如何将参数列表存储到向量？

Question

如何将可变构造函数参数存储到 vector？

我失败的尝试示例：

class Combo 
{
public:
   template <class... Args>
   Combo(Args... args) 
   {
      // this->keys_.push_back(args...);

      // this->keys_.push_back(args)...;

      // this->keys_.push_back(std::forward<Args>(args...));

      //for (uint8_t arg : args...)
      //  this->keys_.push_back(arg);

      // ???
   }

private:
   std::vector<uint8_t> keys_;
};

Answer 1

C++11

for(auto &&i: {args...}) keys.push_back(std::move(i));

C++17

(keys.push_back(args), ...);

哦，抱歉，我错过了显而易见的事情：

template<class... Args> Combo(Args... args): keys_{uint8_t(args)...} {}

Answer 2

你可以这样写：

template <class... Args>
Combo(Args... args)
{
    (keys_.push_back(args), ...);
}

Answer 3

在c++17 using fold expression，你可能会

#include <vector>
#include <utility> // std::forward

class Combo
{
public:
   template <class... Args>
   Combo(Args&&... args) 
   {
      keys_.reserve(sizeof...(Args));  // reserve memory for unwanted reallocation
      (keys_.emplace_back(std::forward<Args>(args)), ...);
   }

private:
   std::vector<uint8_t> keys_;
};

但是，这将允许传递 uint8_t 以外的类型，并且对于那些可以隐式转换为 uint8_t 的类型，将进行隐式转换。

这不是我们想要的行为。因此我建议 static_assert 如下。

#include <type_traits> // std::is_same_v

template <class... Args>
Combo(Args&&... args)
{
   // to make sure that the args all are of type `uint8_t`
   static_assert((std::is_same_v<uint8_t, Args> && ...), "Args should be uint8_t");

   keys_.reserve(sizeof...(Args));  // reserve some memory for unwanted reallocation
   (keys_.emplace_back(std::forward<Args>(args)), ...);
}

这会给你以下错误

Combo obj{ 1, 2, 3, 4.f };
//                  ^^^^ --> float

Answer 4

template<typename... Args>
Combo(Args &&... args): keys_ { std::forward<Args>(args)... } {}

甚至更好

Combo(std::initializer_list<uint8_t> keys): keys_(keys) {}

如何将参数列表存储到向量？

How to store arguments list to vector?

c++

templates

constructor

variadic-templates

stdvector