initializing class A (with a reference member) within class B - c++

Question

i'm trying to make a simple currency converter and I have some problems with my class designed to make the calculations.
I want to create an object of this class in my MainWindow to use the total variable for displaying the result in a widget on my appi form.
Member functions aren't all here yet, but the problem I have is that i get a
'Calculator' : no appropriate default constructor available error in mainwindow.cpp
While I do know what that means, I don't know how to avoid (solve) the issue because i kind of need the reference on the map to get the echange rates needed.
.cpp
#include <QObject>
#include <QString>
#include <QDebug>
#include <ui_mainwindow.h>
class Calculator
{
public:
explicit Calculator(QMap<QString,double> &currency_map);
void multiply(double x, double y);
void getValues(QString strFrom, QString strTo);
private:
double total, firstCurr, secondCurr;
QMap<QString,double> &map;
};
.h
#include "calculator.h"
Calculator::Calculator(QMap<QString,double> &currency_map):map(currency_map)
{
total = 0;
firstCurr = 0;
secondCurr= 0;
}
void Calculator::getValues(QString strFrom, QString strTo)
{
QMapIterator<QString, double> i(map);
while(i.hasNext())
{
if(i.key() == strFrom)
firstCurr=i.value();
if(i.key() == strTo)
secondCurr = i.value();
}
}
void Calculator::multiply(double x, double y)
{
total = x * y;
}
and now im trying to create an object of that class in my MainWindow class:
#include <QMainWindow>
#include <ui_mainwindow.h>
#include "calculator.h"
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QMap<QString,double> &currency_map, QWidget *parent = 0);
~MainWindow();
private:
Ui::MainWindow *ui;
parser currency_parser;
Calculator calc; // error
};
MainWindow.h
MainWindow::MainWindow(QMap<QString, double> &currency_map, QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
ui->from_Combox->addItems(currency_parser.currency_list);
ui->to_Combox->addItems(currency_parser.currency_list);
}
the idea behind was that i have a Map in my main in which i save all my data needed and pass the map around to classes that use it.
i'll post my main even tho im not sure if it's needed
#include "downloader.h"
#include "mainwindow.h"
#include "parser.h"
#include "calculator.h"
#include <QApplication>
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QMap<QString,double> currency_map;
downloader d;
d.Do_download();
parser p;
p.read_line(currency_map);
MainWindow w(currency_map);
w.show();
return a.exec();
};

Answer 1

The error occurs, because you don't specify the appropriate constructor for class Calculator in the MainWindow constructors member initializer list (the compiler want's to use the default constructor then):
MainWindow::MainWindow(QMap<QString, double> &currency_map, QWidget *parent)
: QMainWindow(parent)
, ui(new Ui::MainWindow)
, calc(currency_map) // <<<<<<<<<<<<<<
{
// ...
}