C++11, 14, 17对tuple元素的访问详情

#include <iostream>
#include <tuple>

#define CPP11 (__cplusplus < 201401L)
#define CPP14 (__cplusplus < 201701L and __cplusplus > 201401L)
#define CPP17 (__cplusplus >= 201702L)

class Moo {
  Moo() { ::printf("%s\n", __PRETTY_FUNCTION__); }
  Moo(Moo const &) { ::printf("%s\n", __PRETTY_FUNCTION__); }
  Moo(Moo &&) { ::printf("%s\n", __PRETTY_FUNCTION__); }
  Moo &operator=(Moo const &) noexcept {
    ::printf("%s\n", __PRETTY_FUNCTION__);
    return *this;
  }

  Moo &operator=(Moo &&) noexcept {
    ::printf("%s\n", __PRETTY_FUNCTION__);
    return *this;
  }

  ~Moo() { ::printf("%s\n", __PRETTY_FUNCTION__); }
};

int main() {
  std::cout << "c++ version:" << __cplusplus << std::endl;
  auto &&tp = std::make_tuple<Moo, const char *, int>(Moo(), "hello world", 3);

#if CPP11
  auto &&first = std::get<0>(tp);
  auto &&second = std::get<1>(tp);
  auto &&third = std::get<2>(tp);
#endif

#if CPP14
  auto &&first = std::get<Moo>(tp);
  auto &&second = std::get<const char *>(tp);
  auto &&third = std::get<int>(tp);
#endif

#if CPP17
  auto [a, b, c] = tp;
  auto &[first, second, third] = tp;
#endif

  return 0;
}

{
    std::tuple<int, double, std::string> first;    // 创建一个空的元组，需要指定元组元素的数据类型，调用各个成员的默认构造函数进行初始化。
    std::tuple<int, double, std::string> second(first);  // 拷贝构造
    std::tuple<int, char> third(10, 'a');        // 创建并初始化，使用小括号初始化
    std::tuple<int, std::string, double> fourth{42, "Test", -3.14};  // 创建并初始化，使用新的大括号初始化列表方式初始化
    std::tuple<int, char> fifth(std::make_tuple(20, 'b'));     // 移动构造，使用模板库的make_tuple
    first = std::make_tuple(1, 3.14, "tuple");       // 移动赋值
    int i_third = 3;
    std::tuple<int&> sixth(std::ref(i_third));   // 创建一个元组，元组的元素可以被引用
} 

std::get<N>()

{
    int n = 1;
    auto t = std::make_tuple(10, "Test", 3.14, std::ref(n), n);
    // get尖括号中的值必须是一个整型常量表达式。从0开始计数，意味着get<0>是第一个成员。
    std::cout << "The value of t is "  << "("
               << std::get<0>(t) << ", " << std::get<1>(t) << ", "
               << std::get<2>(t) << ", " << std::get<3>(t) << ", "
               << std::get<4>(t) << ")\n";
    // 由于get返回指定元素的引用，所以可用来修改指定位置的元素的值。此处因为第4个元素是引用类型，所以被引用的值也会改变
    std::get<3>(t) = 9;
    std::cout << n << std::endl;
}

{
    std::tuple<char, int, long, std::string> first('A', 2, 3, "4");
    int i_count = std::tuple_size<decltype(first)>::value;  // 使用std::tuple_size计算元组个数
    std::cout << "the number of elements of a tuple:" << i_count << "\n";
}

{ // std::tie: function template, Constructs a tuple object whose elements are references
  // to the arguments in args, in the same order
  // std::ignore: object, This object ignores any value assigned to it. It is designed to be used as an
  // argument for tie to indicate that a specific element in a tuple should be ignored.
    
    int myint;
    char mychar;
 
    std::tuple<int, float, char> mytuple;
 
    mytuple = std::make_tuple(10, 2.6, 'a');          // packing values into tuple
 
    std::tie(myint, std::ignore, mychar) = mytuple;   // unpacking tuple into variables
 
    std::cout << "myint contains: " << myint << '\n';
    std::cout << "mychar contains: " << mychar << '\n';
}

{
    std::tuple<int, std::string> third(9, std::string("ABC"));
    
    // 得到元组第1个元素的类型，用元组第一个元素的类型声明一个变量
    std::tuple_element<1, decltype(third)>::type val_1;
    
    // 获取元组的第一个元素的值
    val_1 = std::get<1>(third);
    std::cout << "val_1 = " << val_1.c_str() << "\n";
}

{
    std::tuple<char, int, double> first('A', 1, 2.2f);
    // 组合到一起, 使用auto， 自动推导
    auto second = std::tuple_cat(first, std::make_tuple('B', std::string("-=+")));
    // 组合到一起，可以知道每一个元素的数据类型时什么 与 auto推导效果一样
    std::tuple<char, int, double, char, std::string> third = std::tuple_cat(first, std::make_tuple('B', std::string("-=+")));
    // 输出合并后的元组内容
    int index = 0;
    std::cout << index++ << " = " << std::get<0>(second) << "\n";  // 0 = A
    std::cout << index++ << " = " << std::get<1>(second) << "\n";  // 1 = 1 
    std::cout << index++ << " = " << std::get<2>(second) << "\n";  //  2 = 2.2
    std::cout << index++ << " = " << std::get<3>(second) << "\n";  // 3 = B
    std::cout << index++ << " = " << std::get<4>(second).c_str() << "\n"; // 4 = -=+
}