what does "error : a nonstatic member reference must be relative to a specific object" mean?

int CPMSifDlg::EncodeAndSend(char *firstName, char *lastName, char *roomNumber, char *userId, char *userFirstName, char *userLastName)
{
	...

	return 1;
}

extern "C"
{
	__declspec(dllexport) int start(char *firstName, char *lastName, char *roomNumber, char *userId, char *userFirstName, char *userLastName)
	{
		return CPMSifDlg::EncodeAndSend(firstName, lastName, roomNumber, userId, userFirstName, userLastName);
	}
}

On line return CPMSifDlg::EncodeAndSend I have an error : Error : a nonstatic member reference must be relative to a specific object.

What does it mean?

EncodeAndSend is not a static function, which means it can be called on an instance of the class CPMSifDlg. You cannot write this:

 CPMSifDlg::EncodeAndSend(/*...*/);  //wrong - EncodeAndSend is not static

It should rather be called as:

 CPMSifDlg dlg; //create instance, assuming it has default constructor!
 dlg.EncodeAndSend(/*...*/);   //correct 

Only static functions are called with class name.

classname::Staicfunction();

Non static functions have to be called using objects.

classname obj;
obj.Somefunction();

This is exactly what your error means. Since your function is non static you have to use a object reference to invoke it.

CPMSifDlg::EncodeAndSend() method is declared as non-static and thus it must be called using an object of CPMSifDlg. e.g.

CPMSifDlg obj;
return obj.EncodeAndSend(firstName, lastName, roomNumber, userId, userFirstName, userLastName);

If EncodeAndSend doesn't use/relate any specifics of an object (i.e. this) but general for the class CPMSifDlg then declare it as static:

class CPMSifDlg {
...
  static int EncodeAndSend(...);
  ^^^^^^
};