由于我对自己的发现不满意，因此我尝试自己制定解决方案，并最终编写了一个小型库，该库允许在参数包上制定通用操作。我的解决方案具有以下特点：
- 允许迭代参数包的所有或部分元素，可能通过计算它们在包上的索引来指定；
- 允许将参数包的计算部分转发到可变函子；
- 只需要包含一个相对较短的头文件；
- 广泛使用完美转发以允许大量内联并避免不必要的复制/移动以最小化性能损失；
- 迭代算法的内部实现依赖于空基类优化来最小化内存消耗；
- 扩展和适应很容易（相对而言，考虑到它是模板元编程）。
我将首先展示该库可以做什么，然后发布它的实现。
用例
这是一个如何使用for_each_in_arg_pack()函数迭代包的所有参数并将输入中的每个参数传递给某些客户端提供的仿函数的示例（当然，如果参数包包含值，仿函数必须具有通用调用运算符异构类型）：
// Simple functor with a generic call operator that prints its input. This is used by the
// following functors and by some demonstrative test cases in the main() routine.
struct print
{
template<typename T>
void operator () (T&& t)
{
cout << t << endl;
}
};
// This shows how a for_each_*** helper can be used inside a variadic template function
template<typename... Ts>
void print_all(Ts&&... args)
{
for_each_in_arg_pack(print(), forward<Ts>(args)...);
}
上面的print仿函数也可以用于更复杂的计算。特别是，以下是如何迭代包中参数的子集（在本例中为sub-range）：
// Shows how to select portions of an argument pack and
// invoke a functor for each of the selected elements
template<typename... Ts>
void split_and_print(Ts&&... args)
{
constexpr size_t packSize = sizeof...(args);
constexpr size_t halfSize = packSize / 2;
cout << "Printing first half:" << endl;
for_each_in_arg_pack_subset(
print(), // The functor to invoke for each element
index_range<0, halfSize>(), // The indices to select
forward<Ts>(args)... // The argument pack
);
cout << "Printing second half:" << endl;
for_each_in_arg_pack_subset(
print(), // The functor to invoke for each element
index_range<halfSize, packSize>(), // The indices to select
forward<Ts>(args)... // The argument pack
);
}
有时，人们可能只想将参数包的一部分转发给其他可变参数函子，而不是遍历其元素并将每个元素单独传递给非可变参数函子。这是forward_subpack()算法允许做的事情：
// Functor with variadic call operator that shows the usage of for_each_***
// to print all the arguments of a heterogeneous pack
struct my_func
{
template<typename... Ts>
void operator ()(Ts&&... args)
{
print_all(forward<Ts>(args)...);
}
};
// Shows how to forward only a portion of an argument pack
// to another variadic functor
template<typename... Ts>
void split_and_print(Ts&&... args)
{
constexpr size_t packSize = sizeof...(args);
constexpr size_t halfSize = packSize / 2;
cout << "Printing first half:" << endl;
forward_subpack(my_func(), index_range<0, halfSize>(), forward<Ts>(args)...);
cout << "Printing second half:" << endl;
forward_subpack(my_func(), index_range<halfSize, packSize>(), forward<Ts>(args)...);
}
对于更具体的任务，当然可以通过索引包中的特定参数来检索它们。这是nth_value_of()函数允许做的事情，连同它的助手first_value_of()和last_value_of()：
// Shows that arguments in a pack can be indexed
template<unsigned I, typename... Ts>
void print_first_last_and_indexed(Ts&&... args)
{
cout << "First argument: " << first_value_of(forward<Ts>(args)...) << endl;
cout << "Last argument: " << last_value_of(forward<Ts>(args)...) << endl;
cout << "Argument #" << I << ": " << nth_value_of<I>(forward<Ts>(args)...) << endl;
}
另一方面，如果参数包是同质的（即所有参数都具有相同的类型），则下面的公式可能更可取。元函数允许确定参数包中的is_homogeneous_pack<>所有类型是否都是同质的，主要用于static_assert()语句中：
// Shows the use of range-based for loops to iterate over a
// homogeneous argument pack
template<typename... Ts>
void print_all(Ts&&... args)
{
static_assert(
is_homogeneous_pack<Ts...>::value,
"Template parameter pack not homogeneous!"
);
for (auto&& x : { args... })
{
// Do something with x...
