#include "mainwindow.h"
#include "currencyconverter.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    CurrencyConverter *currency = new CurrencyConverter(parent);

    setCentralWidget(currency);
}

and my main .pro file looks like this:

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++11

# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Please consult the documentation of the
# deprecated API in order to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0
INCLUDEPATH += \
              ../CurrencyConverter

LIBS += \
            -L../CurrencyConverter

SOURCES += \
    main.cpp \
    mainwindow.cpp

HEADERS += \
    mainwindow.h

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

When compiling, I'm still getting undefined reference to CurrencyConverter at line where currency pointer is initiaze. the first bullet seems not working for me.
for the second bullet, what you do you mean by "build the files needed within the test sub project." ho do you do that in .pro file?

@Bonnie is right. Your current issue is that you don't build the class along your test hence your error.

You have several way of solving this:

• make a library out of the components of your application and link against it. You app project will mainly be a main.cpp file creating the needed objects using that library as well.
• stay as you are currently and build the files needed within the test sub project.

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class View; }
QT_END_NAMESPACE

class Controller;

class View : public QMainWindow
{
    Q_OBJECT

public:
    View(QWidget *parent = nullptr);
    ~View();
    void setController(Controller *controller);

public slots:
    void set_amount(const QString& val);
    void get_recieve();

private:
    Ui::View *ui;
    Controller *m_controller;
    friend class testConverter;
};

and its cpp

View::View(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::View)
    , m_controller(nullptr)
{
    ui->setupUi(this);
    ui->amount_el->setText(QString::number(0.00));

    connect(ui->amount_el, SIGNAL(textChanged(const QString &)),
            this, SLOT(set_amount(const QString &)));

    connect(ui->convert_btn, SIGNAL(clicked()),
            this, SLOT(get_recieve()));
}

The parent by default is nullptr