./libmylib.so: undefined reference to `submarinex::LIB::kCount'

Question

I have 3 files in a directory. I will build ll.cc to libmylib.so, and build main.cc to myexe.

Use these command to build

g++ -Wall -g -fPIC -std=c++11 ll.cc -shared -o libmylib.so
g++ -Wall -g -std=c++11 main.cc -L. -lmylib -o myexe

But, g++ report a error when build myexe :

./libmylib.so: undefined reference to `submarinex::LIB::kCount'
collect2: error: ld returned 1 exit status

Files:

ll.h

namespace submarinex {

class LIB {
 public:
  void Print();

 private:
  static const int kCount = 100;
};

}  // namespace submarinex

ll.cc

#include "ll.h"

#include <algorithm>
#include <iostream>

namespace submarinex {

void LIB::Print() {
  int min = std::min(101, kCount);
  std::cout << min << std::endl;

  // std::cout << kCount << std::endl;
}

}  // namespace submarinex

main.cc

#include "ll.h"

int main(int argc, char **argv) {
  submarinex::LIB lib;
  lib.Print();

  return 0;
}

Case 1: If use these 2 lines in Print, will report an error when link object of main.cc

  int min = std::min(101, kCount);
  std::cout << min << std::endl;

Case 2: If use this line in Print, will success

  std::cout << kCount << std::endl;

If change

 static const int kCount = 100;

to

 const int kCount = 100;

it also be OK regardless of using Case1 or Case2.

I don't know how to solve this problem.

Answer 1

I think there is no definition of the kCount variable in your source code.

So, your variable must be defined somewhere in your source code, ll.cc.

In the ll.h

namespace submarinex {
    class LIB {
        private:
            const static int kCount =100;
        public:
            void Print();
    };
}  // namespace submarinex

Then, you can define a kCount variable in the ll.cc

#include "ll.h"

#include <algorithm>
#include <iostream>

namespace submarinex 
{
    const int LIB::kCount; //<== here

    void LIB::Print() {

        int min = std::min(101, kCount);
        std::cout << min << std::endl;

        // std::cout << kCount << std::endl;
    }   

}  // namespace submarinex

This is my gcc version information

$ gcc -v
Using built-in specs.
COLLECT_GCC=C:\DEV\COMP\msys32\mingw32\bin\gcc.exe
COLLECT_LTO_WRAPPER=C:/DEV/COMP/msys32/mingw32/bin/../lib/gcc/i686-w64-mingw32/6.3.0/lto-wrapper.exe
Target: i686-w64-mingw32
Configured with: ../gcc-6.3.0/configure --prefix=/mingw32 --with-local-prefix=/mingw32/local --build=i686-w64-mingw32 --host=i686-w64-mingw32 --target=i686-w64-mingw32 --with-native-system-header-dir=/mingw32/i686-w64-mingw32/include --libexecdir=/mingw32/lib --enable-bootstrap --with-arch=i686 --with-tune=generic --enable-languages=c,lto,c++,objc,obj-c++,fortran,ada --enable-shared --enable-static --enable-libatomic --enable-threads=posix --enable-graphite --enable-fully-dynamic-string --enable-libstdcxx-time=yes --disable-libstdcxx-pch --disable-libstdcxx-debug --disable-isl-version-check --enable-lto --enable-libgomp --disable-multilib --enable-checking=release --disable-rpath --disable-win32-registry --disable-nls --disable-werror --disable-symvers --with-libiconv --with-system-zlib --with-gmp=/mingw32 --with-mpfr=/mingw32 --with-mpc=/mingw32 --with-isl=/mingw32 --with-pkgversion='Rev1, Built by MSYS2 project' --with-bugurl=https://sourceforge.net/projects/msys2 --with-gnu-as --with-gnu-ld --disable-sjlj-exceptions --with-dwarf2
Thread model: posix
gcc version 6.3.0 (Rev1, Built by MSYS2 project)

Compiled with MinGW32 shell.

g++ -Wall -g -fPIC -std=c++11 -I. ll.cc -shared -o libmylib.a
g++ -Wall -g -std=c++11 -I. -L. -lmylib main.cc -o myexe

I hope this helps you.

Regards,

Answer 2

std::min use two params const int& (not int) and you pass a reference to submarinex::LIB::kCount without definition (you just declared in .h file).

In this case, you can add this line in ll.cc

namespace submarinex {
    static const int LIB::kCount = 100; //define it here
    void LIB::Print() {
        std::min(101, kCount);
    }
}