Undefined reference to function call?

Question

I have 2 class as follows each has header file class one has a function as follows:

  int call_thread() 
  {
    pthread_create(&thread, NULL, &Print_data, NULL);
    return 0;
  }

I am trying to call this method in class 2:

void position::tick(schedflags_t flags) 
{
    call_thread();
    }

I always get an error undefined reference to 'call_thread()'. I also tried to declare it as static but it gave me an error: that is "" Static function declared but not defined"". What am I missing? Note: I included the header file of class 1 ofcourse.

How are you compiling? Seems to me that compiler can't find the .cpp file with the definition. — olevegard, Jun 18 '13 at 11:44
If `call_thread()` is a mere function, it MUST be declared at least once in a `.h`-file or even `.cpp`-file — bash.d, Jun 18 '13 at 11:45
the call_thread is declared in the h file of the 1st class and the second class just includes the h file of the 1st — Zeyad, Jun 18 '13 at 11:48
@olevegard I am comping using a g++ and including the -lpthread — Zeyad, Jun 18 '13 at 11:49
@Zeyad what's the full compiler command? Does it include the .cpp file in which the function is defined? — olevegard, Jun 18 '13 at 11:51
There are three different meanings to "static" in C++, I guess you are confused there. Please distill a minimal example from your code, from there it should be obvious. Also, read the FAQ concerning the differences between a function and a memberfunction, that info is essential! — doomster, Jun 18 '13 at 11:51
g++ pos.cpp -o position -lpthread this is what i used to compile exactly — Zeyad, Jun 18 '13 at 11:53
You need all *cpp files on that commandline, #include doesn't solve that for you. — doomster, Jun 18 '13 at 11:54
possible duplicate of [What is an undefined reference/unresolved external symbol error and how do I fix it?](http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix). In this case, the problem is probably [this one](http://stackoverflow.com/a/12574400/20984). — Luc Touraille, Jun 18 '13 at 11:59
@Zeyad Does pos.cpp contain the definition for the function? — olevegard, Jun 18 '13 at 12:10

score 2 · Answer 1 · answered Jun 18 '13 at 12:18

My guess is that you have declared and defined call_thread() in Class1:

class Class1
{
    public:
    int call_thread(){...}
}

Then you are trying to call this method in class position:

void position::tick(schedflags_t flags) 
{
    call_thread();
}

call_thread() is a Class1 member function and you need either an instance of Class1 to call it or a class name if it's static member:

void position::tick(schedflags_t flags) 
{
    //for static member
    Class1::call_thread(); 

    //for instance member
    Class1 object;
    object.call_thread();
}

flazzari · Answer 2 · 2013-06-19T20:09:36.647

1

If you have classes you should have objects so you can call the public methods (functions).

Try something like below:

in Class1.h:

class Class1{ 
public:
  Class1(); //constructor
  ~Class1(); //destructor

  int call_thread();
}

Then in Class2 you should have an object, something like:

void position::tick(schedflags_t flags) 
{
  Class1 obj;
  obj.call_thread();
}

edited Jun 19 '13 at 20:09

answered Jun 18 '13 at 12:32

flazzari

173
5

Undefined reference to function call?

2 Answers2