std::thread, class constructor and destructor

Question

When testing threads in C++11 I have created the following example:

#include <iostream>
#include <thread>

class Foo {
public:
    Foo(void) {
        std::cout << "Constructor called: " << this << std::endl;
    }
    ~Foo(void) {
        std::cout << "Destructor called: " << this << std::endl;
    }
    void operator()() const {
        std::cout << "Operatior called: " << this << std::endl;
    }
};

void test_normal(void) {
    std::cout << "====> Standard example:" << std::endl;
    Foo f;
}

void test_thread(void) {
    std::cout << "====> Thread example:" << std::endl;
    Foo f;
    std::thread t(f);
    t.detach();
}


int main(int argc, char **argv) 
{
    test_normal();
    test_thread();

    for(;;);
}

Which prints the following:

Why is the destructor called 6 times for the thread? And why does the thread report different memory locations?

EDIT When adding move and copy constructor output:

Answer 1

The function object will be moved or copied. You didn't account in your output for any of these.

Answer 2

Add a copy constructor and move constructor to your class.

Foo(Foo const&) { std::cout << "Copy Constructor called: " << this << std::endl; }
Foo(Foo&&) { std::cout << "Move Constructor called: " << this << std::endl; }

Now if you run the code the output (on gcc 4.7.2) looks like this:

====> Standard example:
Constructor called: 0xbff696ff
Destructor called: 0xbff696ff
====> Thread example:
Constructor called: 0xbff696ff
Copy Constructor called: 0xbff696cf
Move Constructor called: 0x93a8dfc
Destructor called: 0xbff696cf
Destructor called: 0xbff696ff
Operator called: 0x93a8dfc
Destructor called: 0x93a8dfc

As you can see, the number of calls to the destructor matches the number of calls to the various constructors.

I suspect gcc manages to elide a few of the copy / move construction calls that MSVC seems to be making, so there are fewer calls to destructor than your example.

---

Furthermore, you can avoid the copy construction completely by std::move ing the Foo object to the thread constructor.

In test_thread change the thread construction line to

std::thread t(std::move(f));

Now the output looks like this:

====> Standard example:
Constructor called: 0xbfc23e2f
Destructor called: 0xbfc23e2f
====> Thread example:
Constructor called: 0xbfc23e2f
Move Constructor called: 0xbfc23dff
Move Constructor called: 0x9185dfc
Destructor called: 0xbfc23dff
Destructor called: 0xbfc23e2f
Operator called: 0x9185dfc
Destructor called: 0x9185dfc

Answer 3

Because your Foo is on the stack, not heap. This means that you allocate a new one inside test_thread, then it gets copied when you call std::thread(f) and again inside thread(f).

You would need to instead create a pointer to allocate on the heap, and pass that so that the object isn't copied each time, using the heap (new) to allocate it.

Answer 4

Compiler adds default move and copy constructors if you do not do it yourself, check this

https://ideone.com/wvctrl

#include <iostream>
#include <thread>

class Foo {
public:
    Foo(Foo&& f) {
        std::cout << "Constructor Foo&& called: " << this << std::endl;
    }
    Foo(const Foo& f) {
        std::cout << "Constructor const Foo& called: " << this << std::endl;
    }
    Foo(void) {
        std::cout << "Constructor called: " << this << std::endl;
    }
    ~Foo(void) {
        std::cout << "Destructor called: " << this << std::endl;
    }
    void operator()() const {
        std::cout << "Operatior called: " << this << std::endl;
    }
};

void test_normal(void) {
    std::cout << "====> Standard example:" << std::endl;
    Foo f;
}

void test_thread(void) {
    std::cout << "====> Thread example:" << std::endl;
    Foo f;
    std::thread t(f);
    t.detach();
}


int main(int argc, char **argv) 
{
    test_normal();
    test_thread();

    for(;;);
}

it shows that all ctors pair with dtors.

Also look into this SO:

Rule-of-Three becomes Rule-of-Five with C++11?