Compiling C++ threads

Question

I'm trying to use threads on my C++ application.

My code is:

#include <iostream>
#include <thread>

class C
{
public:

    void * code( void * param )
    {
        std::cout << "Code thread executing " << std::endl;
        return NULL;
    }
};

int main()
{
    C c;
    std::thread t ( &C::code, &c );
    t.join();
}

When compiling, I got those errors:

In file included from /opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/bits/move.h:57:0,
                 from /opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/bits/stl_pair.h:61,
                 from /opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/bits/stl_algobase.h:65,
                 from /opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/bits/char_traits.h:41,
                 from /opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/ios:41,
                 from /opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/ostream:40,
                 from /opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/iostream:40,
                 from C.cpp:1:
/opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/type_traits: In instantiation of 'struct std::_Result_of_impl<false, false, std::_Mem_fn<void* (C::*)(void*)const>, C*>':
/opt/centos/devtoolset-1.0/root/usr/lib/gcc/x86_64-redhat-linux/4.7.0/../../../../include/c++/4.7.0/type_traits:1857:12:   required from 'class std::result_of<std::_Mem_fn<void* (C::*)(void*)const>(C*)>'

and a lot more...

I'm compiling with:

g++ -std=c++0x  C.cpp

The compiler version:

$g++ --version
g++ (GCC) 4.7.0 20120507 (Red Hat 4.7.0-5)

What am I doing wrong?

Answer 1

std::thread is not the same thing as POSIX thread, it doesn't have to take a void* argument and return a void*. The thread constructor can take any callable as long as you specify the right arguments.

The specific error in this case is that you are attempting to start a thread that effectively calls c.code() (technically INVOKE(&C::code, &c)) , but that is an invalid call since C::code takes one argument and you are trying to call it with zero. Simply fix the signature on code() to match what you are calling it with:

void code()
{
    std::cout << "Code thread executing " << std::endl;
}

Alternatively, you can provide the void* arg to the thread constructor:

std::thread t ( &C::code, &c, nullptr );
                              ^^^^^^^

Either way, make sure you compile with -pthread.

Answer 2

Make your class C a callable object using operator()

#include <iostream>
#include <thread>

class C
{
public:

    void operator()( void )
    {
                std::cout << "Code thread executing " << std::endl;

        return NULL;
    }
};

int main()
{
    C c;
    std::thread t (c );
    t.join();
}

or turn you class into a callable object

#include <iostream>
#include <thread>
#include <functional>

class C
{
public:

    void * code( void)
    {
                std::cout << "Code thread executing " << std::endl;

        return NULL;
    }
};

int main()
{
    C c;
    std::thread t (std::bind( &C::code, &c ));
    t.join();
}

and switch to --std=c++11