如何初始化C ++类成员

Question

I am new to c++ and I have the following code in VS2010 using Qt:

// test.h
#pragma once

#include <QtWidgets/QMainWindow>
#include "ui_test.h"
#include "testDevice.h"
#include "testControl.h"

class test : public QMainWindow
{
    Q_OBJECT

public:
    test(QWidget *parent = 0) : control(device,0) {}
    ~test();

private:
    Ui::testClass ui;
    testDevice device;
    testControl control;
};


// test.cpp
#include "test.h"

test::test(QWidget *parent) : QMainWindow(parent)
{
    ui.setupUi(this);    
    device.start();  
    control.start();

}    

test::~test()
{

}

// testControl.h
#pragma once

#include <QThread>
#include "testDevice.h"

class testControl : public QThread 
{
    Q_OBJECT

public:
    testControl(testDevice& device, QObject *parent = 0);

protected:
    void run(void);

private:
    testDevice device;

    ~testControl(void);
};

// testControl.cpp

#include "testControl.h"


testControl::testControl(testDevice& thisDevice, QObject *parent) : QThread(parent) 
{
}

void testControl::run(void)
{
}


testControl::~testControl(void)
{
}

VS2010 is saying "no appropriate default constructor available" and it marks:

test::test(QWidget *parent) : QMainWindow(parent)
{

and

test::~test()

as the error source. I have tried to use testControl as a pointer and as a reference but I got many other errors...could someone please help me out with this one and explain to me whats going on?

Answer 1

You are providing an implicitly inline definition of test 's constructor in the test class definition in the header:

test(QWidget *parent = 0) : control(device,0) {}

This is most likely what the compiler is complaining about. Besides that, you go on to provide a different definition in the .cpp file. You can only have one definition.

There are two ways to fix this.

1) implicitly inline definition of the constructor in the class definition. Modify your existing header code to call the appropriate QMainWindow constructor in the initialization list. And remove the constructor definition from the .cpp file:

// in test.h
test(QWidget* parent = 0) : QMainWindow(parent), control(device, 0) 
{
  ui.setupUi(this);    
  device.start();  
  control.start();
}

2) Declare the constructor in the header, and define it in the .cpp :

// in test.h
test(QWidget* parent = 0); // declaration, no definition

// in test.cpp
test::test(QWidget* parent = 0) : QMainWindow(parent), control(device, 0) 
{
  ui.setupUi(this);    
  device.start();  
  control.start();
}

Either of these solutions should work. What you can't have is a combination of the two, which is what your code sample exhibits.

You may also need a definition of test::~test() . In your code sample you only show a declaration.

Answer 2

You have two major issues ongoing:

1) You should probably write this constructor leaving the definition for the constructor in the header behind.

You ran into the trap of two definitions provided, one in the header, and the other in the source file.

You either need to have the declaration in the header, and definition in the source, or the declaration and definition in the header. You cannot have one declaration and two definitions distributed in the header and source files!

You had the empty definition in the header by {} and the other definition with { .... } in the source file. How would the compiler know which one you really wanted? It cannot reliably... even if it picks up either version.

2) Also, your testControl destructor seems to be private. That is not a good idea. Change that destructor to public!

test.h

#pragma once

#include <QtWidgets/QMainWindow>
#include "ui_test.h"
#include "testDevice.h"
#include "testControl.h"

class test : public QMainWindow
{
    Q_OBJECT

public:
    // Only declare, do not define it here
    // Also, for completeness: you should use the "explicit" keyword in here.
    explicit test(QWidget *parent = 0);
    ~test();

private:
    Ui::testClass ui;
    testDevice device;
    testControl control;
};

test.cpp

#include "test.h"

test::test(QWidget *parent = 0)
    : QMainWindow(parent)
    , control(device,0)
{
    ui.setupUi(this);    
    device.start();  
    control.start();
}

test::~test()
{
}

testControl.h

#pragma once

#include <QThread>
#include "testDevice.h"

class testControl : public QThread 
{
    Q_OBJECT

public:
    testControl(testDevice& device, QObject *parent = 0);
    // Change this to being _public_ rather than _private_ as in your code.
    ~testControl(void);

protected:
    void run(void);

private:
    testDevice device;
};

Answer 3

When a class is constructed, all the data members are constructed as well. If there is a default constructor for a particular data member, that one is executed.

If you need to execute a specific non-default constructor, you must use the member initialisation list in order to construct that particular data member.

I suspect that your problem is with either your

testDevice device;
testControl control;

data members, or both.

What are the available constructors for both these classes? If there are no default constructors available, you must initialise them in the members' initialisation list, supplying appropriate parameter arguments.