Accessing a MinWindow function from another function outside MainWindow Qt - c++

Question

I have a Qt gui project and in the "mainwindow.cpp" file I have to define a function that I cannot declare under "mainwindow.h". But I want to call that function (func_sqrt) under MainWindow and show the result value of my func_sqrt in a label. For some reason I need to do that so. But I don't know how to connect that function to the gui objects. My code looks like this:
#include "mainwindow.h"
#include "ui_mainwindow.h"
QString input;
void func_sqrt(int x);
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
func_sqrt(2);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::showtext(QString txt)
{
ui->lbl_value->setText(txt);
}
void func_sqrt(int x)
{
int y;
y = x*x;
}
I added this part to the func_sqrt function, but it doesn't work:
MainWindow *w = new MainWindow;
w->showtext(QString::number(y));

Answer 1

First of all, your func_sqrt currently is a no-op. Next, internally it calculates a square yet its name says sqrt (short for square root), you might want to check whether the name is consistent with semantics. This is for the side notes.
If you need this function available outside of mainwindow.cpp, you can declare it in a separate header and include it everywhere as needed. Alternatively, you can declare it in every file it is used in, the linker will later resolve the actual implementation. For instance, in a file subordinatewindow.cpp:
void func_sqrt(int);
// ...
int x{42};
func_sqrt(x);