Error involving invalid operands of a const char*

[BleaS]

Here's the error, it's on line 16:

16 [censored] Menu 2.0.cpp invalid operands of types `const char*' and `const char[15]' to binary `operator+'

Here's the code:

#include <iostream>
#include <string>

using std::cout;     using std::cin;
using std::string;   using std::endl;

int main()
{
    const int _num_items = 5;
    
    int row = _num_items + 2;
    string::size_type cols;
    
    for (int r=0; r != row; ++r)
     {
        string _menu_option;
        
        if (r != 0)
         {
           _menu_option = "| " + r + ". Menu Option " + r + " |";
           cols = _menu_option.size();
         }
         
         int c = 0;
         
         while (c != cols)
          {
               if (r = 0 ||  r = row)
                {
                  if (c = 0 || c = cols-1)
                   {
                    cout << "|";
                   }
                  else
                   {
                    cout << "-";
                   }
                }
               else
                {
                    cout << _menu_option;
                    c += _menu_option.size();
                }
          }
          cout << endl;
     }
    
    system("PAUSE");
    return 0;
}

I know what a * means, and what a char is, and what defining it as a const means. However, I don't see how that is relevant to a non-constant string that I defined. It's looking as if it doesn't like the + operands in the string, but that's silly since they should work. I'm not sure what's wrong.

I understand that a C++ string is actually just a character array, so I suppose that's why it says character instead of string. But I still don't get why it's failing.