Issue with basename

So I am having an issue using basename for one of my programming assignments for school

I have tried getting a simplier version of it working -- I got it working however, still confused exactly what I am supposed to do in this case

// $Id: util.cpp,v 1.1 2016-06-14 18:19:17-07 - - $
#include <libgen.h>
#include <cstring>
using namespace std;

#include "util.h"

ydc_exn::ydc_exn (const string& what): runtime_error (what) {
}

string exec::execname_; // Must be initialized from main().
int exec::status_ = EXIT_SUCCESS;

void exec::execname (const string& argv0) {
   execname_ = basename (argv0.c_str());
   cout << boolalpha;
   cerr << boolalpha;
   DEBUGF ('Y', "execname = " << execname_);
}

void exec::status (int new_status) {
   new_status &= 0xFF;
   if (status_ < new_status) status_ = new_status;
}

ostream& note() {
   return cerr << exec::execname() << ": ";
}

ostream& error() {
   exec::status (EXIT_FAILURE);
   return note();
}

Just trying to get the code to compile --- my error message in c++ is: ' util.cpp:15:16: error: no matching function for call to 'basename' execname_ = basename (argv0.c_str()); ^~~~~~~~ /usr/include/libgen.h:40:7: note: candidate function not viable: 1st argument ('const std::__1::basic_string, std::__1::allocator

::value_type *' (aka 'const char *')) would lose const qualifier char *basename(char *); ^ 1 error generated. make: *** [util.o] Error 1'