c++ - Crash cause of own template -

why program crashing when use template? i'm doing wrong? that's test program, because actual program big posting here. first qdebug test1 displayed second not.

#include <qcoreapplication> #include <qdebug> #include <qmutex>  class mutexlocker { public:     mutexlocker(qmutex& m) : _m(m) { _m.lock(); }     ~mutexlocker() { _m.unlock(); }  private:     qmutex& _m; };  template<typename t> class threadguard { public:     threadguard() { _mutex = new qmutex(); }      ~threadguard() { delete _mutex; }      void set(const t& other) {         mutexlocker m(*_mutex); q_unused(m);         _r = other;     }      void set(int i, int j) {         mutexlocker m(*_mutex); q_unused(m);         _r[i] = j;     }      t r() const {         mutexlocker m(*_mutex); q_unused(m);         return _r;     }      const threadguard<t>& operator=(const t& other) {         set(other);         return *this;     }  private:     threadguard(const threadguard&) {}      t _r;     qmutex *_mutex; };  int main(int argc, char *argv[]) {     qcoreapplication a(argc, argv);     qvector<int> test1(10);      for(int = 0; < 10; i++){         test1[i] = i*2;     }     for(int = 0; < 10; i++){         qdebug() << test1[i];     }      threadguard<qvector<int> > test2;     test2.r().resize(10);      for(int = 0; < 10; i++){         test2.r()[i] = i*2;     }     for(int = 0; < 10; i++){         qdebug() << test2.r()[i];     }       return a.exec(); } 

i'm using qt 5.4 on ms vista.

thanks in advance!

add method:

t & r() {     mutexlocker m(*_mutex); q_unused(m);     return _r; } 

explanation:

t r() const returns copy of r_. destroyed then. while actual r_ not modified here test2.r().resize(10);. , later on.