Undefined symbols for architecture x86_64 in cpp - c++

Question

I have a programme that I am writing in Qt Creator, and I am having some compilation issues. This is the error it gives:
Undefined symbols for architecture x86_64:
"VehicleSizer::VehicleSizer(double, double, double, double, double, double, double, double, int)", referenced from:
_main in MSSTO_SimulationTester.cpp.o
Reading other questions I thought it would be due to the mismatch between the declared function definitions in the header and the source file, or that the static variables must be initialised, but I don't think that is the case here. Here is my main file:
#include "Vehicle/vehicleSizer.h"
#include "Vehicle/vehicleSizerAbstract.h"
#include <iostream>
#include <stdio.h>
#include <cmath>
#include <string>
#include <boost/bind.hpp>
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>
int main( )
{
{
double totalBurnTimeUp = 100.;
double massFlowUpper = 80.;
double chamberPressure = 200000.;
double exitDiameterUp = 1.2;
double oxidizerOverFuel = 3.4;
double upperStageDiameter = 6;
double payloadMass = 1000;
double __landingburn = 1.0;
int __engines = 5;
UpperStage = std::shared_ptr< VehicleSizerAbstract >(
new VehicleSizer(totalBurnTimeUp,
__landingburn,
massFlowUpper,
chamberPressure,
exitDiameterUp,
oxidizerOverFuel,
upperStageDiameter,
payloadMass,
__engines));
std::shared_ptr<VehicleSizerAbstract> Vehicle;
}
return 0;
}
Here is my abstract header file
#ifndef VEHICLESIZERABSTRACT_H
#define VEHICLESIZERABSTRACT_H
#define _USE_MATH_DEFINES
#include <iostream>
#include <fstream>
#include <cmath>
class VehicleSizerAbstract
{
public:
// some declared virtual functions that return doubles
virtual ~VehicleSizerAbstract(){}
protected:
// some declared variables
};
#endif // VEHICLESIZERABSTRACT_H
Here is my header file
#ifndef VEHICLESIZER_H
#define VEHICLESIZER_H
#define _USE_MATH_DEFINES
#include <iostream>
#include <fstream>
#include <cmath>
#include "vehicleSizerAbstract.h"
class VehicleSizer : public VehicleSizerAbstract
{
public:
VehicleSizer(double m_ascentBurnTime, double m_landingBurnTime,
double m_massFlow, double m_chamberPressure,
double m_exhaustDiameter, double m_mixtureRatio,
double m_vehicleDiameter, double m_payloadMass,
int m_mainEngineAmount);
// some declared functions that return doubles
private:
// some declared void functions
};
#endif // VEHICLESIZER_H
Here is my cpp file:
#include "vehicleSizer.h"
VehicleSizer::VehicleSizer(double m_ascentBurnTime, double m_landingBurnTime,
double m_massFlow, double m_chamberPressure,
double m_exhaustDiameter, double m_mixtureRatio,
double m_vehicleDiameter, double m_payloadMass,
int m_mainEngineAmount)
{
// some functions
}
What am I missing? I have been looking through all of the similar questions and corresponding answers here but nothing seems to apply or to work. Thank you so much!