Question

Could someone please advise?

If const in C++ defaults to internal linkage, why do I get multiple definition errors in the code below?

First, the file dem.h:

#ifndef _DEM_H_
    #define _DEM_H_
    class Dem {
        public:
            static const int i;
    };

    const int Dem::i = 10;
#endif

Them imp1.cpp:

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

using namespace std;
extern int foo();

int main() {
        cout << foo() << endl;
}

and imp2.cpp:

#include "dem.h"

int foo() {
    return Dem::i ;
}

I compile with the following command and results:

$ g++ imp1.cpp imp2.cpp
/tmp/ccmGt0OY.o:imp2.cpp:(.rdata+0x0): multiple definition of `Dem::i'
/tmp/cc5sN7dz.o:imp1.cpp:(.rdata+0x0): first defined here
collect2: ld returned 1 exit status

Answer 1

From C++11 [basic.link], paragraph 5:

In addition, a member function, static data member, a named class or enumeration of class scope, or an unnamed class or enumeration defined in a class-scope typedef declaration such that the class or enumeration has the typedef name for linkage purposes (7.1.3), has external linkage if the name of the class has external linkage.

Because your class has external linkage, so does your static data member.