Question

I have a class that resembles this:

However, after the initial constructor, the copy constructor is being called 10 times.
If I don’t do the thread creation step. It gets called 4 times which is what I’d expect.
Why is that, and how do I avoid it?

Should I avoid using std::vector in this case and just do new delete instead?

#include <cstdio>
#include <vector>

class A
{
    public:
        A() { printf("hello\n"); }
        ~A() { printf("Goodbye\n"); }
        A(const A&)
        {
            printf("copy constructing\n");
        }

        Thread() { }
};


int main()
{
    std::vector<A> a(4, A);

    for (int i = 0; i < a.size(); i++){
        threads_.create_thread(boost::bind(&A::Thread, a[i]));
    }
}

Ok, I found the problem.

This:

        threads_.create_thread(boost::bind(&A::Thread, a[i]));

Should be:

        threads_.create_thread(boost::bind(&A::Thread, &a[i]));

Answer 1

Take a look at Boost.Ref

for (int i = 0; i < a.size(); i++){
     threads_.create_thread(boost::bind(&A::Thread, boost::ref(a[i]) ));
}

this is from the Boost.Bind :

The arguments that bind takes are
copied and held internally by the
returned function object. For example,
in the following code:

int i = 5;

bind(f, i, _1);

a copy of the value of i is stored
into the function object. boost::ref
and boost::cref can be used to make
the function object store a reference
to an object, rather than a copy:

int i = 5;

bind(f, ref(i), _1);

bind(f, cref(42), _1);