Compile error about std::future - c++

Question

Code like this:
#include <iostream>
#include <future>
#include <thread>
#include <unistd.h>
int foo(int n) {
n = n*1000;
sleep(1);
return n;
}
int main(void) {
std::packaged_task<int (int)> task(std::bind(foo, 3));
std::future<int> f(task.get_future());
std::thread trd(std::move(task));
std::cout << f.get() << std::endl;
return 0;
}
gcc report:
In file included from /usr/include/c++/4.8.2/future:38:0,
from a.cpp:2:
/usr/include/c++/4.8.2/functional: In instantiation of ‘struct std::_Bind_simple<std::packaged_task<int(int)>()>’:
/usr/include/c++/4.8.2/thread:137:47: required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = std::packaged_task<int(int)>; _Args = {}]’
a.cpp:16:33: required from here
/usr/include/c++/4.8.2/functional:1697:61: error: no type named ‘type’ in ‘class std::result_of<std::packaged_task<int(int)>()>’
typedef typename result_of<_Callable(_Args...)>::type result_type;
^
/usr/include/c++/4.8.2/functional:1727:9: error: no type named ‘type’ in ‘class std::result_of<std::packaged_task<int(int)>()>’
_M_invoke(_Index_tuple<_Indices...>)
^
make: *** [a] Error 1
My gcc version is 4.8.2 on fedora 20

Answer 1

The function foo is declared as:
int foo(int);
It has the function type int(int) (taking a parameter int and returning int).
However, the resulting callable returned by std::bind when you bind 3 to the first parameter has a different function type: int(), e.g.:
auto func = std::bind(foo, 3) // Bind 3 to the first parameter.
func(); // Calling func requires no parameter.
Solution
The template parameter specified when declaring the std::packaged_task should be specified as int(), e.g.:
std::packaged_task<int()> task{std::bind(foo, 3)};
Alternatively don't bind the parameter to 3 when constructing the std::packaged_task, instead supply it directly when creating the std::thread object:
std::packaged_task<int(int)> task{foo}; // Don't bind 3
auto f = task.get_future();
std::thread trd{std::move(task), 3}; // Supply 3 here instead.
std::cout << f.get() << std::endl;
Furthermore
Make sure to call trd.join() before returning from function main.
When using std::thread also use the sleep function from the Standard Library instead of the non-portable sleep, e.g.:
std::this_thread::sleep_for(std::chrono::seconds(1));
Also when using std::move you should include the header <utility> in case the other headers don't include it.