thread - 傳遞引用參數

當給 thread 的執行函數傳遞指針參數時，沒有任何問題，可是若是想傳遞引用，按照普通函數的調用方法會遇到編譯失敗：

 1 #include <iostream>
 2 #include <thread>
 3 #include <string>
 4 
 5 int main()
 6 {
 7     std::string a("Hello");
 8     std::cout << "address = " << &a << "\n";
 9     std::thread t([](std::string& a)
10         {
11             std::cout << "in thread address = " << &a << "\n";
12         }, a);
13     t.join();
14 }

編譯：g++ -std=c++11 -pthread test.cpp
失敗了：

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>&}?

這裏相似於 std::bind，std::thread 和 std::bind 採用了相同的機制，必須使用 std::ref 來包裝：

 1 #include <iostream>
 2 #include <thread>
 3 #include <string>
 4 
 5 int main()
 6 {
 7     std::string a("Hello");
 8     std::cout << "address = " << &a << "\n";
 9     std::thread t([](std::string& a)
10         {
11             std::cout << "in thread address = " << &a << "\n";
12         }, std::ref(a));
13     t.join();
14 }