C ++ Visual Studio 2008中未声明的标识符

Question

I have a C++ project in Visual Studio 2008.

In the project I have several forms and several non-form classes. One non-form specifically called Import_LP.h that is a class with several methods all of which are written in the header file with nothing in the resource file.

I have no problem with #include Import_LP in any of the form classes and creating objects and referencing any of its methods, however any other class I try to #include it into, it gives me a

syntax error : undeclared identifier 'Import_LP'

on the line it is referenced occurs ie Import_LP^ importLP;

I come from a java/c# background is there something I'm missing with the linking here?

Answer 1

I was able to resolve the problem by random chance.

So apparently you can't have two files include each other?

I was doing

#include Window.h

in Import_LP.h

and

#include Import_LP.h

in Window.h.

I would appreciate it if someone could explain to me why you can't do this.

Answer 2

If you have include guards, it goes like this: the preprocessor includes Import_LP.h, which says "only include me once", then includes Window.h, which tries to include Import_LP.h, but doesn't because of the include guard. So Window.h starts parsing the window class, but fails because the Import_LP.h class header hasn't fully loaded yet.

The solution is to predeclare the classes:

Window.h:

#ifndef WINDOW_H  //works best if this is first
#define WINDOW_H
#pramga once

class Import_LP;
class Window {
    Import_LP* member; //member has to be a pointer
    void func();
};
#include "Import_LP.h"
inline void Window::func() {  
}
#endif WINDOW_H

Import_LP.h:

#ifndef IMPORT_LP_H //works best if this is first
#define IMPORT_LP_H 
#pramga once

class Window;
class Import_LP {
    void func(Window& parent); //parameter has to be a pointer or reference
};
#include "Window.h"
inline void Import_LP::func(Window* parent) {  
}
#endif IMPORT_LP_H 

This will only allow you to reference the other by pointer or reference until the actual include, but that should be doable. Technically you only have to do this to one or the other headers.

Answer 3

Sounds like the type Import_LP is undefined, your challenge is to figure out why. First thing to do is to read the contents of import_LP.h and figure out how Import_LP is declared. One possible approach would be to open one of the good files, right click on a use of Import_LP and select "Go To Declaration". That should move the focus to the declaration of Import_LP that applies in that specific case.

It could be the declaration is surrounded by a #ifdef that somehow prevents it being compiled in your other files. Or it could be something else, we don't really have enough details to work with.

Update:

So apparently you can't have two files include each other? . . . I would appreciate it if someone could explain to me why you can't do this.

You can end up defining types and variables more than once when you do that.

There are techniques like #include guard and #pragma once which may be used to mitigate these sorts of problems.