Difference between Pointer Datatypes C

Question

I'm new to C/C++ Programming and I'm still learning some differences between pointer datatypes. But I struggled with these one:

int *const x1;
const int *x2;
char *x3[3];
char (*x4)[3];
double* x5, x6;

I think the first one should be a integer variable that points to a constant memory address
And x2 should be a integer Pointer which is const, right?

I don't know if thats right. So can anyone explain me the differences between them?

Note: that `int *a` is a pointer, `int a[2]` is also a pointer (array) that pointes to 2 values. `int *a[2]` is a pointer to array. It may help you a bit to understand — Hearner, Jun 13 '16 at 11:24
@Hearner: `int a[2]` is **not** a pointer (although it may *decay* to a pointer in certain cases). — Paul R, Jun 13 '16 at 11:30
As mentioned: No, `x1` is a constant pointer to a non-constant integer. And `x2` is a non-constant pointer to a constant integer. And so on. For most cases, you can just read the declarations backwards. I think this is a basic feature of most good tutorials to these languages, but maybe your one missed it out... — underscore_d, Jun 13 '16 at 11:38
@Cheiron I _think_ Sourav was indicating that trying to apply identical concepts to both languages is likely to result in mishaps - by subtly using the metaphor of how the postincrement is unsequenced and therefore, if one were literally trying to evaluate the result of dividing a variable named `C` by (itself postincremented), anything could happen. — underscore_d, Jun 13 '16 at 11:42

score 3 · Accepted Answer · answered Jun 13 '16 at 11:30

int *const x1;  // const pointer to int
                // x1 is const pointer, so it cannot point to anything else (once initilized),
                // but what it points to can changed

const int *x2;  // pointer to const int
                // x2 can points to something else,
                // but what it points to cannot be changed

char *x3[3];    // array of three pointers to char
                // (array of pointers)

char (*x4)[3];  // pointer to array of three char
                // (single pointer)

double* x5, x6; // x5 is pointer to double, x6 is just a double

The last example should not be written like that. The `*` belongs to the variable name, thus the star should be with the name as for the other declarations. — too honest for this site, Jun 13 '16 at 12:48

score 2 · Answer 2 · answered Jun 13 '16 at 11:27

2

For problems such as this you can check with cdecl, e.g. for your third example:

char *x3[3];

cdecl returns:

declare x3 as array 3 of pointer to char

and for your fourth example:

char (*x4)[3];

cdecl returns:

declare x4 as pointer to array 3 of char

answered Jun 13 '16 at 11:27

Paul R

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cdecl does not work with multiple declarations (e.g. `double* x5, x6;`). – Holt Jun 13 '16 at 11:30
3

@Holt: true - you would need to apply a little common sense to that one and not get caught out by the associativity of the `*` - I wasn't planning to do *all* the OP's homework for him though. ;-) – Paul R Jun 13 '16 at 11:32
1

Thank you for this website. Did not know that there is such a thing. – Velox Jun 13 '16 at 11:36

score 2 · Answer 3 · answered Jun 13 '16 at 11:30

You read C declarators backwards. "int * const" is therefore a constant pointer to int (you can change the int, but not make the pointer point elsewhere after initialization); whereas "const int *" is a pointer to a constant int (change the pointer all you like, but not the int it's pointing to).

"The C declarator syntax is an experiment that failed." according to Bjarne Stroustrup.

score 2 · Answer 4 · answered Jun 13 '16 at 11:31

 int *const x1;

constant pointer to int, that means the pointer cannot be changed to point anything.

const int *x2;

pointer to int constants. Here the integer is constant, which means you cannot change the value.

char *x3[3];

array (of length 3) of pointers to char

char (*x4)[3];

pointer to an array (of length 3) of char

double* x5, x6;

x5 is pointer to double, x6 is just a double variable

Difference between Pointer Datatypes C

4 Answers4