C++Talk.NET

dstevel · Guest

The signature for strtol is:

strtol( const char*, char**, int)

So.. if we start with a passed "const char*" (pointer to const char),
then we can't create a non-const char pointer pointer to that const
char pointer as in:

void func( const char* a )
{
// ERROR: invalid conversion from `const char**' to `char**'
char** pa = &a;
int i = strtol( a, pa, 10 );
}

And here is another way of saying the same thing without temporary
variables.

void func( const char* a )
{
// ERROR: invalid conversion from `const char**' to `char**'
// ERROR: initializing argument 2 of `long int strtol...
int i = strtol( a, &a, 10 );

}

But you CAN do:

void func( const char* a )
{
int i = strtol( a, (char**)&a, 10 ); // OK
}

AND you can also do this:

void func( const char* a )
{
char* tmp;
int i = strtol( a, &tmp, 10 ); // OK
a = tmp;
}

This indirectly allows us to modify the original const char* a through
the new pointer tmp since tmp will point into the character array a. It
doesn't involve a hard cast, but it seems just as dangerous or even
more so because it's not obvious what just happened. The calling
function could see a change in the string pointed to by a, even though
it's passed as pointer to const.

Can someone help me figure out why this is OK?