Pointer to a pointer (2D array) as input parameter

Question

I started studying C++ at university 2 months ago, and I'm trying to figure out how to pass a dynamic 2d array (pointer to pointer) to a function as an input parameter. I have this dynamic 2d array:

int **p;
p = new int*[R];
for(i=0; i<R; i++)
    p[i] = new int[C];

Now I want to pass this pointer p to a function as an input parameter, using const. I mean that I want the function not to be able to modify the elements of the matrix.

I tried like that:

void func(const int **p) {
}

But i get this error:

main.cpp:19:11: error: invalid conversion from 'int**' to 'const int**' [-fpermissive]

main.cpp:9:6: error: initializing argument 1 of 'void func(const int**)' [-fpermissive]

I tryied using typedef and it works, but it's not constant. If i do like that:

typedef int** abc;

void func(const abc p);

main() {
abc p;
...
func(p);
}

The source compiles but function 'func' is able to modify values of "p matrix"; I want p to be an input parameter, it has to be read-only!

Please, how can I pass a pointer-to-pointer to a function flagging the elements as read-only?

Thank you in advance for you help and sorry for my bad english.

tmw · Accepted Answer · 2015-12-13 20:46:35Z

3

Your const int **p should be a const int * const *p. The conversion from const int ** to int ** isn't allowed to be implicit, because it could violate the constness of the values being pointed to. To see why, look at the following code extract taken from the C standard (§6.5.16.1):

const char **cpp;
char *p;
const char c = 'A';

cpp = &p;       // constraint violation
*cpp = &c;      // valid
*p = 0;         // valid

If cpp = &p was a valid assignment, the subsequent code would be able to set the value of c to 0, despite the value being a const char.

edited Dec 13, 2015 at 20:46

answered Dec 13, 2015 at 20:38

tmw

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Olivier Pellier-Cuit · Accepted Answer · 2015-12-13 20:31:29Z

2

void func(const int * const *p) {
}

answered Dec 13, 2015 at 20:31

Olivier Pellier-Cuit

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1 Comment

PeppeDAlterio Over a year ago

Could you please also provide me a short explaination of this? Thank you so much.

marom · Accepted Answer · 2015-12-13 20:27:21Z

1

Try this:

typedef const int *cint_ptr_t  ;

void func(const cint_ptr_t  *p) {
}

Keep in mind that you want to be const the elements of your 2D array and the pointers to the beginning of each row

answered Dec 13, 2015 at 20:27

marom

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