如何对可变参数模板函数的异构参数包进行通用计算?

安迪(Andy Prowl)

前提:

在使用了可变参数模板后,我意识到,要完成一些超出平凡的元编程任务的事情,很快就会变得很麻烦。特别是,我发现自己希望的执行方式上的参数包一般操作迭代拆分循环在一个std::for_each样的方式,等等。

在观看了C ++和Beyond 2012的Andrei Alexandrescu的关于static ifC ++(从D编程语言中借用的结构)的可取性的演讲之后,我感到有些东西static for也将派上用场-我觉得这些static结构中的更多可以带来利益。

于是我开始想知道如果有一种方法来实现这样的一个可变参数模板函数的参数包(伪代码):

template<typename... Ts>
void my_function(Ts&&... args)
{
    static for (int i = 0; i < sizeof...(args); i++) // PSEUDO-CODE!
    {
        foo(nth_value_of<i>(args));
    }
}

它将在编译时转换为如下形式:

template<typename... Ts>
void my_function(Ts&&... args)
{
    foo(nth_value_of<0>(args));
    foo(nth_value_of<1>(args));
    // ...
    foo(nth_value_of<sizeof...(args) - 1>(args));
}

原则上,static_for将允许进行更精细的处理:

template<typename... Ts>
void foo(Ts&&... args)
{
    constexpr s = sizeof...(args);

    static for (int i = 0; i < s / 2; i++)
    {
        // Do something
        foo(nth_value_of<i>(args));
    }

    static for (int i = s / 2; i < s; i++)
    {
        // Do something different
        bar(nth_value_of<i>(args));
    }
}

或像这样的更具表现力的习语:

template<typename... Ts>
void foo(Ts&&... args)
{
    static for_each (auto&& x : args)
    {
        foo(x);
    }
}

相关工作:

我在网上做了一些搜索,发现确实存在一些东西

  • 该链接描述了如何将参数包转换为Boost.MPL向量,但这仅达到目标的一半(如果不是更少的话)。
  • 关于SO的这个问题似乎需要类似且略相关的元编程功能(将参数包分成两半)-实际上,关于SO的几个问题似乎与此问题相关,但是我没有答案已阅读,令人满意地解决了恕我直言;
  • Boost.Fusion定义了将参数包转换为元组的算法,但我希望:
    1. 不创建不必要的临时变量来保存可以(并且应该)完美地转发给某些通用算法的参数;
    2. 有一个小型的,自包含的库可以执行此操作,而Boost.Fusion可能包含的内容远远超过解决此问题所需的内容。

题:

是否有一种相对简单的方法(可能通过一些模板元编程)来实现我正在寻找的目标,而又不会受到现有方法的限制?

安迪(Andy Prowl)

由于我对发现的结果不满意,因此我尝试自己制定一个解决方案,并最终编写了一个小型库,该允许对参数包进行通用运算。我的解决方案具有以下功能:

  • 允许迭代参数包的所有或某些元素,可能通过计算它们在参数包上的索引来指定
  • 允许将参数包的计算部分转发给可变参数。
  • 只需要包含一个相对较短的头文件;
  • 广泛使用完美的转发以进行大量内联,并避免不必要的复制/移动,以将性能损失降至最低;
  • 迭代算法的内部实现依靠空基类优化来最大程度地减少内存消耗。
  • 扩展和适应很容易(相对而言,考虑到它是模板元编程)。

我将首先展示如何使用该库,然后发布其实现

用例

这是一个示例,说明如何使用该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客户端提供的某些函子(当然,如果参数包包含值,则函子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)...);
}

print上面函子也可以用于更复杂的计算中。特别是,这是一个如何迭代包中参数子集(在这种情况下,是子范围)的方式:

// 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,一个人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它们分别传递给非可变函子。这是forward_subpack()算法允许执行的操作:

// 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,当然可以通过索引来检索包中的特定参数这是该nth_value_of()函数及其助手first_value_of()允许执行的操作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,如果自变量包是同构的(即所有自变量具有相同的类型),则可能更希望使用以下公式。所述is_homogeneous_pack<>元函数允许确定在参数包的所有类型是否是同质的,并且主要是指在所使用static_assert()的语句:

// 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,因此它们也可以与上述算法结合使用。但是,直到C ++支持通用lambda为止,这仅适用于同类参数包。以下示例还显示了homogeneous-type<>元函数的用法,该元函数返回同构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)...);

这基本上是库允许的操作,但是我相信它甚至可以扩展来执行更复杂的任务。

实施方式

现在是实现,它本身有点棘手,所以我将依靠注释来解释代码,并避免将本文过长(也许已经很久了):

#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),但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”部分中提到的解决方案之外这个问题。

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