deque::push_back with threads reacts strangely

Question

I've tried different things, but push_back of threads in a deque container reacts strangely.

why is this happening?? Is this because of the copy/move constructors?

Here the output of the program...

progress[0]:-1
progress[1]:-1
executing threads...
progress[0]:100
progress[1]:100
================================
progress[0]:-1
progress[1]:-1
executing threads...
progress[0]:-1
progress[1]:100

As shown by the output, the result defers between using

for(size_t i = 0; i < 2; ++i) {
    deq.push_back(th(&progress[i]));
}

and

deq.push_back(th(&progress[0]));
deq.push_back(th(&progress[1]));

This is the source code...

class th {
    public:
        void func() {
            *this->progress = 100;
        }

        th(int* prog) :
            progress(prog), 
            m_thread(std::thread(&th::func, this)) {};

        // COPY
        th(th const& other);
        th& operator=(th const& other);

        // MOVE
        th(th&&) = default;
        // th& operator=(th&& other) {
        //  if(this != &other){
        //  }
        //  return *this;
        // }

        void join() { m_thread.join(); }
        int *progress;

    private:
        std::thread m_thread;
};

int main(void) {

    {
        std::vector<int> progress;
        progress.push_back(-1);
        progress.push_back(-1);
        std::deque<th> deq;

        std::cout << "progress[0]:" << progress[0] << std::endl;
        std::cout << "progress[1]:" << progress[1] << std::endl;

        std::cout << "executing threads..." << std::endl;

        deq.push_back(th(&progress[0]));
        deq.push_back(th(&progress[1]));


        for (std::deque<th>::iterator it = deq.begin(); it != deq.end(); it++) {
            it->join();
            // deq.erase(it);
        }


        std::cout << "progress[0]:" << progress[0] << std::endl;
        std::cout << "progress[1]:" << progress[1] << std::endl;    
    }

    std::cout << "================================" << std::endl;

    {
        std::vector<int> progress;
        progress.push_back(-1);
        progress.push_back(-1);
        std::deque<th> deq;

        std::cout << "progress[0]:" << progress[0] << std::endl;
        std::cout << "progress[1]:" << progress[1] << std::endl;

        std::cout << "executing threads..." << std::endl;

        for(size_t i = 0; i < 2; ++i) {
            deq.push_back(th(&progress[i]));
        }

        for (std::deque<th>::iterator it = deq.begin(); it != deq.end(); it++) {
            it->join();
            // deq.erase(it);
        }

        std::cout << "progress[0]:" << progress[0] << std::endl;
        std::cout << "progress[1]:" << progress[1] << std::endl;
    }

    exit(EXIT_SUCCESS);
}

And how can I use the erase member function of deque without the compiler complaining

    /home/user/test/obj/main.o: In function `__gnu_cxx::_Mutable_BidirectionalIteratorConcept<th*>::__constraints()':
main.cpp:(.text._ZN9__gnu_cxx37_Mutable_BidirectionalIteratorConceptIP2thE13__constraintsEv[__gnu_cxx::_Mutable_BidirectionalIteratorConcept<th*>::__constraints()]+0x40): undefined reference to `th::operator=(th const&)'
/home/user/test/obj/main.o: In function `__gnu_cxx::_ConvertibleConcept<th, th>::__constraints()':
main.cpp:(.text._ZN9__gnu_cxx19_ConvertibleConceptI2thS1_E13__constraintsEv[__gnu_cxx::_ConvertibleConcept<th, th>::__constraints()]+0x20): undefined reference to `th::th(th const&)'
/home/user/test/obj/main.o: In function `__gnu_cxx::_OutputIteratorConcept<th*, th>::__constraints()':
main.cpp:(.text._ZN9__gnu_cxx22_OutputIteratorConceptIP2thS1_E13__constraintsEv[__gnu_cxx::_OutputIteratorConcept<th*, th>::__constraints()]+0x64): undefined reference to `th::operator=(th const&)'
/home/user/test/obj/main.o: In function `th* std::__copy_move_backward<true, false, std::random_access_iterator_tag>::__copy_move_b<th*, th*>(th*, th*, th*)':
main.cpp:(.text._ZNSt20__copy_move_backwardILb1ELb0ESt26random_access_iterator_tagE13__copy_move_bIP2thS4_EET0_T_S6_S5_[th* std::__copy_move_backward<true, false, std::random_access_iterator_tag>::__copy_move_b<th*, th*>(th*, th*, th*)]+0x5c): undefined reference to `th::operator=(th const&)'
/home/user/test/obj/main.o: In function `th* std::__copy_move<true, false, std::random_access_iterator_tag>::__copy_m<th*, th*>(th*, th*, th*)':
main.cpp:(.text._ZNSt11__copy_moveILb1ELb0ESt26random_access_iterator_tagE8__copy_mIP2thS4_EET0_T_S6_S5_[th* std::__copy_move<true, false, std::random_access_iterator_tag>::__copy_m<th*, th*>(th*, th*, th*)]+0x44): undefined reference to `th::operator=(th const&)'
/home/user/test/obj/main.o: In function `__gnu_cxx::_Mutable_ForwardIteratorConcept<th*>::__constraints()':
main.cpp:(.text._ZN9__gnu_cxx31_Mutable_ForwardIteratorConceptIP2thE13__constraintsEv[__gnu_cxx::_Mutable_ForwardIteratorConcept<th*>::__constraints()]+0x3c): undefined reference to `th::operator=(th const&)'
collect2: ld returned 1 exit status
make: *** [link] Error 1

Thank you!

Answer 1

You pass the value of this (i.e. the current address of the object) to the constructor of std::thread. After moving the th object into the deque, this value of this is no longer valid, as the object now has been moved to a different location. Yet the thread still uses the old value. Try making th non-moveable and using emplace_back rather than push_back.

class th {
    public:
        void func() {
            *this->progress = 100;
        }

        explicit th(int* prog) :
            progress(prog), 
            m_thread(std::thread(&th::func, this)) {};

        // COPY
        th(th const& other) = delete;
        th(th && other) = delete;
        th& operator=(th const& other) = delete;
        th& operator=(th &&) = delete;

        void join() { m_thread.join(); }
        int *progress;

    private:
        std::thread m_thread;
};

std::vector<int> progress;
progress.push_back(-1);
progress.push_back(-1);
std::deque<th> deq;
deq.emplace_back (&progress[0]);
// etc.