Undefined reference to function when linking in C++

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list.h:

C++

class list {
  private:
  int mCount = 5;

  public:
  int getCount();
};

core_list.cpp:

C++

#include "list.h"

int list::getCount() {
  return mCount;
}

main.cpp:

C++

#include "list.h"
#include <iostream>
int main() {
  list newList;
  std::cout << newList.getCount();
  getchar();
  return 0;
}

Makefile:

default:
  g++ -c *.cpp -O2 -Wall
  g++ -o Game.exe *.o
  rm -f *.o *~

Gives me:

main.o:main.cpp:(.text.startup+0x6d): undefined reference to `list::getCount()'
collect2.exe: error: ld returned 1 exit status
Makefile:2: recipe for target 'default' failed
mingw32-make: *** [default] Error 1

But if I modify the Makefile:

<pre lang="text">
default:
  g++ -c *.cpp -O2 -Wall
  #g++ -o Game.exe *.o
  rm -f *.o *~

So it only compiles and doesn't link, it compiles. It just won't link
for whatever reason. Thank you for the help.

What I have tried:

I have tried renaming the .cpp file to the same as its respective .h file, but to no avail. I have also modified the Makefile and noticed that it does compile, it just won't link.

Posted 22-Nov-22 6:42am

CodeSie Programming

Updated 22-Nov-22 8:16am

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Rick York · Answer 1 · 2022-11-22T07:14:00

As an experiment, take the implementation of list::getCount() and put it in your main.cpp file. You should see it compile and link correctly. If so, that means the linker is not seeing the object file from core_list.cpp.

The reason it does not link when you comment that line out in the Makefile is because that is the line that does the linking. If anything, you should comment out the next line that removes the object files. That will let you see if there is an object file generated for core_list.cpp.

CPallini · Answer 2 · 2022-11-22T07:33:00

Solution 2

Your source codes and makefile work fine on my Windows 11 system (MinGW32-Make 3.82.90, g++ 10.3.0).

I suggest you to try, via the command line:

g++ -c *.cpp -O2 -Wall

and then

g++ -o Game.exe *.o

If the last command line gives link errors, then try

g++ -o Game.exe main.o list.o

Posted 22-Nov-22 7:33am

CPallini

Updated 22-Nov-22 7:41am

v2

k5054 · Answer 3 · 2022-11-22T08:16:00

Check what .cpp files you have in your directory. You may be linking in something you're not expecting when you're using wildcards. Better yet, use the power of make:

make

CXXFLAGS = -Wall -Wextra -O2

Game.exe: core_list.o main.o
       $(CXX) $^ -o $@

clean:
       rm Game.exe *.o *~

Now make knows that Game.exe depends on list.o and main.o, and knows how to create them without you adding specific rules. Better, if you modify list.cpp, make will only compile that source file, and then link using the previously compiled main.o, which for large projects can be a huge time saver. Even better, you could tell make that the .o files depend on list.h so they should be recompiled if you modify core_list.h

make

CXXFLAGS = -Wall -Wextra -O2
core_list.o:  core_list.cpp list.h
main.o: main.cpp list.h

Game.exe: core_list.o main.o
      $(CXX) $^ -o $@

clean:
      rm Game.exe *.o *~

Undefined reference to function when linking in C++

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