通过索引访问可变参数模板中的类型 [英] Access a type in a variadic template by index
问题描述
我想通过索引获取可变参数模板中的类型.索引被指定为模板参数.我设法找到了一个可行的"hack",但我认为这不符合可变参数模板编程的精神.此外,它使用额外的内存.
I would like to obtain a type in a variadic template by index. The index is specified as a template argument. I managed to find a 'hack' that works, but I believe that it is not in the spirit of variadic template programming. Besides, it uses extra memory.
下面是带有一些解释的代码:
Here is the code with some explanations:
template <typename... InputPortTypes>
class PipelineReceiver
{
protected:
// This tuple is used for storing types only
// Hence, I would like to get rid of it, but I am not sure how.
std::tuple<
std::function<std::unique_ptr<InputPortTypes> (int)>...
> InputPortsTuple;
// This vector is used for storing the actual objects
// This is needed to be able to access/change its elements
// during run time later on.
// The vector is used for storage of function pointers (i.e. of type std::function)
// that represent methods of another object upstream the pipeline.
std::vector<boost::any> InputPortsVector;
public:
PipelineReceiver()
{
// create an empty vector of the required size
InputPortsVector.resize(sizeof...(InputPortTypes));
}
void connectPorts(int InputPortIndex, boost::any c_OutputPort)
{
// connect ports
InputPortsVector[InputPortIndex] = c_OutputPort;
}
// this function needs to be modified to avoid using InputPortsTuple
template<int N>
void getInputPortValue(void)
{
std::cout <<
*boost::any_cast<decltype(std::get<N>(this -> InputPortsTuple))>(
InputPortsVector[N]
)(0) <<
std::endl;
}
};
我想删除对象 InputPortsTuple
并将其替换为某种形式的递归过程,以推断 getInputPortValue
中的类型.
I would like to remove the object InputPortsTuple
and replace it with some form of a recursive procedure for inferring the types in getInputPortValue
.
理想情况下,我希望 N
是动态参数,而不是模板参数.但是,我不确定是否可行.
Ideally, I would like N
to be a dynamic parameter instead of a template argument. However, I am not sure if this is possible.
推荐答案
You could simply abuse std::tuple_element
:
typename std::tuple_element<N, std::tuple<InputPortTypes...>>::type
注意:如果可以使用C ++ 14,
Note: if you can use C++14,
std::tuple_element_t<N, std::tuple<InputPortTypes...>>
是执行相同操作的更好方法.不过,并非所有普通的编译器都知道它.
is a nicer way to do the same thing. Not all common compilers know it yet, though.
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