why doesn't signal gets emiited?

[ChrisW67]

i dont understand why are my signals not working in this multi threaded app

Have a look at the comments I have inserted into your code snippets

file.h:

#include <QObject>
 
 
class FThread;
 
class File : public QObject
{
    Q_OBJECT
public:
    File(QObject *parent = 0);
    FThread *fthread;   
    // ^^^^^ Where does this pointer ever get an instance of FThread attached to it?
public slots:
    void print();
 
};

#include <QObject>


class FThread;

class File : public QObject
{
    Q_OBJECT
public:
    File(QObject *parent = 0);
    FThread *fthread;   
    // ^^^^^ Where does this pointer ever get an instance of FThread attached to it?
public slots:
    void print();

};

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file.cpp:

#include "file.h"
#include <QDebug>
#include "fthread.h"
 
File::File(QObject *parent) :
    QObject(parent)
{
 
    connect(fthread,SIGNAL(done()),this,SLOT(print()));   
    // ^^^^^ you use fthread here but is has not been initialised either 0 (NULL)
    // or to be a valid pointer.
    // On the way to crash city
 
}
void File::print(){
    qDebug()<<"printing from File object";
    fthread.exit();   
    // ^^^^^ given that fthread is a pointer I am surprised this even compiles
}

#include "file.h"
#include <QDebug>
#include "fthread.h"

File::File(QObject *parent) :
    QObject(parent)
{

    connect(fthread,SIGNAL(done()),this,SLOT(print()));   
    // ^^^^^ you use fthread here but is has not been initialised either 0 (NULL)
    // or to be a valid pointer.
    // On the way to crash city

}
void File::print(){
    qDebug()<<"printing from File object";
    fthread.exit();   
    // ^^^^^ given that fthread is a pointer I am surprised this even compiles
}

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NOTE:

i get following output:

App started
The program has unexpectedly finished.
...path/Thread exited with code 0

means it is exiting without error

Which part of "The program has unexpectedly finished." means "exiting without error"?

Have you single-stepped the program to see at what point the program terminates unexpectedly?