array of char , array of integers and pointers to constant

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2.50/5 (2 votes)

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I have two questions

Question Number 1

Part1.c
int *p;
int a[]={1};
p=a;
printf("%d",p);

part2.c
char *q;
char b[]="welcome";
q=b;
printf("%s",b);

in part1.c it prints the address of variable where in part2.c it prints the values that is string? Only the return type is different how?

Question No 2

C#

int a=10;
int const *p=&a;
a=20;
p=&a;
printf("%d",*p);

Above program is pointer to constant . Here i have changed the value of a but it is not showing errors why? Does pointer to constant mean that variable should be declared as const (i.e)

const int a =10;

?

Posted 30-Jan-13 4:01am

steven8Gerrard

Updated 30-Jan-13 4:05am

v2

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Matthew Faithfull · Accepted Answer · 2013-01-30T04:17:00

I hope this isn't somebody's homework. Anyway in Q1 the difference between part one and part two is largely down to the format strings "%s" and "%d"

Given "%s" the printf code expects a char*, it get one so it prints the string. given "%d" the printf code expects an int. It gets an int* but an int* is automatically convertible to an int, it's just a 4 byte integer value ( assuming a 32bit system ) so you get the address printed as an integer. Pass *p if you want to see the value.

Q2. constness is not broken because the value of p never changes. Try for yourself deleting the line
p=&a;
You'll see it makes no difference to the result. 'p' is just a pointer to where 'a' is stored( the address of a ). The value in 'a' can change as many times as you like, it's still stored in the same place so 'p' doesn't change. *p is a way of saying ( the value of what p points to ) and that will change everytime 'a' changes

If pointers are confusing you then I'd advise getting a really clear understanding of them before throwing 'const' into the mix. constness is neither as easy as it should be nor the best implemented part of the C++ language. Don't get me wrong it's a useful idea just not one that fits very comfortably into C++ which makes it a slippery thing to get a grip on.

H.Brydon · Accepted Answer · 2013-02-03T19:05:00

Solution 1 is good but doesn't differentiate the different flavors of const. Consider the following:

C++

int i1 = 0;
const int i2 = 0;

char* c1 = "...";
char* const c2 = "...";
char const* c3 = "...";
const char* c4 = "...";
const char* const c5 = "...";

I think you have i1 and i2 figured out. Pointers complicate it a little.

c1 is a pointer to char that can be modified at will. c2 is a const pointer to char (can't change where c2 points but you can change what it points to). c3 and c4 are the same thing. c3/c4 are pointer to const char (you can change where c3/c4 points but can't change what they point to). c5 is a const pointer to const char (can't change where it points or what it points to).

Introduction of 'typedef' into the mix invites a whole new dimension.