c++ error: cannot convert basic_string

I'm new to C++, been programming a while in C though. Trying to read in a string and then convert the string into int by using strtol. Iam using the gcc compiler. And I get the following error message: "c++ error: cannot convert 'std::__cxx11::string {aka std::__cxx11::basic_string}' to 'const char*' for argument '1' to 'long int strtol(const char*, char**, int)' int c = strtol(str[j], &p, 10);". I have tried different types of conversions but really like the strtol for future reference. Is there something to do with my vector string?

#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;
using std::string;

int main()
{
  int a = 0;
  int i = 0;
  int size = 0;
  int* big;

  cin>>a;
  size = a*2;

  int sizes[size];
  string *str = new string[size];
  while(i < a){
    cin>>str[i];
  i++;
}

i = 0;
while(i < a){
  cout << str[i] << endl; // just for checking
  i++;
}



for (int j =0; j<size-1;j++){
  char* p;
  char* q;
  int c = strtol(str[j], &p, 10);
  if (!*p) {
     sizes[j] = *p;
  }else{
     sizes[j] = *p/2;
  }
}

return 0;
}

Thanks in advance!