Error invalid use of this in non member function

I had working on a class and started writing everything in the same .cpp file. However, after a while I could see the class getting bigger and bigger so I decided to split it into a .h and a .cpp file.

gaussian.h file:

class Gaussian{
    private:
        double mean;
        double standardDeviation;
        double variance;
        double precision;
        double precisionMean;
    public:
        Gaussian(double, double);
        ~Gaussian();
        double normalizationConstant(double);
        Gaussian fromPrecisionMean(double, double);
        Gaussian operator * (Gaussian);
        double absoluteDifference (Gaussian);
};

gaussian.cpp file:

#include "gaussian.h"
#include <math.h>
#include "constants.h"
#include <stdlib.h>
#include <iostream>

Gaussian::Gaussian(double mean, double standardDeviation){
    this->mean = mean;
    this->standardDeviation = standardDeviation;
    this->variance = sqrt(standardDeviation);
    this->precision = 1.0/variance;
    this->precisionMean = precision*mean;
} 

//Code for the rest of the functions...

double absoluteDifference (Gaussian aux){
    double absolute = abs(this->precisionMean - aux.precisionMean);
    double square = abs(this->precision - aux.precision);
    if (absolute > square)
        return absolute;
    else
        return square;
}

However, I can’t get this to compile. I try running:

g++ -I. -c -w gaussian.cpp

But I get:

gaussian.cpp: In function ‘double absoluteDifference(Gaussian)’:
gaussian.cpp:37:27: error: invalid use of ‘this’ in non-member function
gaussian.h:7:16: error: ‘double Gaussian::precisionMean’ is private
gaussian.cpp:37:53: error: within this context
gaussian.cpp:38:25: error: invalid use of ‘this’ in non-member function
gaussian.h:6:16: error: ‘double Gaussian::precision’ is private
gaussian.cpp:38:47: error: within this context

Why can’t I use this?? I am using it in the fromPrecisionMean function and that compiles. Is it because that function returns a Gaussian? Any extra explanation will be really appreciated, I am trying to learn as much as I can! Thanks!

You just tried to copy some code without understanding what was going on.

It looks like you saw a code like this :

#include <QtGui>
#include <QWidget>
 
class MyWidget : public QWidget
{
    Q_OBJECT
public:
 
protected:
    void paintEvent(QPaintEvent *event)
    {
        //create a QPainter and pass a pointer to the device.
        //A paint device can be a QWidget, a QPixmap or a QImage
        QPainter painter(this);   // <- Look at this !!
        //               ^^^^ Allowed    

        // ...
    }

signals:

public slots:
 
};
 
int main( int argc, char **argv )
{
    QApplication app( argc, argv );
 
    MyWidget myWidget;
    myWidget.show();
 
    return app.exec();
}

Here you can see that in the function paintEvent of the class MyWidget we use the this pointer. We can because we use it in a non-static member of the class. Here this is of type MyWidget*.

Look at the standard :

9.3.2 The this pointer [class.this]

In the body of a non-static member function, the keyword this is a prvalue expression whose value is the address of the object for which the function is called. The type of this in a member function of a class X is X*.

So you cannot use the this pointer inside the main function, this is no sense.

And other thing, like it is said in the comment inside the code, you just have to pass a pointer to a device to the QPainter constructor. It can be a QWidget, a QPixmap or a QImage.

Maybe you should read this for a beginning : http://www.cplusplus.com/doc/tutorial/classes/

Digia, Qt and their respective logos are trademarks of Digia Plc in Finland and/or other countries worldwide.

Вы просто пытались скопировать какой-то код, не понимая, что происходит.

Похоже, вы видели такой код:

#include <QtGui>
#include <QWidget>
 
class MyWidget : public QWidget
{
    Q_OBJECT
public:
 
protected:
    void paintEvent(QPaintEvent *event)
    {
        //create a QPainter and pass a pointer to the device.
        //A paint device can be a QWidget, a QPixmap or a QImage
        QPainter painter(this);   // <- Look at this !!
        //               ^^^^ Allowed    

        // ...
    }

signals:

public slots:
 
};
 
int main( int argc, char **argv )
{
    QApplication app( argc, argv );
 
    MyWidget myWidget;
    myWidget.show();
 
    return app.exec();
}

Здесь вы можете видеть, что в функции paintEvent класса class MyWidget мы используем this указатель. Мы можем, потому что мы используем его в нестатическом члене класса. Здесь this имеет тип MyWidget*.

Посмотрите стандарт:

9.3.2 Указатель this [class.this]

В разделе тело нестатической функции-членаключевое слово this является выражением prvalue, значением которого является адрес объекта, для которого вызывается функция. Тип this в функции-члене класса X — X*.

Таким образом, вы не можете использовать this указатель внутри main функция, в этом нет смысла.

И еще, как сказано в комментарии внутри кода, вам просто нужно передать указатель на устройство в QPainter конструктор. Это может быть QWidget, чтобы QPixmap или QImage.

Может быть, вы должны прочитать это для начала: http://www.cplusplus.com/doc/tutorial/classes/

Are you using Qt Creator? If so you should start by adding a new class. Let’s call it Press, then Qt creator will create press.h and press.cpp for your. Press. h will look like this:
@class Press : public QWidget
{
Q_OBJECT
public:
explicit Press(QWidget *parent = 0);

signals:

public slots:

};
@

press.cpp will look like this
@
#include «press.h»

Press::Press(QWidget *parent) :
QWidget(parent)
{
}
@

When you add any private member to your .h file like

@ QPushButton* saveButton;@

You should initialize it in your class constructor’s initializer list. That would give you a .h file that now looks like this

@
#include <QWidget>

class QPushButton;

class Press : public QWidget
{
Q_OBJECT
public:
explicit Press(QWidget *parent = 0);

signals:
    
public slots:
    
private:
    QPushButton* saveButton;

};
@

and a .cpp file that looks like this:

@
#include «press.h»
#include <QPushButton>

Press::Press(QWidget *parent) :
QWidget(parent),
saveButton (new QPushButton(«Save»,this))
{
}
@

Now your saveButton pointer has a legitimate value when the class constructor runs. Of course you have to run the class constructor. So somewhere in main if you want to use the Press class you need the include and a statement that looks like this is you want a solid instance of your object.

@
Press myPress;
@

THEN when you want to use Save in a connect statement you would use a statement like this:

@
QObject::connect(saveButton,SIGNAL(clicked()), &myPress, SLOT(Save()));
@

That line assumes saveButton is a pointer. With savePointer as a solid object main would end up looking something like this

@
Press myPress;
QPushButton saveButton;
QObject::connect(&saveButton,SIGNAL(clicked()), &myPress, SLOT(Save()));
@

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Edit:Fixed memory leaks.