CODEWITHSUNDEEP
c++qt
Cheesi
Cheesi

Reputation: 483

"Undefined reference to"-error / static member var in C++/Qt

First of all, I'm not really familiar with C++/Qt, so I tried it, as i would do it in C#... I also have some problems with pointers..

The story: I use the QSerialPort to read and write on a serial 232 port. Of course, there should be only one instance, otherwise there would be access errors. So my idea was to define a static member variable to hold the object.

The problem: I always get the error "undefined reference to SerialManager::obj"

The source code:

serialmanager.h

#include <QSerialPort>
class SerialManager
{
public:
    static QSerialPort* getObj();
private:
    static QSerialPort* obj;
}

serialmanager.cpp

#include "serialmanager.h"
QSerialPort *obj = new QSerialPort();

QSerialPort* SerialManager::getObj()
{
    if(!obj->isOpen())
    {
        obj->setPortName("/dev/ttyO1"); //error line
        obj->setBaudRate(QSerialPort::Baud57600);
        //and so on...
    }
    return obj;
}

Upvotes: 0

Views: 103

Answers (2)

Toby Speight
Toby Speight

Reputation: 30709

In the header file, you are declaring SerialManager::obj in the header file.

In your implementation file, you are defining obj, notSerialManager::obj.

The fix is to change

QSerialPort *obj = new QSerialPort();

to

QSerialPort *SerialManager::obj = new QSerialPort();

You may find you want to initialise it to nullptr, and construct on demand when you need it (that might solve problems with dependencies, and/or improve program start times, if you have many static objects to construct).

If you really want to only ever construct a single object, you could also make the pointer (but not the pointed-to object) constant:

private:
    static QSerialPort *const obj;

That can safeguard against accidentally assigning again (but can make it hard to substitute a different object for your unit tests).

Upvotes: 2

SurvivalMachine
SurvivalMachine

Reputation: 8356

You are missing the class's name, fix it by changing

QSerialPort *obj = new QSerialPort();

into

QSerialPort *SerialManager::obj = new QSerialPort();

Upvotes: 1

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