具有可变函数参数的 C++ 多态性

Question

我正在与您分享我在使用可变函数参数的类中遇到的一个问题。它是下面代码中显示的类 Thread。它是 std::thread 的包装器，以便使用函数模式。

我想对这个函数使用多态性，将类 Thread 继承到一个新类 Functor 中，但是 gcc 返回以下错误:

#include <thread>
#include <iostream>

using namespace std;

template<class... Args>
class Thread
{
public:
    virtual void operator()(Args...) = 0;

    void run(Args... args)
    {
    std::thread t(std::forward< Thread<Args...> >(*this), std::forward<Args>(args)...);
    t.join();
    }
};

template<class... Args>
class Functor : public Thread<Args...>
{
public:
    // generates the errors bellow
    virtual void operator()(Args... /*args*/)
    {
    }

    // doesnot work since the pure virtual function wants another prototype of function.
    // void operator()(int)
    // {
    // }
};

int main()
{
    int a = 12;
    Functor<int> f;
    f.run(ref(a));

    return 0;
}

from t-Thread-args2.cpp:1:
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/tuple: In instantiation of ‘struct std::_Head_base, false>’:
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/tuple:215:12:   required from ‘struct std::_Tuple_impl, int>’
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/tuple:507:11:   required from ‘class std::tuple, int>’
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/functional:1601:39:   required from ‘struct std::_Bind_simple(int)>’
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/thread:133:9:   required from ‘std::thread::thread(_Callable&&, _Args&& ...) [with _Callable = Thread; _Args = {int}]’
t-Thread-args2.cpp:14:83:   required from ‘void Thread::run(Args ...) [with Args = {int}]’
t-Thread-args2.cpp:42:17:   required from here
/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.2/../../../../include/c++/4.7.2/tuple:166:13: error: cannot declare field ‘std::_Head_base, false>::_M_head_impl’ to be of abstract type ‘Thread’
t-Thread-args2.cpp:7:7: note:   because the following virtual functions are pure within ‘Thread’:
t-Thread-args2.cpp:10:18: note:     void Thread::operator()(Args ...) [with Args = {int}]

我不太明白这个错误，因为纯虚函数在派生类中定义得很好。但是，在将函数 run() 移动到派生类 (Functor) 时，它起作用了。

提前致谢， 蚕儿

Answer 1

根据 [thread.thread.constr]§3，std::thread 构造函数的第一个参数的类型是 F&&，要求 F 是 MoveConstructible。在您的例子中，F 是 Thread，它不是 MoveConstructible。

换句话说，std::thread 需要按值存储仿函数，而您将仿函数作为抽象的 Thread 转发。