Multiply Characters

Question

If I declare a global variable in the header file and include it in two .cpp files, the linker gives an error saying that the character is many times defined. My question is: why does this only happen for certain types of objects (e.g. int) and not for others (e.g. enum)?

The test code used is shown below:

test.h

#ifndef TEST_HEADER
#define TEST_HEADER

namespace test
{           
        int i_Test1 = -1;
        int i_Test2 = -1;
};

#endif // TEST_HEADER

Class1.h

#ifndef CLASS_1_HEADER
#define CLASS_1_HEADER

class class1
{
public:
        void count();
};

#endif //CLASS_1_HEADER

class1.cpp

#include <iostream>
#include "class1.h"
#include "test.h"

void class1::count()
{
        std::cout << test::i_Test1 << std::endl;
}

class2.h

#ifndef CLASS_2_HEADER
#define CLASS_2_HEADER

class class2
{
public:
        void count();
};

#endif //CLASS_2_HEADER

class2.cpp

#include "class2.h"
#include <iostream>
#include "test.h"

void class2::count()
{
        std::cout << test::i_Test2 << std::endl;
}

main.cpp

#include "class1.h"
#include "class2.h"

int main(int argc, char** argv)
{
        class1 c1;
        class2 c2;
        c1.count();
        c2.count();
        return -1;
}

Building this code with

g++ main.cpp class1.cpp class2.cpp -o a

outputs the following result:

ld: fatal: symbol test::i_Test1' is multiply-defined: (file /var/tmp//ccwWLyrM.o type=OBJT; file /var/tmp//ccOemftz.o type=OBJT); ld: fatal: symbol test :: i_Test2 'repeatedly defined: (file / var / tmp // ccwWLyrM.o type = OBJT; file /var/tmp//ccOemftz.o type = OBJT); ld: fatal: error file processing. There is no output written to collect2: ld returned 1 exit status

If I changed the test.h file as shown below:

test.h (with listing)

#ifndef TEST_HEADER
#define TEST_HEADER

namespace test
{
        enum val
        {
                i_Test1 = 5,
                i_Test2
        };
        //int i_Test1 = -1;
        //int i_Test2 = -1;
};

#endif // TEST_HEADER

" " , :

5
6

Answer 1

, - . (, , ) , , ( (ODR)). :

• ( )
• ( ) . ( )

ODR. , , / (, ODR - , undefined).

- . , . , ( "extern" ) .cpp ( "extern" . const , "extern" , , ).