convertion form char** to const char** error

[mihai]

Hi,

I have a question regarding the conversion (in c or c++) from char**
to const char**.

The fallowing code refuses to compile with g++ (and others):

char **p;
const char **q = static_cast<const char**>(p);

or, exact the same situation:

void f(const char **p) {}
int main() {
f(p);
return 0;
}

And I don't understand why. It should work because is a conversion for
a non const pointer to a const pointer.

Thank you,
Mihai.

 

[Juha Nieminen]

char **p;
const char **q = static_cast<const char**>(p);

Rather ironically const_cast will work. (It's ironic because
const_cast is usually used to *remove* constness, not to add it.)

 

[Juha Nieminen]

http://c-faq.com/ansi/constmismatch.html

 

[James Kanze]

I have a question regarding the conversion (in c or c++) from
char** to const char**.

The fallowing code refuses to compile with g++ (and others):

char **p;
const char **q = static_cast<const char**>(p);

or, exact the same situation:

void f(const char **p) {}
int main() {
f(p);
return 0;
}

And I don't understand why. It should work because is a
conversion for a non const pointer to a const pointer.

No it's not. It's a conversion between two different types of
non-const pointers. Allowing it would break const:

char* p ;
char** pp = &p ;
char const** pcp = pp ;
char const* pc = "doh!" ;
*pcp = pc ;
*p = '\0' ;

Thus, it's forbidden.