Qt：从类 MainWindow 访问函数：其他文件中的公共 QMainWindow

Question

So I have in mainwindow.h :

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

    void SetBoxTest(const QString &Text);

[...]

and in mainwindow.cpp :

void MainWindow::SetBoxTest(const QString &Text) {
    ui->plainTextEdit->setPlainText(Text);
}

1. I want access SetBoxTest in other .cpp file. I included mainwindow.h and now what? How to properly access SetBoxTest function?

2. Is accessing UI in that way is correct?

3. Also I saw this const QString &Text somewhere, why shouldn't I just put QString Text for such function type (which sets text in text box)? What's better?

EDIT: When I try to do it like :

MainWindow.SetBoxTest(DataString);

or

MainWindow.SetBoxTest(DataString);

It says I'm missing ; before . missing ; before .

Answer 1

1. What are you trying to do exactly ? If you want to modify your MainWindow PlainTextEdit from an other UI file, you can emit a signal.

2. Yes.

3. This is a so called « lvalue-reference to const ». It denotes a reference to a const object (here a QString). The fact is that if you just write :

    void SetBoxTest(QString Text);

Since your QString is passed by value, it will be copied . With a reference, it won't be copied at all (a reference is just an alias). A reference is then more efficient than passing by value.

However, Qt tries to optimize copies by using what they call Implicit Sharing

Answer 2

1. if your MainWindow object name is window, just do window.SetBoxTest(); or uses -> if you are using a pointer
2. ui->plainTextEdit ... I dont see ui defined... did you used qt creator to create a form?
3. const QString &Text is passing by reference .

Learn the C++ fundamentals, most of these points have nothing to do specifically with qt

Answer 3

I want access SetBoxTest in other .cpp file. I included "mainwindow.h" and now what? How to properly access SetBoxTest function?

In addition to including "mainwindow.h", you just need a pointer to your main window, and then you can call window->SetBoxTest("Hello World");

Is accessing UI in that way is correct?

This is a pretty loaded question. My opinion is yes that is good, much better than letting other classes access your main window's UI directly.

Also I saw this "const QString &Text" somewhere, why shouldn't I just put "QString Text" for such function type (which sets text in text box)? What's better?

Generally, const QString &text is better because you are passing a reference, and that takes less time than QString text , which is passing a copy. See here for an explanation.

Answer 4

1. is in the other cpp file a instance of mainwindow running?
2. maybe ... we will see what your first answer is ;)
3. the const QString& is just more secure. if you remove the const from your code, it wont change anything but it ensures you, that you cant change it/get a compile error if you try. the reference is so you have a qstring and give it to the function ... not just create it cuz you need it.

//edit: and the reference will maybe save you some memory cause it wont copy the whole string just the pointer.

soo long zai