Extern / Globals

Hey,

I was trying to do some simple stuff using global variables but my main file was getting huge! I broke it up and I had like

main.cpp

Code:

---

int global1;
int global2;
int global3;

int main()
{
    return 0;
}

---

and then blah.h

Code:

---

extern "C++"
{
    int global1;
    int global2;
    int global3;
}

---

and then blah.cpp

Code:

---

#include "blah.h"

void blah() { /* uses globals */ }

---

Unfortunately though I get that the variables are already defined as if the extern declaration is a new definition!

Code:

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1>------ Build started: Project: EmptyProject, Configuration: Debug Win32 ------
1>Compiling...
1>Events.cpp
1>Linking...
1>Events.obj : error LNK2005: "struct ID3DXMesh * g_pMesh" (?g_pMesh@@3PAUID3DXMesh@@A) already defined in EmptyProject.obj
...
1>Debug\EmptyProject.exe : fatal error LNK1169: one or more multiply defined symbols found

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Is this not the right way to use extern? I thought that's how you refer to global symbols -- with extern?

Quote:

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Originally Posted by Tonto

I was trying to do some simple stuff using global variables but my main file was getting huge!

---

Why are you using global variables in the first place? If you absolutely must use global variables, then declare them as extern in the header:

Code:

---

extern int global1;
extern int global2;
extern int global3;

---

Then define them in exactly one source file.

I thought I had tried that @_@

I'll avoid the use of them somehow -- singleton or something --

Thanks for the quick response

Quote:

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Originally Posted by Tonto

I thought I had tried that @_@

I'll avoid the use of them somehow -- singleton or something --

Thanks for the quick response

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A singleton is really just a clever disguise for a global variable...

I know :(

Quote:

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Originally Posted by Tonto

I know :(

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I usually bundle up my "global" variables into a class or struct, and pass this object around by reference to any function which needs access to it. It's painful, but a net gain. If you ever find yourself having to multi-thread your code, it makes life easier.

Quote:

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Originally Posted by Tonto

I thought I had tried that @_@

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Linkage specification blocks (extern "whatever" {}) do not prevent variable declarations from being definition.
Curiously enough, a direct linkage specifiation on the variable does.

Code:

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{}

---

Quote:

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Originally Posted by CornedBee

Linkage specification blocks (extern "whatever" {}) do not prevent variable declarations from being definition.
Curiously enough, a direct linkage specifiation on the variable does.

Code:

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{}

---

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It's got nothing to do with the curly braces. It's related to whether the optional string parameter is given. You would see the same thing if you compared

Code:

---

extern int x;

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and

Code:

---

extern "C" int y;    // equivalent to extern "C" {int y;}

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That string qualifier "C" changes the meaning.

No, that's my point.

Code:

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extern "C" int x;

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and

Code:

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extern "C" { int x; }

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are *not* equivalent. C++ 3.1/2 says so:

Quote:

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A declaration is a definition unless it declares a function without specifying the function's body, it contains the extern specifier, or a linkage-specification[24] and neither an initializer nor a function-body, ...

[24] Appearing inside the braced-enclosed declaration-seq in a linkage-specification does not affect whether a declaration is a definition.

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What about static class variables? Act like global variables inside a namespace, without the hassle of extern? You can tell I've never read the specification.:eek:

Quote:

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What about static class variables? Act like global variables inside a namespace, without the hassle of extern?

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You mean you find it more hassle to put extern before a declaration than to put that declaration into a class, which means that every use of the variable must be qualified with the class since using declarations don't extract static members from classes?