thread - 传递引用参数

当给 thread 的执行函数传递指针参数时，没有任何问题，但是如果想传递引用，按照普通函数的调用方法会遇到编译失败：

#include <iostream>
#include <thread>
#include <string>

int main()
{
    std::string a("Hello");
    std::cout << "address = " << &a << "\n";
    std::thread t([](std::string& a)
        {
            std::cout << "in thread address = " << &a << "\n";
        }, a);
    t.join();
}

编译：g++ -std=c++11 -pthread test.cpp
失败了：

In file included from /usr/include/c++/4.7/bits/move.h:57:0,
                 from /usr/include/c++/4.7/bits/stl_pair.h:61,
                 from /usr/include/c++/4.7/bits/stl_algobase.h:65,
                 from /usr/include/c++/4.7/bits/char_traits.h:41,
                 from /usr/include/c++/4.7/ios:41,
                 from /usr/include/c++/4.7/ostream:40,
                 from /usr/include/c++/4.7/iostream:40,
                 from test.cpp:1:
/usr/include/c++/4.7/type_traits: In instantiation of ?.truct std::_Result_of_impl<false, false, main()::<lambda(std::string&)>, std::basic_string<char, std::char_traits<char>, std::allocator<char> > >?.
/usr/include/c++/4.7/type_traits:1857:12:   required from ?.lass std::result_of<main()::<lambda(std::string&)>(std::basic_string<char>)>?
/usr/include/c++/4.7/functional:1563:61:   required from ?.truct std::_Bind_simple<main()::<lambda(std::string&)>(std::basic_string<char>)>?
/usr/include/c++/4.7/thread:133:9:   required from ?.td::thread::thread(_Callable&&, _Args&& ...) [with _Callable = main()::<lambda(std::string&)>; _Args = {std::basic_string<char, std::char_traits<char>, std::allocator<char> >&}]?
test.cpp:12:7:   required from here
/usr/include/c++/4.7/type_traits:1834:9: error: no match for call to ?.main()::<lambda(std::string&)>) (std::basic_string<char>)?
test.cpp:9:17: note: candidates are:
In file included from /usr/include/c++/4.7/bits/move.h:57:0,
                 from /usr/include/c++/4.7/bits/stl_pair.h:61,
                 from /usr/include/c++/4.7/bits/stl_algobase.h:65,
                 from /usr/include/c++/4.7/bits/char_traits.h:41,
                 from /usr/include/c++/4.7/ios:41,
                 from /usr/include/c++/4.7/ostream:40,
                 from /usr/include/c++/4.7/iostream:40,
                 from test.cpp:1:
/usr/include/c++/4.7/type_traits:1834:9: note: void (*)(std::string&) {aka void (*)(std::basic_string<char>&)} <conversion>
/usr/include/c++/4.7/type_traits:1834:9: note:   candidate expects 2 arguments, 2 provided
test.cpp:9:33: note: main()::<lambda(std::string&)>
test.cpp:9:33: note:   no known conversion for argument 1 from ?.td::basic_string<char>?.to ?.td::string& {aka std::basic_string<char>&}?

这里类似于 std::bind，std::thread 和 std::bind 采用了相同的机制，必须使用 std::ref 来包装：

#include <iostream>
#include <thread>
#include <string>

int main()
{
    std::string a("Hello");
    std::cout << "address = " << &a << "\n";
    std::thread t([](std::string& a)
        {
            std::cout << "in thread address = " << &a << "\n";
        }, std::ref(a));
    t.join();
}