QThread vs QThread* and QThread: Destroyed while thread is still running warning

[Axtroz]

Greetings.
I am trying to understand the concept behind using threads and have subclassed QThread the following way:

// Header:
 
#include <QtDebug>
 
class TestThread : public QThread
{
    public:
        void run();
};
 
// C++ source:
 
void TestThread::run() {
    qDebug() << "Message from thread";
    exec();
}
 
// main.cpp:
 
#include "testthread.h"
 
int main(int argc, char** argv) {
    TestThread t1, t2;
    t1.start();
    t2.start();
    qDebug() << "Message from main function";
    return 0;
}

// Header:

#include <QtDebug>

class TestThread : public QThread
{
    public:
        void run();
};

// C++ source:

void TestThread::run() {
    qDebug() << "Message from thread";
    exec();
}

// main.cpp:

#include "testthread.h"

int main(int argc, char** argv) {
    TestThread t1, t2;
    t1.start();
    t2.start();
    qDebug() << "Message from main function";
    return 0;
}

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It returns 2 warnings QThread: Destroyed while thread is still running warning
and then it crashes.
However, if I replace

TestThread t1,t2;

TestThread t1,t2;

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with

TestThread *t1 = new TestThread, *t2 = new TestThread;

TestThread *t1 = new TestThread, *t2 = new TestThread;

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it works. My question is: why?