Why Qstring constant value?

Question

Qt

Newbie. I'm trying to dig into someone else's code
specifically in this piece:

public:
    Get(const QString &peer, const QString &objectId, QObject *parent = 0);

also I will not understand why in a prototype of method Get constant parameters of lines are specified. Any thoughts?

#pragma once

#include <QObject>
#include <QStringList>

class QUdpSocket;
class Get : public QObject
{
    Q_OBJECT

    QString peer;
    QString objectId;

    QUdpSocket *socket;

    QStringList response;
public:
    Get(const QString &peer, const QString &objectId, QObject *parent = 0);

    void execute();

    QStringList getResponse() const;

private Q_SLOTS:
    void readPendingDatagram();
};

Answer 1

There is such a rule: "const by default", i.e. const should be used wherever possible. In this case, it QStringis passed by reference and is scheduled to be read-only, so it is made const.
In addition, for copied objects constit is not necessary, it is enough to pass them by value (the argument will be copied and only a copy of it will be available to us). But QStringit can be quite large, and the method calls are quite frequent, so just a reference is passed. It becomes possible to change the object through this link. To disable this again, the method specifies constfor the reference argument.