QVector append costum data type

pauledd

Hi there

This might be a more C++ related question but since it uses the QVector I will try it here.

I've made a costum data type made of a simple struct that holds three short int's

struct RGB
{
   short red;
   short green;
   short blue;
};

then I created a QVector of type RGB

QVector<RGB> vec;

Now I want to append data to the QVector like this

blue = 3882;
red = 9293;
green = 3828;
vec.append(RGB(red,green,blue));

I dont know how this kind of use is called but I use that very often in Qt with its own data types.
How can I make my own RGB data type able to do this in that way?

Of cause when I try to compile this I get an error:

aha_1980

@pauledd said in QVector append costum data type:

struct RGB
{
short red;
short green;
short blue;
};

Most likely you simply have to add a constructor to initialize the members to your struct:

struct RGB
{
  RGB(short red, short green, short blue) : red(red), green(green), blue(blue) {}

   short red;
   short green;
   short blue;
};

Happy New Year!

VRonin

You are just using the wrong brackets: vecImage.append(RGB(red,green,blue)); should be vecData.append(RGB{red,green,blue}); see http://en.cppreference.com/w/c/language/struct_initialization

I tend to agree, however, with @aha_1980 in saying that a specific constructor is more robust or at least use the named version

pauledd

Ok first
@aha_1980
I dont know why but this doesnt work. Here's my complete code:
mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>

struct RGB
{
    RGB(short red, short green, short blue) : red(red), green(green), blue(blue) {}
    short red;
    short green;
    short blue;
};

namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
    Q_OBJECT

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

private:
    Ui::MainWindow *ui;
    QVector<RGB> vec;
};

#endif // MAINWINDOW_H

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    short red = 23;
    short green = 22;
    short blue = 28;
    vec.append(RGB(red,green,blue));
}

MainWindow::~MainWindow()
{
    delete ui;
}

I get compile error:

/usr/include/qt5/QtCore/qvector.h:319: Fehler: no matching function for call to ‘RGB::RGB()’
             new (from++) T();
             ^

And @VRonin
if I use no constructor as suggested by aha_1980 and I use the curly brackets my app crashes, with constructor it does not crash, but compiles the same error as above.

mrjj

Hi
it tells you
"no matching function for call to ‘RGB::RGB()’"

Which means, it cannot find a default constructor that takes no arguments.
which vector want for resize and moving stuff around. ( i think )
so you need to add that to your RGB class if it wants it.

btw, Qt has QRgb :)

VRonin

RGB(short red, short green, short blue) : red(red), green(green), blue(blue) {} should be RGB(short red=0, short green=0, short blue=0) : red(red), green(green), blue(blue) {}

I use the curly brackets my app crashes

That means you have another problem somewhere else that is unrelated to this one. Check the stack trace to see what went wrong

pauledd

ok, maybe I should go through this first:

Creating a Custom Type

*Before we begin, we need to ensure that the custom type we are creating meets all the requirements imposed by QMetaType. In other words, it must provide:

    a public default constructor,
    a public copy constructor, and
    a public destructor.*

http://doc.qt.io/qt-5/custom-types.html

pauledd

@mrjj said in QVector append costum data type:

btw, Qt has QRgb :)

Yes :D but I would take this as a lesson because I am sure I would stumble over
the problem in the future.

pauledd

Ok , sorry I found I already asked something simmilar:
https://forum.qt.io/topic/71761/how-to-append-struct-to-qlist/9
I'll try to solve by my own and report back.

pauledd

I solved it.
My little struct exploded into a class as described in the above linked thread.
I added a standart/normal/copy constructor and a descructor,
a assignment operator and this, to me, mysterious "encapsulate members". I can just guess this is some kind of getter/setter accessing
the private members.

class RGB
{
public:
    RGB(){}                                             // standart constructor
    ~RGB(){}                                            // destructor
    RGB(const RGB &other)
        :r_(other.r_),g_(other.g_),b_(other.b_){}       // copy constructor
    RGB(short r, short g, short b)                      // normal constructor
        :r_(r), g_(g),b_(b){}
    RGB& operator = (const RGB& other)                  // assignment operator
    {
        r_ = other.r_;
        b_ = other.b_;
        g_ = other.g_;
        return *this;
    }
    const short& r() const {return r_;}                 // encapsulate members
    const short& g() const {return g_;}
    const short& b() const {return b_;}
    void setR(const short& val){r_ = val;}
    void setG(const short& val){g_ = val;}
    void setB(const short& val){b_ = val;}

private:
    short r_;
    short g_;
    short b_;

};

@mrjj said in QVector append costum data type:

btw, Qt has QRgb :)

If I read correctly QColor Class, the whole thing was not actually not just an exercise for me because QColor seems to not support 16bit colors like 0-65536 as such but only "getRgbF" with values from 0.0 to 1.0. Shure I could use something like 0.22816 but I would like to keep the same range as used in OpenCV Datatype cv::Vec3s.

The long and the short of it:
I can now use my RGB type directly to append a new value
to my QVector:

mrjj

mysterious "encapsulate members".

Hi, nothing mysterious about it. :)
It just means hide the details.
This give you the freedom to later change the class internal working without
breaking the program/ requires modifications outside class.

VRonin

Your default constructor leaves the values initialised with junk, that's not a great idea usually
Your destructor, copy constructor and assignment operators are all doing the default. =default them or remove them altogether
short is one of those rare cases where passing/returning by value is generally more efficient than by reference/pointer (16bits vs >=32bit)

class RGB
{
public:
    ~RGB()=default;                                            // destructor
    RGB(const RGB &other)=default;       // copy constructor
    RGB(short r=0, short g=0, short b=0)                      // normal constructor
        :r_(r), g_(g),b_(b){}
    RGB& operator = (const RGB& other)=default;                  // assignment operator
    short r() const {return r_;}                 // encapsulate members
    short g() const {return g_;}
    short b() const {return b_;}
    void setR(short val){r_ = val;}
    void setG(short val){g_ = val;}
    void setB(short val){b_ = val;}

private:
    short r_;
    short g_;
    short b_;

}

pauledd

Thanks for your information/hints! I will correct the remaining suggestions, and this should be solved.