C++ begineer : using namespace error

Question

I am really new in C++, and I can not solve the compilation error below.

data_structure.h

#include <stdint.h>
#include <list>

namespace A {

        class B {
            public:

            bool        func_init(); // init
            };

};

data_structure.cpp

#include "data_structure.h"

using namespace A;

bool B::func_init(){

    std::cout << "test init" << std::endl;
    return true;

}

main.cpp

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

using namespace A;

int main( int argc, char **argv ) {

    A::B s;
    s.func_init();

    return 0;
}

I have an error as the following

undefined reference to `A::B::func_init()'

Please kindly advice why I can not get the func_init, eventhough it is declared as public? I have put also the correct namespace there.

Would appreciate for any response.

score 5 · Accepted Answer · answered Apr 10 '13 at 14:43

5

That's a linker error, so you're probably not compiling all your source files, or linking them together, or making some weird use of the C compiler (I see your files have the extension .c, some compilers treat them as C-source).

answered Apr 10 '13 at 14:43

Luchian Grigore

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my mistake, it was cpp file from the beginning, not c files. but I still have this problems of compliation – xambo Apr 10 '13 at 14:46
@xambo how are you compiling? – Luchian Grigore Apr 10 '13 at 14:48
1

@xambo there you go - you also need to compile `data_structure.cpp`. – Luchian Grigore Apr 10 '13 at 14:51

batfan47 · Answer 2 · 2013-04-10T15:10:02.847

2

g++ main.cpp data_structure.cpp -o test should do it.

However I did need to add #include <iostream> to your data_structure.cpp file to resolve

data_structure.cpp: In member function ‘bool A::B::func_init()’:
data_structure.cpp:7:5: error: ‘cout’ is not a member of ‘std’
data_structure.cpp:7:33: error: ‘endl’ is not a member of ‘std’

and make it compile.

edited Apr 10 '13 at 15:10

answered Apr 10 '13 at 14:56

batfan47

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score 1 · Answer 3 · answered Apr 10 '13 at 15:08

The definition of a function has to be in the namespace that declares the function. A using declaration just pulls in names from the namespace; it doesn't put you inside it. So you have to write data_structure.cpp like this:

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

namespace A {
bool B::func_init() {
    std::cout << "test init" << std::endl;
    return true;
}
}

Or, if you prefer, you can use the namespace name explicitly in the function definition:

bool A::B::func_init() {
    std::cout << "test init" << std::endl;
    return true;
}

+1 That was my understanding, although I did manage to compile code equivalent to OP's using g++ 4.4.3, which I really wasn't expecting to work. — juanchopanza, Apr 10 '13 at 15:26

score 0 · Answer 4 · answered Apr 10 '13 at 14:56

Have you tried not putting

using namespace A;

in your data_structure.cpp file and instead putting:

#include "data_structure.h"

bool A::B::func_init() {
   std::cout << "test init" << std::endl;
   return true;
}

I have the feeling that when you use using namespace A; doesn't let you append function definitions to the namespace, but only tells the compiler/linker where to look for Types or Functions...

Another theory: Have you tried nesting your CPP code in the same namespace?

#include "data_structure.h"

namespace A {
   bool B::func_init() {
      std::cout << "test init" << std::endl;
      return true;
   }
}

C++ begineer : using namespace error

4 Answers4