Header Guards and LNK4006

Question

Hi,

I have a character array defined in a header

//header.h
const char* temp[] = {"JeffSter"};

The header if #defined guarded and has a #pragma once at the top. If this header is included in multiple places, I get an LNK4006 - char const * * temp already defined in blahblah.obj. So, I have a couple of questions about this

1. Why does this happen if I have the guards in place? I thought that they prevented the header from being read in after the first access.
2. Why do the numerous enums in this header not also give the LNK4006 warnings?
3. If I add static before the signature, I don't get the warning. What are the implications of doing it this way.
4. Is there a better way to do this that avoids the error, but lets me declare the array in the header. I would really hate to have a cpp file just for an array definition.

Answer 1

Include guards protect you from including the same header into the same file repeatedly - but not from including it in distinct files.
What happens is that the linker sees temp in more then one object file - you can solve that by making temp static or putting it into an unnamed namespace:

static const char* temp1[] = {"JeffSter"};
// or
namespace {
    const char* temp2[] = {"JeffSter"};
}

Alternatively you can use one source file which defines temp and just declare it as extern in the header:

// temp.cpp:
const char* temp[] = {"JeffSter"};

// header.h:
extern const char* temp[];

Answer 2

(1) Header guards have absolutely nothing to do with preventing multiple definitions in your entire program. The purpose of header guards is to prevent multiple inclusion of the same header file into the same translation unit (.cpp file). They work as intended in your case.

(2) The rule that governs multiple-definition issues in C++ is called One Definition Rule (ODR). ODR is defined differently for different kins of entities. For example, types can (and most always have to) be defined in every translation unit where they are used, which is why your enum definition does not result in an error. But objects with external linkage have to be defined in one and only one translation unit, which is why your definition of temp causes an error. Don't define objects with external linkage in header files.

(3) By adding static you give your object internal linkage. This will define an independent temp object in each translation unit into which your header file is included, which is perfectly OK with ODR. You could also do

const char* const temp[] = { "JeffSter" }; 

with the same result. const objects in C++ have internal linkage by default.

(4) This depends on whether you need an object with external linkage (i.e. one for the entire program) or an object with internal linkage (unique to each translation unit). If you need the latter, use static or extra const (if that works for you) as shown above.

If you need the former (external linkage), you should put a non-defining declaration into the header file

extern const char* temp[];

and move the definition into one and only one .cpp file

char* const temp[] = { "JeffSter" }; 

The above declaration in the header file will work for most purposes. However, it declares temp as an array of unknown size - an incomplete type. If you wish to declare it as an array of known size, you have to specify the size manually

extern const char* temp[1];

and remember to keep it in-synch between the declaration and definition.

Answer 3

Why does this happen if I have the guards in place? I thought that they prevented the header from being read in after the first access.

Include guards make sure that a header is included only once in one file (translation unit). For multiple files including the header, you want the header to be included in each file.

By defining, as opposed to declaring variables with external linkage (global variables) in your header file, you can only include the header in once source file. If you include the header in multiple source files, there will be multiple definitions of a variable, which is not allowed in C++.

So, as you have found out, it is a bad idea to define variables in a header file for precisely the reason above.

Why do the numerous enums in this header not also give the LNK4006 warnings?

Because, they don't define "global variables", they're only declarations about types, etc. They don't reserve any storage.

If I add static before the signature, I don't get the warning. What are the implications of doing it this way.

When you make a variable static, it has static scope. The object is not visible outside of the translation unit (file) in which it is defined. So, in simple terms, if you have:

static int i;

in your header, each source file in which you include the header will get a separate int variable i, which is invisible outside of the source file. This is known as internal linkage.

Is there a better way to do this that avoids the error, but lets me declare the array in the header. I would really hate to have a cpp file just for an array definition.

If you want the array to be one object visible from all your C++ files, you should do:

extern int array[SIZE];

in your header file, and then include the header file in all the C++ source files that need the variable array. In one of the source (.cpp) files, you need to define array:

int array[SIZE];

You should include the header in the above source file as well, to allow for catching mistakes due to a difference in the header and the source file.

Basically, extern tells the compiler that "array is defined somewhere, and has the type int, and size SIZE". Then, you actually define array only once. At link stage, everything resolves nicely.