:

typedef <whateverT;
T *deconstify(const T *);
const T read_only_obj = { value_that_cannot_change };
*deconstify(&read_only_obj) = oops_a_bug;

Or, do you avoid const, after which you cannot even *call* the
function even though you would have done it safely? For instance:

T *nonconst(T *); /* nonconst() does not modify it though */
const T *ptr;
ptr = nonconst(&read_only_obj); /* alas, this gets a diagnostic */

The Standard C Library takes the former approach: functions like
strchr() "deconstify" their argument:

const char ro[] = "room!";

*strchr(ro, 'r') = 'b'; /* boom! */

(which may indeed "go boom", i.e., crash, and does on some of my
systems).

One of the many reasons C++ has overloaded functions is to sidestep
this problem. In C++, the types of arguments -- including whether
they are const-qualified -- determine which function is actually
called, so we simply write two completely separate functions:

const int *operate(const int *);
int *operate(int *);

Then:

const int obj1 = 0
int obj2 = 0;

*operate(&obj1) = 42;

will not compile, because &obj1 has type "const int *", so this
calls operate(const int *), which returns "const int *", not
"int *". Of course, this opens the door to abuse: the two
separate functions might well do entirely different things.
Ideally, the const-qualified version of the function does the
same thing as the non-const-qualified version, and in a language
better than C++, we could avoid writing two functions by simply
declaring that the return type matches the argument type:

flexible operate(arg) [
constraint: typeof(operate) is typeof(arg);
constraint: typeof(arg) is choice { int *, const int * };
] {
...
return arg;
}

which also of course lets us write "overloaded" functions without
repeating them six ways from Sunday:

/*
* Substring operation: find "needle" in "haystack"
*/
flexible substring(haystack, needle) [
constraint: typeof(operate) is typeof(haystack);
constraint: typeof(needle) is typeof(haystack);
constraint: typeof(haystack) is choice {
char *, const char *,
Unicode16 *, const Unicode16 *,
Unicode32 *, const Unicode32 *
};
] {
... code to find substring ...
}

(To achieve the above, C++ generally uses "templates" instead of,
or combined with, "overloaded" functions, but there is really
no need for both.)

(I made up the above syntax on the fly, so it is probably full
of flaws.)
--
In-Real-Life: Chris Torek, Wind River Systems
Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603
email: forget about it http://web.torek.net/torek/index.html
Reading email is like searching for food in the garbage, thanks to spammers.
Dec 30 '07 #5
Chris Torek wrote:

[snip]
Ideally, the const-qualified version of the function does the
same thing as the non-const-qualified version, and in a language
better than C++, we could avoid writing two functions by simply
declaring that the return type matches the argument type:
[snip]
which also of course lets us write "overloaded" functions without
repeating them six ways from Sunday:

/*
* Substring operation: find "needle" in "haystack"
*/
flexible substring(haystack, needle) [
constraint: typeof(operate) is typeof(haystack);
constraint: typeof(needle) is typeof(haystack);
Why? What's wrong with allowing substring(L"unicode string", "string")?
constraint: typeof(haystack) is choice {
char *, const char *,
Unicode16 *, const Unicode16 *,
Unicode32 *, const Unicode32 *
};
] {
... code to find substring ...
}
--
Army1987 (Replace "NOSPAM" with "email")
Jan 2 '08 #6

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

Similar topics

2
by: joe | last post by:
hi, after reading some articles and faq, i want to clarify myself what's correct(conform to standard) and what's not? or what should be correct but it isn't simply because compilers don't...
13
by: matthias_k | last post by:
Hi, I've never thought about this before, but since you are able to overload a function only by changing the const-ness of the formal parameters, some annoying side effects will arise. For...
11
by: x-pander | last post by:
given the code: <file: c.c> typedef int quad_t; void w0(int *r, const quad_t *p) { *r = (*p); }
9
by: sarathy | last post by:
Hi, Can anyone just help me with what exactly is constant expression. I read the section on Constant expression in K&R2. i am not fully clear with it.
10
by: d3x0xr | last post by:
---- Section 1 ---- ------ x.c int main( void ) { char **a; char const *const *b; b = a; // line(9)
0
by: d3x0xr | last post by:
Heh, spelled out in black and white even :) Const is useles... do NOT follow the path of considering any data consatant, because in time, you will have references to it that C does not handle,...
4
by: junky_fellow | last post by:
Hi guys, Consider the following statements. const int *ptr; /* ie the contents pointed to by ptr cannot be changed */ My question is how/who prevents the contents from being modified ?...
4
by: =?utf-8?b?QXNiasO4cm4gU8OmYsO4?= | last post by:
Suppose I have a function that takes a pointer as its input, but does not change what the pointer points to. In that case, the const qualifier can be used to indicate that the variable pointed to...
1
by: mcbhagyaraj | last post by:
sir, i am getting doubt on const qualifier . Que: void main() { const int f(int ); int b = 4; int a = f(&b); printf("%d", a); // o/p i...
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...
0
by: ryjfgjl | last post by:
ExcelToDatabase: batch import excel into database automatically...
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: jfyes | last post by:
As a hardware engineer, after seeing that CEIWEI recently released a new tool for Modbus RTU Over TCP/UDP filtering and monitoring, I actively went to its official website to take a look. It turned...
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...
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: Defcon1945 | last post by:
I'm trying to learn Python using Pycharm but import shutil doesn't work
1
by: Shællîpôpï 09 | last post by:
If u are using a keypad phone, how do u turn on JavaScript, to access features like WhatsApp, Facebook, Instagram....
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...

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.