How to use protected function setLocalPort?

I should use setlocalport for my socket connection but the property is protected and i have an error of compilation.

This is in qt application.

m_pSocket = new QTcpSocket();

m_pSocket->setLocalPort(m_iLocalPort);

error: ‘void QAbstractSocket::setLocalPort(quint16)’ is protected

If you want to use protected member like a public one, then you should provide a custom class that is the child of the class whose protected method you intend to use. There is nothing that would forbid you to create a child class inheriting QTcpSocket, and then using the protected method you want. Example for the QTcpSocket case that has been described here can be the following.

// Let us define CustomTcpSocket, i.e. the class inheriting QTcpSocket
#pragma once
#include <QTcpSocket>


class CustomTcpSocket
    : public QTcpSocket
{
    Q_OBJECT
public:
    CustomTcpSocket(QObject* parent = nullptr);
    virtual ~CustomTcpSocket();

    // This method will be used to call QTcpSocket::setLocalPort which is protected.
    void SetLocalPort(quint16 port);
};

Then, we provide the implementation itself.

#include "CustomTcpSocket.h"

CustomTcpSocket::CustomTcpSocket(QObject* parent)
    : QTcpSocket(parent)
{
}

CustomTcpSocket::~CustomTcpSocket()
{
}

void CustomTcpSocket::SetLocalPort(quint16 port)
{
    // Since method is protected, and scope is the child one, we can easily call this method here.
    QAbstractSocket::setLocalPort(port);
}

Now we can easily use this newly created class in the following way.

auto customTcpSocketInstance = new CustomTcpSocket();
customTcpSocketInstance->SetLocalPort(123456);

Through usage of polymorphism, instances of CustomTcpSocket should be accepted by other Qt's APIs. However, there is no guarantee it will work as you would expect it to. Qt developers wanted this method to be protected for some of the reasons. So, use it with caution.