通过引用共享对象（在Qt中）

Question

I would like to share an object (a QDir) between several pages of a QWizard, so that if one page changes the value of this directory, this carries through to all other pages.nSince this object is not optional, I would like to share it by passing a reference rather than a pointer so I do not need to worry about a possible null pointer (is this correct? I've read opinions both ways). Here is a minimal test case:

test.hpp:

#include <QApplication>
#include <QWizard>
#include <QDir>
#include <QVBoxLayout>
#include <QWizardPage>
#include <QLineEdit>

class TestWizard : public QWizard {
    Q_OBJECT
public:
    TestWizard(QWidget* parent = 0);
    QDir directory;
};

class TestPage : public QWizardPage {
    Q_OBJECT

public:
    TestPage(QDir& _directory, int _id, QWidget* parent = 0);
    void initializePage();

public slots:
    void setDirectory(QString new_dir);

private:
    QVBoxLayout* layout;
    QDir directory;
    QLineEdit* dir_edit;
    int id;

};

test.cpp:

#include "test.hpp"
#include <iostream>

TestWizard::TestWizard(QWidget* parent) : QWizard(parent){
    directory = QDir();
    addPage(new TestPage(directory, 0));
    addPage(new TestPage(directory, 1));
    addPage(new TestPage(directory, 2));
}
TestPage::TestPage(QDir& _directory, int _id, QWidget* parent)
    : QWizardPage(parent){

    directory = _directory;
    id = _id;

    layout = new QVBoxLayout;
    dir_edit = new QLineEdit;
    layout->addWidget(dir_edit);
    this->setLayout(layout);
    connect(dir_edit, SIGNAL(textChanged(QString)),
        this, SLOT(setDirectory(QString)));
}
void TestPage::initializePage(){
    std::cout << id << ": " << directory.absolutePath().toUtf8().constData()
              << std::endl;
}
void TestPage::setDirectory(QString new_dir){
    directory.setPath(new_dir);
}
int main(int argc, char* argv[]){
    QApplication app(argc, argv);
    TestWizard wizard;
    wizard.show();
    return app.exec();
}

Clicking through this and editing the text gives the result that changing one text only affects the directory variable on one page. I know how to do this with a pointer, but I would like to understand why this does not work.

It looks like I have a basic misconception somewhere about what is behind this and I'd like to clear it up before I do something stupid :)

Answer 1

I think I have been a bit of an idiot, but just to confirm:

test.cpp:

TestPage::TestPage(QDir& _directory, int _id, QWidget* parent)
    : QWizardPage(parent){

    directory = _directory; // This is where I'm going wrong
...