Question

I have this .h file

#ifndef VISUALSETTINGS_H
#define VISUALSETTINGS_H

#include <QPair>
#include <QDataStream>

class VisualSettings
{
    public:
        VisualSettings();
        VisualSettings(QPair<int, int>, QPair<int, int>);
        QPair<int, int> getUpper();
        void setUpper(QPair<int, int>);
        QPair<int, int> getLower();
        QPair<int, int> setLower(QPair<int, int>);

    private:
        QPair<int, int> upper;
        QPair<int, int> lower;
};

QDataStream &operator<<(QDataStream &out, const VisualSettings &vs);
QDataStream &operator>>(QDataStream &in, VisualSettings &vs);

#endif // VISUALSETTINGS_H

And i have this .cpp(a part of it)

#include "visualsettings.h"

VisualSettings::VisualSettings()
{
    upper.first = 0;
    upper.second = 0;
    lower.first = 0;
    lower.second = 0;
}

VisualSettings::VisualSettings(QPair<int, int> u, QPair<int, int> l)
{
    upper.first = u.first;
    upper.second = u.second;
    lower.first = l.first;
    lower.second = l.second;
}

QPair<int, int> VisualSettings::getUpper()
{
   return upper;
}

I am testing this class like this:

QPair<int, int> q1(2,3);
QPair<int, int> q2(4, 5);

VisualSettings v(q1, q2);

//QPair<int, int> q3 = v.getUpper();

//cout << v.getUpper().first;// << " - " << vs.getUpper().second << endl;

I get this error for this constructor:

main.obj:-1: error: LNK2019: unresolved external symbol “public: __thiscall VisualSettings::VisualSettings(struct QPair,struct QPair)” (??0VisualSettings@@QAE@U?$QPair@HH@@0@Z) referenced in function _main

If i use VisualSettings vs() when i do vs.getUpper() i get the error:

C:\Users\Alex\testing-build-desktop-Qt_4_8_0_for_Desktop_-MSVC2010_Qt_SDK__Debug..\testing\main.cpp:23: error: C2228: left of ‘.getUpper’ must have class/struct/union

I noticed that even if a declare a QList and use list.at(0) i get the same error(must have…).

What could be the problem?

LATER EDIT
The answer from @johnathon helped me, but I have another problem. I overwrite the operators << and >> like this:

QDataStream &operator<<(QDataStream &out, const VisualSettings &vs)
{
    out << vs.getUpper().first << vs.getUpper().second;
    out << vs.getLower().first << vs.getLower().second;

    return out;
}

QDataStream &operator>>(QDataStream &in, const VisualSettings &vs)
{
    QPair uaux(0, 0);
    in >> uaux.first >> uaux.second;
    vs.setUpper(uaux);

    QPair laux(0, 0);
    in >> laux.first >> laux.second;
    vs.setLower(laux);

    return in;
}

I get this error:

C:\Users\Alex\testing-build-desktop-Qt_4_8_0_for_Desktop_-MSVC2010_Qt_SDK__Debug..\testing\visualsettings.cpp:42: error: C2662: ‘VisualSettings::getUpper’ : cannot convert ‘this’ pointer from ‘const VisualSettings’ to ‘VisualSettings &’
Conversion loses qualifiers

C:\Users\Alex\testing-build-desktop-Qt_4_8_0_for_Desktop_-MSVC2010_Qt_SDK__Debug..\testing\visualsettings.cpp:42: error: C2228: left of ‘.first’ must have class/struct/union

What could be the problem?

Also I have another question. If at another class i use an VisualSettings object, in .h i declare it like VisualSettings vs, in the constructor of that class, can i say vs = new VisualSettings(1,2) or vs(1,2); so the question is how i call the constructor of VisualSetting in the constructor of another class(if the VisualSettigs object is variable of the class)?

LATER LATER EDIT:

I still got errors at my program and I don’t know how to solve it. So I wanted to overwite operator >>

class VisualSettings
{
............
};

QDataStream &operator<<(QDataStream &out, const VisualSettings &vs);
QDataStream &operator>>(QDataStream &in, VisualSettings &vs);

In the .cpp

QDataStream &operator<<(QDataStream &out, const VisualSettings &vs)
{
    out << vs.getUpper().first << vs.getUpper().second;
    out << vs.getLower().first << vs.getLower().second;

    return out;
}

I get this error:

C:\Users\Alex\testing-build-desktop-Qt_4_8_0_for_Desktop_-MSVC2010_Qt_SDK__Release..\testing\visualsettings.cpp:44: error: C2662: ‘VisualSettings::getLower’ : cannot convert ‘this’ pointer from ‘const VisualSettings’ to ‘VisualSettings &’
Conversion loses qualifiers
C:\Users\Alex\testing-build-desktop-Qt_4_8_0_for_Desktop_-MSVC2010_Qt_SDK__Release..\testing\visualsettings.cpp:44: error: C2228: left of ‘.first’ must have class/struct/union

I have another class

class Playlist
{
................
};

bool caseArtistLessThan(const SongInfo &s1, const SongInfo &s2);

In .cpp i have:

bool caseArtistLessThan(const SongInfo &s1, const SongInfo &s2)
{
    QString str1(ID3_GetArtist(&s1.getTag()));
    QString str2(ID3_GetArtist(&s2.getTag()));

    return str1 < str2;
}

I get the error:

C:\Users\Alex\testing-build-desktop-Qt_4_8_0_for_Desktop_-MSVC2010_Qt_SDK__Release..\testing\playlist.cpp:20: error: C2662: ‘SongInfo::getTag’ : cannot convert ‘this’ pointer from ‘const SongInfo’ to ‘SongInfo &’
Conversion loses qualifiers
C:\Users\Alex\testing-build-desktop-Qt_4_8_0_for_Desktop_-MSVC2010_Qt_SDK__Release..\testing\playlist.cpp:20: error: C2662: ‘SongInfo::getTag’ : cannot convert ‘this’ pointer from ‘const SongInfo’ to ‘SongInfo &’
Conversion loses qualifiers

Can somebody help me solve this problems? I don’t understand why it doesn’t works. I’ve seen a lot of examples and I’ve done just like in that examples. What could be the problem?

Answer 1

About second error. You cannot pass const parameter to operator >> as you will be modifying it. const means no modification allowed to object.

Some other notes:

QPair has assign operator so you don’t need to call

upper.first = u.first;
upper.second = u.second;

instead use

upper = u;

There is also such thing as initialization list and you may assign it even like this:

VisualSettings::VisualSettings(QPair<int, int> u, QPair<int, int> l)
    : upper(u)
    , lower(l)
{
}