Passing object by reference and Multithreading

Question

I have a small problem and wondering if someone can help. I tried to demonstrate my problem in the most simple way possible. I am trying to pass an object by reference to multiple threads. Every thread calls "doSomething" which is a member function that belongs to the object "Example". The "doSomething" function should increment the counter. My gcc version is 4.4.7

The questions:

why is the value of the variable "counter" is not incremented although I passed the object by reference to the thread function.

The code:

#include <iostream>
#include <thread>

class Exmaple {
    private:
        int counter;

    public:
            Exmaple() { 
            counter = 0;
        }    

        void doSomthing(){
            counter++;
        }

        void print() {
            std::cout << "value from A: " << counter << std::endl;
        }

};

// notice that the object is passed by reference
void thread_task(Exmaple& o) {
    o.doSomthing();
    o.print();
}    

int main()
{
    Exmaple b;
    while (true) {
        std::thread t1(thread_task, b);
        t1.join();
    }    
    return 0;
}

The output:

value from A: 1
value from A: 1
value from A: 1
value from A: 1
value from A: 1
value from A: 1
value from A: 1
value from A: 1
value from A: 1

Answer 1

while (true) {
    std::thread t1(thread_task, b);
    t1.join();
}  

Two things you need to know here:

• Use std::ref to pass a reference.
• Infinite loop is Undefined Behavior in C++;

Working Example below:

#include <iostream>
#include <thread>

class Exmaple {
    private:
        int counter;

    public:
            Exmaple() { 
            counter = 0;
        }    

        void doSomthing(){
            counter++;
        }

        void print() {
            std::cout << "value from A: " << counter << std::endl;
        }

};

// notice that the object is passed by reference
void thread_task(Exmaple& o) {
    o.doSomthing();
    o.print();
}    

int main()
{
    Exmaple b;
    for(int i =0; i < 10; i++) {
        std::thread t1(thread_task, std::ref(b));
        t1.join();
    }    
    return 0;
}

Output:

value from A: 1
value from A: 2
value from A: 3
value from A: 4
value from A: 5
value from A: 6
value from A: 7
value from A: 8
value from A: 9
value from A: 10

See it Live.

Though going further you should also consider data race

Answer 2

Note that in std::thread t1(thread_task, b) you pass b by value to the constructor of std::thread (as you call the constructor here, and not thread_task directly). A solution could be to either to wrap b by an std::ref-object, or change your code to pass a pointer:

void thread_task(Exmaple* o) {
    o->doSomthing();
    o->print();
}

int main()
{
    Exmaple b;
    while (true) {
        std::thread t1(thread_task, &b);
        t1.join();
    }
    return 0;
}

Answer 3

I'm not very familiar with multithreading, but you're passing b by value to the thread, and not by reference. The value of b is then passed by reference to thread_task, so the value is always 1.

According to the documentation, you have to write your thread like so to pass objects by reference:

std::thread t1(thread_task, std::ref(b));