}
cout << endl;
}
最后，由于lambda只是函子的语法糖，它们也可以与上述算法结合使用；但是，在C++ 支持泛型 lambda之前，这仅适用于同构参数包。以下示例还显示了homogeneous-type<>元函数的用法，它返回同质包中所有参数的类型：
// ...
static_assert(
is_homogeneous_pack<Ts...>::value,
"Template parameter pack not homogeneous!"
);
using type = homogeneous_type<Ts...>::type;
for_each_in_arg_pack([] (type const& x) { cout << x << endl; }, forward<Ts>(args)...);
这基本上是图书馆允许做的事情，但我相信它甚至可以扩展到执行更复杂的任务。
执行
现在是实现，这本身有点棘手，所以我将依靠注释来解释代码并避免使这篇文章太长（也许它已经是）：
#include <type_traits>
#include <utility>
//===============================================================================
// META-FUNCTIONS FOR EXTRACTING THE n-th TYPE OF A PARAMETER PACK
// Declare primary template
template<int I, typename... Ts>
struct nth_type_of
{
};
// Base step
template<typename T, typename... Ts>
struct nth_type_of<0, T, Ts...>
{
using type = T;
};
// Induction step
template<int I, typename T, typename... Ts>
struct nth_type_of<I, T, Ts...>
{
using type = typename nth_type_of<I - 1, Ts...>::type;
};
// Helper meta-function for retrieving the first type in a parameter pack
template<typename... Ts>
struct first_type_of
{
using type = typename nth_type_of<0, Ts...>::type;
};
// Helper meta-function for retrieving the last type in a parameter pack
template<typename... Ts>
struct last_type_of
{
using type = typename nth_type_of<sizeof...(Ts) - 1, Ts...>::type;
};
//===============================================================================
// FUNCTIONS FOR EXTRACTING THE n-th VALUE OF AN ARGUMENT PACK
// Base step
template<int I, typename T, typename... Ts>
auto nth_value_of(T&& t, Ts&&... args) ->
typename std::enable_if<(I == 0), decltype(std::forward<T>(t))>::type
{
return std::forward<T>(t);
}
// Induction step
template<int I, typename T, typename... Ts>
auto nth_value_of(T&& t, Ts&&... args) ->
typename std::enable_if<(I > 0), decltype(
std::forward<typename nth_type_of<I, T, Ts...>::type>(
std::declval<typename nth_type_of<I, T, Ts...>::type>()
)
)>::type
{
using return_type = typename nth_type_of<I, T, Ts...>::type;
return std::forward<return_type>(nth_value_of<I - 1>((std::forward<Ts>(args))...));
}
// Helper function for retrieving the first value of an argument pack
template<typename... Ts>
auto first_value_of(Ts&&... args) ->
decltype(
std::forward<typename first_type_of<Ts...>::type>(
std::declval<typename first_type_of<Ts...>::type>()
)
)
{
using return_type = typename first_type_of<Ts...>::type;
return std::forward<return_type>(nth_value_of<0>((std::forward<Ts>(args))...));
}
// Helper function for retrieving the last value of an argument pack
template<typename... Ts>
auto last_value_of(Ts&&... args) ->
decltype(
std::forward<typename last_type_of<Ts...>::type>(
std::declval<typename last_type_of<Ts...>::type>()
)
)
{
using return_type = typename last_type_of<Ts...>::type;
return std::forward<return_type>(nth_value_of<sizeof...(Ts) - 1>((std::forward<Ts>(args))...));
}
//===============================================================================
// METAFUNCTION FOR COMPUTING THE UNDERLYING TYPE OF HOMOGENEOUS PARAMETER PACKS
// Used as the underlying type of non-homogeneous parameter packs
struct null_type
{
};
// Declare primary template
template<typename... Ts>
struct homogeneous_type;
// Base step
template<typename T>
struct homogeneous_type<T>
{
using type = T;
static const bool isHomogeneous = true;
};
// Induction step
template<typename T, typename... Ts>
struct homogeneous_type<T, Ts...>
{
// The underlying type of the tail of the parameter pack
using type_of_remaining_parameters = typename homogeneous_type<Ts...>::type;
// True if each parameter in the pack has the same type
static const bool isHomogeneous = std::is_same<T, type_of_remaining_parameters>::value;
// If isHomogeneous is "false", the underlying type is the fictitious null_type
using type = typename std::conditional<isHomogeneous, T, null_type>::type;
};
// Meta-function to determine if a parameter pack is homogeneous
template<typename... Ts>
