Digits to words problem

[SpottyBlue]

I was practising the question before the final semester exam, and I have a problem.

#include <iostream>
#include <string.h>
#include <cmath>

using namespace std;

int num; // A number needs to be entered
int o_num; // Original number
int r;
string str1 = "", str2 = "", temp = "";

void DigitToWord()
{
    r = num % 10;

    switch (r)
    {
    case 0:
        str1 = "zero ";
        break;

    case 1:
        str1 = "one ";
        break;

    case 2:
        str1 = "two ";
        break;

    case 3:
        str1 = "three ";
        break;

    case 4:
        str1 = "four ";
        break;

    case 5:
        str1 = "five ";
        break;

    case 6:
        str1 = "six ";
        break;

    case 7:
        str1 = "seven ";
        break;

    case 8:
        str1 = "eight ";
        break;

    case 9:
        str1 = "nine ";
        break;
    }

    temp = str1;
    str2 = str2 + temp;
}

int main()
{

    do
    {
        cout << "Enter a positive integer (not more than 10000): ";
        cin >> num;
    } while (num < 0 || num > 10000);

    o_num = num;
    for (; num != 0; num = num / 10)
    {
        DigitToWord();
    }

    cout << endl << o_num << " : " << str2 << endl;

    system("pause");
    return 0;
}

When I was creating the code in VS 2013, I saw a red wavy line on "<<" before "str2" (line 78).

I got an error called:

Error 1 error C2679: binary '<<' : no operator found which takes a right-hand operand of type 'std::string' (or there is no acceptable conversion)

If I input 756, the program must show an output like this:

756 : seven five six

[NathanOliver]

Nevermind. I Though I saw something but realised I was wrong after posting.

[deceptikon]

You need to include <string> for the string class, not <string.h>. The latter is for functions inherited from C.