Question

I’m new to C++, switching over from Java for a class I’m taking in college. In our first assignment we have to write several makefiles for our code. My first makefile isn’t working – it always complains about an error in “TempControlTest: undefined reference to ‘BangBangControl::XXXX’, where XXXX a function in BangBangControl (I get errors for every function in BangBangControl). I’ve been looking around online and all the makefile examples look slightly different so I can’t tell what I’m doing wrong. I’ll post all my code below, if anyone could just give me a little help, that would be AWESOME. Also, this is due tonight, so any quick help is appreciated so much. If you’re still here, thanks very much!

HeatingUnit.h:

#ifndef HEATINGUNIT_H
#define HEATINGUNIT_H

class HeatingUnit {

private:
        bool on;
        int temp;

public:
        HeatingUnit(bool o, int t);
        void turnOn();
        void turnOff();
        int tick();
};

#endif

HeatingUnit.cpp:

#include "HeatingUnit.h"

HeatingUnit::HeatingUnit(bool o, int t) {
        on = o;
        temp = t;
}

void HeatingUnit::turnOn() {
        on = true;
}

void HeatingUnit::turnOff() {
        on = false;
}

int HeatingUnit::tick() {
        if(on) return ++temp + 1;
        else return --temp;
}

BangBangControl.h:

#ifndef BANGBANGCONTROL_H
#define BANGBANGCONTROL_H

#include "HeatingUnit.h"

class BangBangControl {

private:
        int tempToKeep;
        HeatingUnit unit;

public:
        BangBangControl(int ttk, bool on, int initTemp);
        void setTemp(int t);
        int getTemp();
        void update();
};

#endif

BangBangControl.cpp:

#include "HeatingUnit.h"
#include "BangBangControl.h"

BangBangControl::BangBangControl(int ttk, bool on, int initTemp) 
: unit(on, initTemp) {
        tempToKeep = ttk;
}

void BangBangControl::setTemp(int t) {
        tempToKeep = t;
}

int BangBangControl::getTemp() {
        return tempToKeep;
}

void BangBangControl::update() {
        int currentTemp = unit.tick();
        if(currentTemp > tempToKeep + 2) unit.turnOff();

        else if(currentTemp < tempToKeep - 2) unit.turnOn();
}

TempControlTest.cpp:

include <iostream>
#include "BangBangControl.h"
using namespace std;

int main() {
        BangBangControl bang(100, false, 60);
        for(int i = 0; i < 50; i++) {
                bang.update();
                cout << bang.getTemp() << " ";
                if(i % 10 == 9) cout << endl;
        }
}

And finally, Makefile_Executable:

all: TempControlTest

TempControlTest: TempControlTest.cpp BangBangControl.o
        g++ -o TempControlTest TempControlTest.cpp

BangBangControl.o: BangBangControl.cpp HeatingUnit.o
        g++ -c BangBangControl.cpp

HeatingUnit.o: HeatingUnit.cpp
        g++ -c HeatingUnit.cpp

clean:
        rm -rf *.o TempControlTest

Again, thank you very much!

Answer 1

You aren’t linking your objects together.

You’ve got two lines compiling BangBangControl.cpp and HeatingUnit.cpp to two .o files (although I doubt you need to have BangBangControl.o depend on HeatingUnit.o). But you need to put them together into the final executable (this is called “linking”).

TempControlTest: TempControlTest.cpp BangBangControl.o HeatingUnit.o
        g++ -o TempControlTest TempControlTest.cpp HeatingUnit.o BangBangControl.o

In the line above, TempControlTest.cpp will be itself compiled, and then linked together with the two already-compiled objects on which it seems to depend into one executable.

NB: when you get an “Undefined reference” error, it generally means your code compiles fine but was missing a symbol it needed to be linked against.

NB2: your makefile rules don’t cause the cpp files to depend on the headers they use. That means when you change a header and run “make”, make will tell you everything’s already built. That’s bad.