struct is_homogeneous_pack
{
static const bool value = homogeneous_type<Ts...>::isHomogeneous;
};
//===============================================================================
// META-FUNCTIONS FOR CREATING INDEX LISTS
// The structure that encapsulates index lists
template <unsigned... Is>
struct index_list
{
};
// Collects internal details for generating index ranges [MIN, MAX)
namespace detail
{
// Declare primary template for index range builder
template <unsigned MIN, unsigned N, unsigned... Is>
struct range_builder;
// Base step
template <unsigned MIN, unsigned... Is>
struct range_builder<MIN, MIN, Is...>
{
typedef index_list<Is...> type;
};
// Induction step
template <unsigned MIN, unsigned N, unsigned... Is>
struct range_builder : public range_builder<MIN, N - 1, N - 1, Is...>
{
};
}
// Meta-function that returns a [MIN, MAX) index range
template<unsigned MIN, unsigned MAX>
using index_range = typename detail::range_builder<MIN, MAX>::type;
//===============================================================================
// CLASSES AND FUNCTIONS FOR REALIZING LOOPS ON ARGUMENT PACKS
// Implementation inspired by @jogojapan's answer to this question:
// http://stackoverflow.com/questions/14089637/return-several-arguments-for-another-function-by-a-single-function
// Collects internal details for implementing functor invocation
namespace detail
{
// Functor invocation is realized through variadic inheritance.
// The constructor of each base class invokes an input functor.
// An functor invoker for an argument pack has one base class
// for each argument in the pack
// Realizes the invocation of the functor for one parameter
template<unsigned I, typename T>
struct invoker_base
{
template<typename F, typename U>
invoker_base(F&& f, U&& u) { f(u); }
};
// Necessary because a class cannot inherit the same class twice
template<unsigned I, typename T>
struct indexed_type
{
static const unsigned int index = I;
using type = T;
};
// The functor invoker: inherits from a list of base classes.
// The constructor of each of these classes invokes the input
// functor with one of the arguments in the pack.
template<typename... Ts>
struct invoker : public invoker_base<Ts::index, typename Ts::type>...
{
template<typename F, typename... Us>
invoker(F&& f, Us&&... args)
:
invoker_base<Ts::index, typename Ts::type>(std::forward<F>(f), std::forward<Us>(args))...
{
}
};
}
// The functor provided in the first argument is invoked for each
// argument in the pack whose index is contained in the index list
// specified in the second argument
template<typename F, unsigned... Is, typename... Ts>
void for_each_in_arg_pack_subset(F&& f, index_list<Is...> const& i, Ts&&... args)
{
// Constructors of invoker's sub-objects will invoke the functor.
// Note that argument types must be paired with numbers because the
// implementation is based on inheritance, and one class cannot
// inherit the same base class twice.
detail::invoker<detail::indexed_type<Is, typename nth_type_of<Is, Ts...>::type>...> invoker(
f,
(nth_value_of<Is>(std::forward<Ts>(args)...))...
);
}
// The functor provided in the first argument is invoked for each
// argument in the pack
template<typename F, typename... Ts>
void for_each_in_arg_pack(F&& f, Ts&&... args)
{
for_each_in_arg_pack_subset(f, index_range<0, sizeof...(Ts)>(), std::forward<Ts>(args)...);
}
// The functor provided in the first argument is given in input the
// arguments in whose index is contained in the index list specified
// as the second argument.
template<typename F, unsigned... Is, typename... Ts>
void forward_subpack(F&& f, index_list<Is...> const& i, Ts&&... args)
{
f((nth_value_of<Is>(std::forward<Ts>(args)...))...);
}
// The functor provided in the first argument is given in input all the
// arguments in the pack.
template<typename F, typename... Ts>
void forward_pack(F&& f, Ts&&... args)
{
f(std::forward<Ts>(args)...);
}
结论
当然，即使我对这个问题提供了自己的答案（实际上是因为这个事实），我很想知道是否存在我错过的替代或更好的解决方案——除了“相关作品”部分中提到的那些的问题。