Error: passing ‘const string {aka const std::__cxx11::basic_string

I have the following code for dealing with doubles:

static bool
double_param(const char ** p, double * ptr_val)
{
    char *end;
    errno = 0;
    double v = strtod(*p, &end);
    if (*p == end || errno) return false;
    *p = end;
    *ptr_val = v;
    return true;
}

This code is used to check whether passed double parameter is invalid like so:

if (!double_param(&p, &b1)) //p is pointer and b1 is parameter.
        //throw error;

And, I need to write an equivalent code for dealing with string of 3 characters long. I've gotten this on my own:

static bool
string_param(const char ** p, const string * ptr_val)
{
    char *end;
    errno = 0;
    int v = strtol(*p, &end, 10);
    if (*p == end || errno) return false;
    *p = end;
    *ptr_val = v;
    return true;
}

but I get the following compile error:

error: passing ‘const string {aka const std::__cxx11::basic_string<char>}’ as ‘this’ argument discards qualifiers [-fpermissive]
 (*ptr_val) = v;
            ^

Any suggestions are appreciated to get around this error. Also, please point out the mistake in my code and explain a little so that I can understand better and learn. Thanks in advance.