unresolved external symbol error -
<p>I'm fairly new to C++ and my professor gave me most of the code to start with. I'm supposed to write a program that uses the <code>add_day(int n)</code> function I wrote in Chrono.cpp.
There must be a problem with the <strong>Chrono::Date add_day(int n);</strong> line.</p>

<p>Here is the error: </p>

<blockquote>
  <p>hw4pr2.obj : error LNK2019: unresolved external symbol "class Chrono::Date __cdecl add_day(int)" (?add_day@@YA?AVDate@Chrono@@H@Z) referenced in function _main</p>
</blockquote>

<pre><code>#include "Chrono.h"
#include "std_lib_facilities_3.h"

int main()
{
    Chrono::Date date; //declaring our date variable from the Chrono.cpp file
    Chrono::Date add_day(int n);
    cout &lt;&lt; "Enter a date with the format (year,month,day):\n"; //asks user to input date in that format

    while(cin &gt;&gt; date){ //input date
    if(!cin) //bad input date
    {
        cout &lt;&lt; "Wrong format. Closing...\n";
        return 1;
    }
    ++date; //increment the date with specifications to Chrono.cpp and Chrono.h
    cout &lt;&lt; "Tomorrow is: " &lt;&lt; date &lt;&lt; "\n"; //reads out the date
    cout &lt;&lt; "Two days from now is " &lt;&lt; add_day(2);
}
    keep_window_open();
    return 0;
}
</code></pre>
