Linux Users' Group of Davis (LUGOD): Mailing List Archives - Re: [vox] things that really suck about C!

"People new to C/C++ are sometimes confused about the difference between a const pointer and a pointer to const. A const pointer essentially means you can’t change the pointer variable itself, but you can change the value it points to. A pointer to const means you can change the pointer but not what it points to. You can use them both together and have a const pointer to a const. The code snippet below should make it really clear I hope."

//pointer to a const
void f1()
{
int i = 100;
const int* pi = &i;
//*pi = 200; <- won't compile
pi++;
}

//const pointer
void f2()
{
int i = 100;
int* const pi = &i;
*pi = 200;
//pi++; <- won't compile
}

//const pointer to a const
void f3()
{
int i = 100;
const int* const pi = &i;
//*pi = 200; <- won't compile
//pi++; <- won't compile
}

On Sun, Feb 28, 2010 at 10:49 PM, Carl Boettiger <cboettig@gmail.com> wrote:

On Sun, Feb 28, 2010 at 6:35 PM, Brian Lavender <brian@brie.com> wrote:
I think if anything, C has been a certain detriment to the field of
computer science!

One calls a function and the arguments are passed by value. Call a
function with an array as an argument, and feel free to modify its
contents!

so declaring an array as const prevents this, func(const double * a). I understand that this also helps the compiler make optimizations it cannot do when you don't use const. I think you could still modify the contents of the array by first copying the pointer though,

double * b = a;
b[i] = something new.

So there's also the modifier "restrict", which I believe would prevent this, and again helps out the compiler do smart things. Others can probably confirm/correct this? Is it good practice to use these modifiers as often as possible/appropriate?

Certainly, C++ added the idea of reference, but I think Pascal
simplifies these concepts much better. Yet, Pascal seems to be relegated
to the status as a legacy language!

brian

#include <stdio.h>

#define CAP 10

void mod_array(int a[])
{
a[2] = 5;
}

void trychange(int a)
{
a = 2;
}

void reallychange(int *a)
{
*a = 2;
}

int main() {
int b[CAP];
int c;
int i;

printf("Load array and change a value\n");
for (i=0; i < CAP; i++)
b[i] = i + 20;

mod_array(b);

for (i=0; i < CAP; i++)
printf("b[%d] has value of %d\n",i,b[i]);

c = 10;

printf("c has a value of %d\n",c);
trychange(c);

printf("c has a value of %d after trychange(c)\n",c);

reallychange(&c);

printf("c has a value of %d after reallychange(&c)\n",c);

}

--
Brian Lavender
http://www.brie.com/brian/

"There are two ways of constructing a software design. One way is to
make it so simple that there are obviously no deficiencies. And the other
way is to make it so complicated that there are no obvious deficiencies."

Professor C. A. R. Hoare
The 1980 Turing award lecture
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--
Carl Boettiger
Population Biology, UC Davis
http://two.ucdavis.edu/~cboettig

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