Question

我希望类Foo存储一个函数指针，它在构造时得到，并在某个时刻调用此函数。我看了这两个问题寻求帮助：

根据答案，我们提出了这段代码：

#include <functional>

template < int N, typename... ARGS >
struct apply_func {
   static void applyTuple( std::function<void(ARGS...)>& f,
                          const std::tuple<ARGS...>& t,
                          ARGS... args ) {
    apply_func<N-1>::applyTuple( f, t, std::get<N-1>( t ), args... );
  }
};

template <typename... ARGS>
struct apply_func<0,ARGS...>
{
  static void applyTuple( std::function<void(ARGS...)>& f,
                          const std::tuple<ARGS...>& /* t */,
                          ARGS... args ) {
    f( args... );
  }
};

template < typename... ARGS >
void applyTuple( std::function<void(ARGS...)>& f,
                 std::tuple<ARGS...> const& t ) {
   apply_func<sizeof...(ARGS), ARGS...>::applyTuple( f, t );
}

template<typename... ARGS>
class Foo
{
    std::function<void(ARGS...)> m_f;
    std::tuple<ARGS...> *argument_pack;

    public:
    Foo(std::function<void(ARGS...)> f):m_f(f){}
    void operator()(ARGS... args);
    void run();

};

template<typename... ARGS>
void Foo<ARGS...>::operator()(ARGS... args)
{
    m_f(args...); // this works
}

template<typename... ARGS>
void Foo<ARGS...>::run()
{
    applyTuple<ARGS...>(m_f, *argument_pack); // this doesn't compile
}

void bar(int i, double d){}

int main(void)
{
    Foo<int,double> foo(bar);
    foo(1,1.0); // this works
    foo.run(); // this doesn't compile
}

如果使用'g ++ -std = c ++ 0x'进行编译，则最后一行的下一行将显示以下错误：

    test.cc: In function ‘void applyTuple(std::function<void(ARGS ...)>&, const std::tuple<_Elements ...>&) [with ARGS = {int, double}]’:
test.cc:52:9:   instantiated from ‘void Foo<ARGS>::run() [with ARGS = {int, double}]’
test.cc:61:17:   instantiated from here
test.cc:27:8: error: no matching function for call to ‘apply_func<2, int, double>::applyTuple(std::function<void(int, double)>&, const std::tuple<int, double>&)’
test.cc:27:8: note: candidate is:
test.cc:6:19: note: static void apply_func<N, ARGS>::applyTuple(std::function<void(ARGS ...)>&, const std::tuple<_Elements ...>&, ARGS ...) [with int N = 2, ARGS = {int, double}]
test.cc:6:19: note:   candidate expects 4 arguments, 2 provided
test.cc: In static member function ‘static void apply_func<N, ARGS>::applyTuple(std::function<void(ARGS ...)>&, const std::tuple<_Elements ...>&, ARGS ...) [with int N = 2, ARGS = {int, double}]’:
test.cc:27:8:   instantiated from ‘void applyTuple(std::function<void(ARGS ...)>&, const std::tuple<_Elements ...>&) [with ARGS = {int, double}]’
test.cc:52:9:   instantiated from ‘void Foo<ARGS>::run() [with ARGS = {int, double}]’
test.cc:61:17:   instantiated from here
test.cc:9:9: error: no matching function for call to ‘apply_func<1>::applyTuple(std::function<void(int, double)>&, const std::tuple<int, double>&, const double&, int&, double&)’
test.cc:9:9: note: candidate is:
test.cc:6:19: note: static void apply_func<N, ARGS>::applyTuple(std::function<void(ARGS ...)>&, const std::tuple<_Elements ...>&, ARGS ...) [with int N = 1, ARGS = {}]
test.cc:6:19: note:   candidate expects 2 arguments, 5 provided
test.cc: In static member function ‘static void apply_func<N, ARGS>::applyTuple(std::function<void(ARGS ...)>&, const std::tuple<_Elements ...>&, ARGS ...) [with int N = 1, ARGS = {}]’:
test.cc:9:9:   instantiated from ‘static void apply_func<N, ARGS>::applyTuple(std::function<void(ARGS ...)>&, const std::tuple<_Elements ...>&, ARGS ...) [with int N = 2, ARGS = {int, double}]’
test.cc:27:8:   instantiated from ‘void applyTuple(std::function<void(ARGS ...)>&, const std::tuple<_Elements ...>&) [with ARGS = {int, double}]’
test.cc:52:9:   instantiated from ‘void Foo<ARGS>::run() [with ARGS = {int, double}]’
test.cc:61:17:   instantiated from here
test.cc:9:9: error: no matching function for call to ‘get(const std::tuple<>&)’
test.cc:9:9: note: candidates are:
/usr/include/c++/4.6/utility:133:5: note: template<long unsigned int _Int, class _Tp1, class _Tp2> typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type& std::get(std::pair<_Tp1, _Tp2>&)
/usr/include/c++/4.6/utility:138:5: note: template<long unsigned int _Int, class _Tp1, class _Tp2> const typename std::tuple_element<_Int, std::pair<_Tp1, _Tp2> >::type& std::get(const std::pair<_Tp1, _Tp2>&)
/usr/include/c++/4.6/tuple:531:5: note: template<long unsigned int __i, class ... _Elements> typename std::__add_ref<typename std::tuple_element<__i, std::tuple<_Elements ...> >::type>::type std::get(std::tuple<_Elements ...>&)
/usr/include/c++/4.6/tuple:538:5: note: template<long unsigned int __i, class ... _Elements> typename std::__add_c_ref<typename std::tuple_element<__i, std::tuple<_Elements ...> >::type>::type std::get(const std::tuple<_Elements ...>&)

我错过了什么？谢谢！

Answer 1

我认为问题在于applyTuple将对元组的引用作为其第二个参数：

template < typename... ARGS >
void applyTuple( std::function<void(ARGS...)>& f,
                 std::tuple<ARGS...> const& t ) {
   apply_func<sizeof...(ARGS), ARGS...>::applyTuple( f, t );
}

但是，在run内部，你传入一个指向元组的指针，因为argument_pack是一个指向元组的指针：

std::tuple<ARGS...> *argument_pack;

如果您将run的正文更改为

applyTuple<ARGS...>(m_f, *argument_pack);

然后我认为你的问题应该消失。

希望这有帮助！

Answer 2

你不能说apply_func<N-1>::...。这将使用ARGS = {}实例化结构，因此“候选者需要2个参数，5个提供”错误。

此外，在applyTuple中，您使用了apply_func<sizeof...(ARGS), ARGS...>::applyTuple，它希望接受4个参数：函数f，元组t和{{1} }，但是你无法传递 args ，因此“候选人需要4个参数，2个提供”错误。

作为替代解决方案，您可以使用我的vtmp库并使用

ARGS... args

（注：需要g ++ 4.7）

Answer 3

我解决了。这是三个错误的组合：

用户指出的指针错误：templatetypedef
我不得不在这篇文章中回到原始版本的申请元组：How do I expand a tuple into variadic template function's arguments?。我将所有模板参数更改为ARGS...，因为对于元组，函数和类，它们都是相同的。老实说，我仍然不明白为什么这不起作用......
函数指针必须像我一样存储（void (*m_f) (ARGS...);）而不是std::function<void(ARGS...)>对象。当然，构造函数必须相应地更改

感谢您的回答！

调用函数并传递存储在元组中的参数？

3 个答案: