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error LNK2001 and error LNK2019 (C++) -- Requesting some learning about these errors

Answer 1

+2 A:

This

WeaponsDB::PI = 4*atan(1.0);

assigns a value to PI. It does not create space for it (does not define it).

This creates space for (defines) PI and assigns a value to (initializes) it.

double WeaponsDB::PI = 4*atan(1.0);

You should probably also mark PI as "static const" and not just "static". static makes it owned by the class instead of by the instance. const makes it immutable (and enables various optimizations).

You might also consider using M_PI from math.h instead of recreating the constant.

Edit: parenthetically added the more precise terms: defines, initializes due to WP's comment.

Mr Fooz 2008-12-19 05:34:47

Hmmm...the change double WeaponsDB::PI = 4*atan(1.0);Doesn't seem to work for me (it throw errors regarding redefinition)The reason I don't make it const is I would have to define the value at compile time and I can't define floating points as const for some reason.

Chad 2008-12-19 05:40:35

"it throw errors regarding redefinition" because you didn't obey what someone else commented on your question: "You need to post the part of the .h file that declares the PI and in the .cpp file that defines it." Keep the declaration in the .h file. Put one DEFINITION in one cpp file.

Windows programmer 2008-12-19 06:13:40

I see this answer might not be clear enough though it's correct. "assigns a value to PI. It does not create space for it" = assigns but doesn't define. "creates space for PI and assigns a value to it" = defines and initializes.

Windows programmer 2008-12-19 06:16:05

const forces you to choose the value at or before initialization time, not at compile time. Only integer member initializations can be specified in a .h file. As dribeas and WP have suggested, you need to make sure that you have a single .cpp file that defines it, creating the actual storage space

Mr Fooz 2008-12-19 14:08:15

Answer 2

A:

Declaring a variable in one place and assigning it a value in another place shouldn't cause any errors. Make sure you include WeaponsDB.h at the top of all files that reference objects defined there.

Ex: at the top of Driver.cpp, make sure "#include WeaponsDB.h" appears.

Hope this helps!

Pwninstein 2008-12-19 05:35:17

I have the necessary #include's. I believe it would a unknown member type of error if that was the case.

Chad 2008-12-19 05:41:43

Answer 3

+6 A:

Most often, when the linker is failing to detect a static method it is due to forgetting to really define it somewhere, as it was pointed before:

// header
class X {
   static const int y;
};

// cpp
const int X::y = 1;

But in your case, as you are not only missing static variables but also all the rest of the members my bet is that you are not linking the implementation file(s) (.cpp) in your project. You must provide a compilation unit that defines the symbols that were declared in the header, and make the environment compile and link it together. If the symbols belong to a compilation unit in an external library (from your current project) then you must remember to link with the library.

David Rodríguez - dribeas 2008-12-19 07:15:19

Answer 4

A:

From reading your comments in other people's answers, it looks like you are assigning it a value inside of a function. You need to have:

double WeaponsDB::PI = 4*atan(1.0);

.. in a global scope (meaning not inside of a function) of your implementation file, presumably WeaponsDB.cpp. And in that case, "const"ing it would be appropriate. In fact, you can skip the part about putting it in your .cpp and just do it directly in WeaponsDB.h:

public:
    static const double PI = 4*atan(1.0);

Jim Buck 2008-12-19 18:37:44

Depending on the compiler, only integral types can be assigned to const values in a header file.

Mr Fooz 2008-12-20 05:45:35

Answer 5

A:

An awful lot of static members are missing. Could it be that you concentrate your static members definitions in one file, and then you don't link that file in?

Arkadiy 2008-12-19 18:41:09

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