默认初始化（带值初始化）参数包 [英] Default initialized (with value initialization) parameter pack

问题描述

我可以将参数包初始化为每种类型的相应值初始化吗？

要进一步阐述，的简单函数模板

To elaborate a bit more, take the example of a simple function template

template<typename T>
void f(T arg = T())
{ 
  // eg for T=int, arg is 0 (value initialization) when default initialized
}

是否可以表达其可变对应物，即

Would it be possible to express its variadic counterpart, ie

template<typename... Args>
void F(Args... args /* how can I value initialize the parameter pack? */)
{
}

推荐答案

#include <iostream>
#include <utility>
#include <tuple>
#include <cstddef>
#include <type_traits>

template <typename... Args>
void F(Args... args)
{
    // target function, arbitrary body
    using expander = int[];
    (void)expander{ 0, (void(std::cout << args << " "), 0)... };
    std::cout << std::endl;
}

template <typename... Args, typename... Params, std::size_t... Is>
void F(std::index_sequence<Is...>, Params&&... params)
{
    F<Args...>(std::forward<Params>(params)...
             , std::decay_t<typename std::tuple_element<sizeof...(Params) + Is, std::tuple<Args...>>::type>{}...);
}

template <typename... Args, typename... Params>
auto F(Params&&... params)
    -> std::enable_if_t<(sizeof...(Args) > sizeof...(Params))>
{
    F<Args...>(std::make_index_sequence<sizeof...(Args) - sizeof...(Params)>{}
             , std::forward<Params>(params)...);
}

测试：

#include <string>

int main()
{
    // F(int, char, float = float{}, double = double{})
    F<int, char, float, double>(1, 'c');

    // F(int = int{}, char = char{}, float = float{}, double = double{})     
    F<int, char, float, double>();

    // F(const std::string&, const std::string& = std::string{})
    F<const std::string&, const std::string&>("foo");

    // F(int, int, int)
    F(1, 2, 3);
}

输出： b

Output:

1 'c' 0 0 
0 '\0' 0 0
"foo" ""
1 2 3

DEMO

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