Instance of class shared between static libraries (obj files) - c++

Question

I have a C++ application with the following 3 files:
// sample.h
#ifndef sample_h
#define sample_h
#include <stdio.h>
namespace mynamespace {
class sample {
public:
void myprintf(const char* tmp);
};
}
#endif
// sample.cpp
#include "sample.h"
void mynamespace::sample::myprintf(const char* val) {
printf(val);
}
// main.cpp
#include "sample.h"
int main() {
mynamespace::sample sample1; // How to omit this line?
sample1.myprintf("Hello world!");
}
Is it possible to remove the instantiation of sample1 object from main.cpp, and to have it already available (coming from the static library "sample.obj")?
If I move that line to sample.h, then the error I get during compilation is:
"class mynamespace::sample sample1" already defined in sample.obj
If I move that line to sample.cpp, then the error message is:
'sample1': undeclared identifier
Actually I understand why both errors occur, I just don't know what is the solution.
Thanks

Answer 1

Use static declaration:
in sample.h
namespace mynamespace{
class sample {
public:
static sample sample1;
void myprintf(const char* tmp);
};
static sample& sample1 = sample::sample1;
}
then in sample.cpp
mynamespace::sample mynamespace::sample::sample1;
from main.cpp
access the variable
mynamespace::sample::sample1.myprintf("");
mynamespace::sample1.myprintf("");

Answer 2

Specify an extern storage class in the header file:
namespace mynamespace {
// ...
extern sample sample1;
}
And then define it normally in sample.cpp:
mynamespace::sample sample1;
It's possible that on some compilers/operating systems it will be necessary to specify something else, something that's compiler-specific, in order to get external linkage to a data symbol in a library. This is beyond the scope of the C++ standard. Consult your compiler's documentation for more information.