Undefined reference to global array

Question

I have an array called vel declared inside global.h and defined inside global.cpp. When I try to use it inside a function, get_velocities(), of another class called Robot (inside Robot.cpp), it says:

undefined reference to `vel'

Here are the three files: 1) global.h

#ifndef GLOBAL_H_INCLUDED
#define GLOBAL_H_INCLUDED
#include <array>

using std::cout;
using std::cin;
using std::endl;
using std::array;

static constexpr const int marker_num = 10;
static constexpr const int dim = (2 * marker_num) + 3;
extern array <float, 3> vel;

#endif // GLOBAL_H_INCLUDED

2) global.cpp

#include <iostream>
#include <cstdio>
#include <cmath>
#include "global.h"
#include "WorldState.h"
#include "Robot.h"
#include "Sensor.h"
#include "Marker.h"

array <float, 3> vel = {0.0, 0.0, 0.0};

3) Robot.cpp

#include <iostream>
#include <cstdio>
#include <cmath>
#include "global.h"
#include "WorldState.h"
#include "Robot.h"
#include "Sensor.h"
#include "Marker.h"


Robot::Robot(float a, float b, float c){

    //ctor
    x = a;
    y = b;
    theta = c;
}

void Robot::get_velocities(){

    v_tx = 1.0;
    v_ty = 0.0;
    omega_t = 0.0;

    vel = {v_tx, v_ty, omega_t};

}

Edit:

I did read this question . What I realized was that the global variable requires not just a declaration but also a definition. I have provided this definition inside global.cpp. Also when I include #include "global.cpp" in Robot.cpp, it works (But this is not an acceptable practice). So, I believe this error is due to global.cpp not linking properly.

1) Isn't it a common practice to declare global variables in global.h and keep the definitions in global.cpp? How do I link them properly? I believe that one way is to create a proper make file. However, I am using codeblocks IDE. How do I do it in this IDE?

2) Is it better to eliminate global.cpp and do all definitions for global variables and functions inside the main or the file that uses them?

Is there any compiling warninga if you compile with `-W -Wall` flags? — jraynal, Mar 14 '17 at 18:50
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) — 1201ProgramAlarm, Mar 14 '17 at 18:55
I tried enabling `-Wall`. I received no additional messages. — skr, Mar 14 '17 at 18:56
Why so many includes in `global.cpp` if it just contains a single variable? — Bo Persson, Mar 14 '17 at 19:22
@SyamprasadKR: That's bad coding style. You should include only what you need. — Christian Hackl, Mar 14 '17 at 20:03

score 1 · Accepted Answer · edited May 23 '17 at 12:00

1

My problem was that the Codeblocks IDE was not linking global.cpp. I found the answer here (first answer). I apologize if this question is a duplicate or combination of duplicates.

edited May 23 '17 at 12:00

Community

1
1

answered Mar 14 '17 at 19:55

skr

914
3
18
35

Codeblocks IDE neither compiles nor links anything. It is a graphical interface which invokes the real tools. – Christian Hackl Mar 14 '17 at 20:07
1

I apologize for such mistakes. I am still learning. I guess the proper thing to say was that the setting of the IDE didn't invoke the tools to link required file. – skr Mar 14 '17 at 20:13
1

I have observed that IDEs often hide or obfuscate what's really happening; they keep beginners from understanding the fundamental basics of the build process. I would advise all beginners to start with a very simple text editor like Notepad+ and use the command-line tools. – Christian Hackl Mar 14 '17 at 20:16

score 0 · Answer 2 · answered Mar 14 '17 at 18:51

0

Declare and define the array vel within global.h. Maybe storing them within the same class would help.

answered Mar 14 '17 at 18:51

FutureProgrammer

45
12

I am including `global.h` in many places. It will throw `multiple definition error`. – skr Mar 14 '17 at 18:53

Undefined reference to global array

2 Answers2