C++ Problems with splitting code into multiple files - c++

Question

So, I am working on a project and decided to split the code into multiple files as it was getting too big. However, a got a compilation error. I managed to recreate the error in this simple case:
//main.cpp
#include<iostream>
#include "classa.h"
using namespace std;
int main()
{
return 0;
}
The main does nothing it just includes classa.h
//classa.h
#ifndef CLASSA_H_INCLUDED
#define CLASSA_H_INCLUDED
#include<vector>
using namespace std;
vector<int> primes= {1,2,3,5,7,11,13,17,19};
class classa
{
private:
int a;
public:
int getA();
void setA(int newA);
};
#endif //CLASSA_H_INCLUDED
The class isn't even necessary for the error to occur. However, I wanted to have something in classa.cpp
//classa.cpp
#include "classa.h"
using namespace std;
int classa::getA()
{
return a;
}
void classa::setA(int newA)
{
a=newA;
}
It gives me the following error:
obj\Debug\sources\main.o:main.cpp:(.bss+0x0): multiple definition of `primes'
obj\Debug\sources\classa.o:classa.cpp:(.bss+0x0): first defined here
The problem is that unlike here in my project I cannot use some sort of constant or a define for the global variables as they are things that can be modified by different classes.

Answer 1

Make primes an extern variable, and declare it in your classa.h header, but only defined it once in classa.cpp.
Currently, as your compiler told you, primes exists twice, in main.cpp and in classa.cpp. Keep in mind, #include is merely text substitution.
classa.h:
extern std::vector<int> const primes;
classa.cpp:
std::vector<int> const primes = {1,2,3,5,7,11,13,17,19};
Read more about storage class specifiers here.

Answer 2

If you have .cpp file in overall - then it makes sense to split into
Into .h file:
extern std::vector<int> primes;
Into .cpp file:
using namespace std;
vector<int> const primes = {1,2,3,5,7,11,13,17,19};
"using namespace std" might cause conflicts between 3rd party libraries, but this occurs very rarely - I prefer to use "using namespace std" whenever possible. But if there are conflicts - then you might want to localized "using namespace std" into your own .cpp file. (Where you control what #include's that file has).
But sometimes you might not have .cpp file at all (e.g. only local inline functions or template classes) - then you can initialize vector like this:
__declspec(selectany) std::vector<int> primes = {1,2,3,5,7,11,13,17,19};
This will instruct linker to pick up only one copy (some of them), and discard everything else. This does not play well if you want to #ifdef some of initialization, but this is not normal use case anyway.
I prefer not to use const ever, because it's always pain in neck to get const right after 2-3 level conversions. I write in function comments what is input / what is output and what should not be modified. (Because that one can change over iterations)