Passing Object Class array a strings to function

Those errors means the names Visitors and string were not recognised as types by C++ compiler...

In order to work, the declaration of your type Visitors must preceed the function declaration; either in the same source file or in a header file included with #include pre-processor directive.

Also, C++ string type is defined in the standard header file string, that must be included prior to usage, and the type is cotained in the namespace std, so you either need to use the full name std::string or declare you are using it with the using directive:

#include <string>
#include "visitors.hpp"

using std::string;

int getUniques(Visitors info[],
               string address,
               string startDate,
               string endDate);

and in the visitors.hpp file, your type definition (proably a class, but could also be a struct, an union, an enum or a typedef to some other type)...

#pragma once

class Visitors {
     ... // Your class definition.
};

Note that it is a bad practice to use the using directive in header files, because it will pollute the default namespace of the compilation units that include your header. If that is the case, you should use the full name std::string.