将类添加到基本Qt GUI应用程序时出现Qt链接器错误

我已经挣扎了一段时间，无法解决这个问题。我在我的基本QtGUI应用程序中添加了一个类，但我得到了链接器错误。我不知道是什么原因造成的，希望得到反馈。

链接器错误：

main.obj:-1: error: LNK2019: unresolved external symbol "public: void __thiscall Serial::start(enum QSerialPort::BaudRate)" (?start@Serial@@QAEXW4BaudRate@QSerialPort@@@Z) referenced in function _main
main.obj:-1: error: LNK2019: unresolved external symbol "public: __thiscall Serial::Serial(class QObject *)" (??0Serial@@QAE@PAVQObject@@@Z) referenced in function _main
debugTest.exe:-1: error: LNK1120: 2 unresolved externals

这是项目文件：

#-------------------------------------------------
#
# Project created by QtCreator 2013-11-18T10:08:32
#
#-------------------------------------------------
QT       += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = Test
TEMPLATE = app

SOURCES += main.cpp
        mainwindow.cpp 
    serial.cpp
HEADERS  += mainwindow.h 
    serial.h
FORMS    += mainwindow.ui

serial.h文件：

#ifndef SERIAL_H
#define SERIAL_H
#include <QObject>
#include <QtSerialPort/QSerialPort>
class Serial : public QObject
{
    Q_OBJECT
public:
    explicit Serial(QObject *parent = 0);
    void start(QSerialPort::BaudRate baudRate);
    bool Serial::closePort();
    void Serial::readData();
signals:
public slots:
};
#endif // SERIAL_H

serial.cpp文件：

#include "serial.h"
#include <QObject>
#include <QtSerialPort/QSerialPort>
Serial::Serial(QObject *parent) :
    QObject(parent)
{
    serial = new QSerialPort(this);
    connect(serial, SIGNAL(readyRead()), this, SLOT(readData()));
}
bool Serial::start(QSerialPort::BaudRate baudRate)
{
    if(serial.open(QIODevice::ReadWrite))
    {
        //TODO: Connection OK is green in GUI
        QDebug << "Port opened successfully" << std::endl;
        return true;
    }
    else
    {
        //Open failed.
        QDebug << "Port failed to open" << std::endl;
        //TODO: Show error window in GUI
        return false;
    }
    if(
        serial.setBaudRate(baudRate) &&
        serial.setDataBits(QSerialPort::Data8) &&
        serial.setParity(QSerialPort::NoParity) &&
        serial.setStopBits(QSerialPort::OneStop) &&
        serial.setFlowControl(QSerialPort::NoFlowControl)
      )
    {
        //Connection ok.
        QDebug << "Parameters set OK" << std::endl;
    }
    else
    {
        QDebug << "Parameters set FAILED" << std::endl;
        Serial::closePort();
    }
}
bool Serial::closePort()
{
    serial.close();
    //TODO: Connection OK is red in GUI
}
void Serial::readData()
{
    this->readDataArray = this->serial.readAll();
}