我正在与您分享我使用可变参数函数参数的类所遇到的问题 . 它是以下代码中显示的类Thread . 它是std :: thread的包装器,以便使用函数模式 .
我想在这个函数中使用多态来将类Thread继承到一个新类Functor中,但是gcc会返回以下错误:
#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}]
我真的不理解错误,因为纯虚函数在派生类中得到了很好的定义 . 但是,在将函数run()移动到派生类(Functor)中时,它可以工作 .
提前谢谢,卡纳
3 回答
问题是:
它试图复制
Thread
子对象 . 幸运的是它是抽象的,因此您得到编译错误而不是意外的运行时行为 .您想要一个引用包装器:
我考虑了参与者提供的多个建议,包括使用
std::ref
,并希望与您分享解决我之前代码所遇问题的工作代码版本 .再次感谢 .
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
. 在您的情况下,F
是Thread
,这不是MoveConstructible
.换句话说,
std::thread
需要按值存储仿函数,并且您将仿函数转发为Thread
,这是抽象的 .