将模板化的参考参数传递给线程

Question

我怎样才能使它工作？

#include <iostream>
#include <thread>

using namespace std;

int main()
{
    auto thr = []<typename PrintType>( PrintType &p )
    {
        cout << p << endl;
    };
    string str = "hello world";
    jthread jt( thr, ref( str ) );
}

我什至不明白这里有什么问题。

这是 Windows 上的 clang 13 所说的：

In file included from test.cpp:2:
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\thread:55:9: error: no matching function for call to 'invoke'
        _STD invoke(_STD move(_STD get<_Indices>(_Tup))...);
        ^~~~~~~~~~~
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\yvals_core.h:1385:20: note: expanded from macro '_STD'
#define _STD       ::std::
                   ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\thread:62:17: note: in instantiation of function template specialization 'std::thread::_Invoke<std::tuple<(lambda at test.cpp:8:13), std::reference_wrapper<std::basic_string<char>>>, 0ULL, 1ULL>' requested here
        return &_Invoke<_Tuple, _Indices...>;
                ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\thread:302:19: note: in instantiation of function template specialization 'std::thread::_Start<(lambda at test.cpp:8:13) &, std::reference_wrapper<std::basic_string<char>>>' requested here
            _Impl._Start(_STD forward<_Fn>(_Fx), _STD forward<_Args>(_Ax)...);
                  ^
test.cpp:13:10: note: in instantiation of function template specialization 'std::jthread::jthread<(lambda at test.cpp:8:13) &, std::reference_wrapper<std::basic_string<char>>, 0>' requested here
        jthread jt( thr, ref( str ) );
                ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\type_traits:1482:19: note: candidate template ignored: substitution failure [with _Callable = (lambda at test.cpp:8:13), _Ty1 = std::reference_wrapper<std::basic_string<char>>, _Types2 = <>]: no matching function for call to '_Call'
_CONSTEXPR17 auto invoke(_Callable&& _Obj, _Ty1&& _Arg1, _Types2&&... _Args2) noexcept(
                  ^
C:\Program Files\Microsoft Visual Studio\2022\Community\VC\Tools\MSVC\14.30.30705\include\type_traits:1476:19: note: candidate function template not viable: requires single argument '_Obj', but 2 arguments were provided
_CONSTEXPR17 auto invoke(_Callable&& _Obj) noexcept(noexcept(static_cast<_Callable&&>(_Obj)()))
                  ^
1 error generated.

Answer 1

只要我们坚持将thr lambda 传递给jthread 构造函数，这里的问题就有点无法解决。这个sn-p

auto thr = []<typename PrintType>( PrintType &p )
{
    cout << p << endl;
};
string str = "hello world";
jthread jt(thr, str);

将会失败，因为您不能将非常量引用绑定到 jthread 构造函数中的临时结果（注意：const PrintType& hp 可以工作，但我认为这不是 OP 想要的）。

std::ref 可以使用非模板化的 lambda：

auto thr = [](std::string& p)
{
    std::cout << p << std::endl;
};

std::string str = "hello world";
std::thread jt( thr, std::ref(str) );

但它不适用于模板化 lambda（因为auto thr = [](auto& p) 只会尝试将非 const 左值引用绑定到 reference_wrapper 类型的临时对象）。

我能想到的唯一解决方案是将带有闭包的 lambda 直接传递给线程构造函数：

auto thr = []<class Type>(Type& p)
{
    std::cout << p << std::endl;
};
std::string str = "hello world";
std::thread jt([&str, &thr]() { thr(str); });

Answer 2

TLDR：

    jthread jt(&decltype(thr)::operator()<std::string>, thr, ref(str));

---

您遇到的问题是由于PrintType 被std::reference_wrapper 替换造成的，这不是operator<< 的有效参数。您希望编译器在接收它作为参数时执行从 std::reference_wrapper 到 std::string& 的隐式转换，但编译器不希望在那里出现 std::string&。它期望std::reference_wrapper。

/opt/compiler-explorer/gcc-11.2.0/include/c++/11.2.0/thread: In instantiation of 'static std::thread std::jthread::_S_create(std::stop_source&, _Callable&&, _Args&& ...) [with _Callable = main()::<lambda(PrintType&)>&; _Args = {std::reference_wrapper<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >}]':

如您所见，您最终不会以 std::string& 作为参数，而是使用包装器：std::reference_wrapper<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >。

这段代码编译：

#include <thread>
#include <iostream>

using namespace std;

int main()
{
    auto thr = [](const std::string &p)
    {
        cout << p << endl;
    };
    string str = "hello world";
    jthread jt(thr, ref( str ));
}

因此，合乎逻辑的解决方案是强制编译器以正确的类型实例化模板：

#include <thread>
#include <iostream>
#include <type_traits>

using namespace std;

int main()
{
    auto thr = []<typename PrintType>(PrintType& p)
    {
        cout << p << endl;
    };
    string str = "hello world";

    // explicitly instantiating via a pointer to member function
    jthread jt(&decltype(thr)::operator()<std::string>, thr, ref(str));
}