Question

just a very small program to test how to use the namespace. I divide it into 3 files, since in large product, ns.h is the namespace interface and ns.cpp is the implementation. I cannot put all these stuff into one file.

Here is the code:

//ns.h
#ifndef MY_H
#define MY_H
namespace my
{
    int a=1;
    int b=0;
    void test();
}
#endif

//ns.cpp
#include <iostream>
#include "ns.h"

using namespace my;
//my::a=1;
//my::b=0;
void my::test()
{
    std::cout<<a<<std::endl;
}

//testns.cpp
#include <iostream>
#include "ns.h"
int main()
{
    std::cout<<my::b<<std::endl;
    my::test();
}

If I keep the above code, and compile will get:

testns.obj : error LNK2005: "int my::b" (?b@my@@3HA) already defined in ns.obj
testns.obj : error LNK2005: "int my::a" (?a@my@@3HA) already defined in ns.obj

If I comment the statement #include “ns.h” I will get undefined error.

D:\mfc\testns.cpp(5) : error C2653: 'my' : is not a class or namespace name
D:\mfc\testns.cpp(5) : error C2065: 'b' : undeclared identifier
D:\mfc\testns.cpp(6) : error C2653: 'my' : is not a class or namespace name
D:\mfc\testns.cpp(6) : error C2065: 'test' : undeclared identifier

Kindly help me if you know how to do this. Thanks a lot.

Answer 1

Headers are for declarations, not definitions. That’s nothing to do with the namespace problem.

//ns.h
#ifndef MY_H
#define MY_H
namespace my
{
    extern int a, b; // declared, not defined thanks to 'extern'.
    void test();
}
#endif

//ns.cpp
#include <iostream>
#include "ns.h"

int my::a=1; // now we provide the actual definitions.
int my::b=0;
void my::test()
{
    std::cout << my::a << std::endl;
}

//testns.cpp
#include <iostream>
#include "ns.h"
int main()
{
    std::cout << my::b << std::endl;
    my::test();
}