Thread: global integral constants

>> Well... I'm assuming there is no need to take the address of an integral constant.

From the first post:
>> I don't like the "enum-hack" (for one thing, you can't take the address of it if you need to).

In addition, and someone else can correct me if I'm wrong, but wouldn't a different address mean that the ODR is violated?

>> That would happen with the original code, with a static variable, would it not?
Yes. Which is what I said earlier.

>> But... who talked about taking the address of the variable?
ChaosEngine said (and I already quoted) that the enum-hack was out because you cannot take the address of an enum.

Separately, the different addresses are an indication that different constants are created for each compilation unit. Is this a violation of ODR? Isn't that what the question is all about? Your mention of scope is irrelevent. We're not worried about whether the code will compile and link.

Here is my test code:

Code:

// Header.h

#ifndef HEADER_H
#define HEADER_H

#define USE_UNNAMED_NAMESPACE 1

namespace NS
{
#if USE_UNNAMED_NAMESPACE
    namespace
    {
        const int size = 10;
    }
#else
    enum { size_enum = 10 };
    extern int size;
#endif
}

void Test1();
void Test2();

#endif // HEADER_H

Code:

// Source1.cpp

#include <iostream>
#include "Header.h"

void Test1()
{
    std::cout << "Test1: " << NS::size << ", " << &(NS::size) << '\n';
}

Code:

// Source2.cpp

#include <iostream>
#include "Header.h"

void Test2()
{
    std::cout << "Test2: " << NS::size << ", " << &(NS::size) << '\n';
}

Code:

// main.cpp

#include "Header.h"

#if !USE_UNNAMED_NAMESPACE
int NS::size = NS::size_enum;
#endif

int main()
{
    Test1();
    Test2();
}

Change USE_UNNAMED_NAMESPACE to 0 to see it with the enum/extern hack.

I don't know if that's the only way. You could also make it a member of a class:

Code:

struct NS
{
    static const int size = 10;
};

This would mean that all compilations would get the same address for the constant, it would be available for sizing of arrays, and it wouldn't require two different names (like the enum-extern hack). I'm not sure if it is standard, though, to allow the initialization of the static const like that. There might also be other issues I haven't thought of or mentioned.