Question

I am trying to create a new instance of a class, however I am receiving a LNK2001 unresolved external symbol error when I attempt to compile my code.

As far as I can tell I have written and included the class in exactly the same manner as I included another class, in both cases –

#include "class.h"      // In main.cpp 

class Class {           // In class.h
   private: 
       // etc. 
   public:         
       Class(); 
       ~Class(); 
       // etc.
}; 

#include "class.h"      // In class.cpp

Is there a common / likely cause of these errors, or a good way I might go about finding the source of the issue?

Edit: The error is

“Error 1 error LNK2019: unresolved external symbol “class Max
__cdecl max(void)” (?max@@YA?AVMax@@XZ) referenced in function _main main.obj Racing “

Edit: In both cases, a class is implemented across a .h and a .cpp file included in a project. The error is only appearing with one class.

Answer 1

Somewhere you have written this:

Max max();

What you intended was to declare a variable max of type Max.

C++ thinks you intend to declare a function max which returns an object of type Max. This is what it is looking for.

If you just say this:

Max max;

The issue will go away.

Edit: This only occurs with constructors which take no arguments. If the constructor takes arguments, C++ can see from the parameters (which will be rvalues, e.g. constants or expressions) that it is an instantiation of the class not a function declaration.

Max max(5);  // Clearly cannot be a function, because 5 is an rvalue

Or

Max max(int); // Clearly cannot be an instantiation, because int is a type

But if the constructor takes no arguments, to distinguish between them, you have to drop the brackets if you are instantiating.