静态模板成员函数的实例化？答案

【问题标题】：Instantiation of static template member function?静态模板成员函数的实例化？
【发布时间】：2018-06-04 15:25:04
【问题描述】：

我有一个问题，给定这个 sn-p 的代码：

class Thread{
    private:
    template<class t_Funtion, class ... t_Args>
    struct ThreadExiter
    {
        using CallbackType = decltype(std::bind(std::declval<t_Funtion>(), std::declval<t_Args>()...));

        static CallbackType m_Callback;

        template<class ... t_ConstructorArgs>
        ThreadExiter(t_Funtion p_Function, t_ConstructorArgs ... p_Args) :
            m_Callback(std::forward<t_Funtion>(p_Function), std::forward<t_ConstructorArgs&&>(p_Args) ...)
        {
            // Nothing to do
        }
        ~ThreadExiter()
        {
            m_Callback();
        }
    };

如何实例化静态成员static CallbackType m_Callback;？

我试过了：

template<class t_Funtion, class ... t_Args> 
typename Thread::ThreadExiter<t_Funtion, t_Args...>::CallbackType Thread::ThreadExiter<t_Funtion, t_Args...>::m_Callback

但我得到了：

error: no matching function for call to 'std::_Bind<int (*(Thread*)) Thread*)>::_Bind()'  typename Thread::ThreadExiter<t_Funtion, t_Args...>::CallbackType Thread::ThreadExiter<t_Funtion, t_Args...>::m_Callback;
                                                               ^

【问题讨论】：

标签： c++ c++11 templates stdbind

【解决方案1】：

尝试在ThreadExiter的构造函数中构造静态成员时出错：

ThreadExiter(/*...*/) : m_Callback(/*...*/) {}

来吧：

class Thread
{
private:
    template<class t_Funtion, class ... t_Args>
    struct ThreadExiter
    {
        using CallbackType = decltype(std::bind(std::declval<t_Funtion>(), std::declval<t_Args>()...));

        static CallbackType m_Callback;

        template<class ... t_ConstructorArgs>
        ThreadExiter(t_Funtion p_Function, t_ConstructorArgs ... p_Args)
        {
            m_Callback = CallbackType{std::forward<t_Funtion>(p_Function), std::forward<t_ConstructorArgs&&>(p_Args) ...};
        }
        ~ThreadExiter()
        {
            m_Callback();
        }
    };
};

template<class t_Funtion, class ... t_Args> 
typename Thread::ThreadExiter<t_Funtion, t_Args...>::CallbackType Thread::ThreadExiter<t_Funtion, t_Args...>::m_Callback;

【讨论】：

【解决方案2】：

谢谢！最后我以这种方式解决了（因为我没有复制和赋值构造函数）：

     static CallbackType* m_Callback;
        template<class ... t_ConstructorArgs>
        ThreadExiter(t_Funtion p_Function, t_ConstructorArgs ... p_Args)
        {
            static CallbackType l_Callback(std::forward<t_Funtion>(p_Function), std::forward<t_ConstructorArgs&&>(p_Args) ...);
            m_Callback = &l_Callback;
        }

        ~ThreadExiter()
        {
            m_Callback->operator()();
        }

以这种方式实例化：

Template<class t_Funtion, class ... t_Args>
using CallbackType = decltype(std::bind(std::declval<t_Funtion>(), 
std::declval<t_Args>()...));

template<class t_Funtion, class ... t_Args>
CallbackType<t_Funtion, t_Args...>* Thread::ThreadExiter<t_Funtion, t_Args...>::m_Callback;

【讨论】：