C/C++编程：元组tuple

我就是我 2022-05-20 02:20 231阅读 0赞

> 了解过python的程序员都应该知道元素的概念，纵观传统C++中的容器，除了std::pair外，似乎没有现成的结构能够用来存放不同类型的数据(通常我们会自己定义结构)，但std::pair只能保留两个元素。因此**C++11引入了tuple扩展了pair的概念，能够拥有任意数量的元素**
> 
> 元组是”多元数组“的缩写

#include <tuple>
    #include <iostream>
    #include <variant>
    
    auto get_student(int id)
    {
        if (id == 0)
            return std::make_tuple(3.8, 'A', "John");
        if (id == 1)
            return std::make_tuple(2.9, 'C', "Jack");
        if (id == 2)
            return std::make_tuple(1.7, 'D', "Ive");
        // return type is std::tuple<double, char, std::string>
        return std::make_tuple(0.0, 'D', "null");
    }
    
    template <size_t n, typename... T>
    constexpr std::variant<T...> _tuple_index(const std::tuple<T...>& tpl, size_t i) {
        if constexpr (n >= sizeof...(T))
            throw std::out_of_range("out of range.");
        if (i == n)
            return std::variant<T...>{ std::in_place_index<n>, std::get<n>(tpl) };
        return _tuple_index<(n < sizeof...(T)-1 ? n+1 : 0)>(tpl, i);
    }
    template <typename... T>
    constexpr std::variant<T...> tuple_index(const std::tuple<T...>& tpl, size_t i) {
        return _tuple_index<0>(tpl, i);
    }
    
    template <typename T>
    auto tuple_len(T &tpl) {
        return std::tuple_size<T>::value;
    }
    
    template <typename T0, typename ... Ts>
    std::ostream & operator<< (std::ostream & s, std::variant<T0, Ts...> const & v) { 
        std::visit([&](auto && x){ s << x;}, v); 
        return s;
    }
    
    int main()
    {
        auto student = get_student(0);
        std::cout << "ID: 0, "
                  << "GPA: "   << std::get<0>(student) << ", "
                  << "Grade: " << std::get<1>(student) << ", "
                  << "Name: "  << std::get<2>(student) << '\n';
        
        double gpa;
        char grade;
        std::string name;
        
        // tuple unpack
        std::tie(gpa, grade, name) = get_student(1);
        std::cout << "ID: 1, "
                  << "GPA: "   << gpa   << ", "
                  << "Grade: " << grade << ", "
                  << "Name: "  << name  << '\n';
        
        
        std::tuple<std::string, double, double, int> t("123", 4.5, 6.7, 8);
        std::cout << std::get<std::string>(t) << std::endl;
        // std::cout << std::get<double>(t) << std::endl;   // illegal, runtime error
        std::cout << std::get<3>(t) << std::endl;
        
        // concat
        auto new_tuple = std::tuple_cat(get_student(1), std::move(t));
        
        // iteration
        for(int i = 0; i != tuple_len(new_tuple); ++i) {
            std::cout << tuple_index(new_tuple, i) << std::endl; // runtime indexing
        }
    }
    © 2021 GitHub, Inc.

//创建一个四元素的元组
        // -元素初始化为默认值(基本类型为0)
        tuple<string,int,int,complex<double>> t;
        //创建并初始化一个元组
        tuple<int,float,string> t1(41,6.3,"nico");
    
        //使用make_tuple()创建元组：自动根据value建立tuple，因此不需要明确指出元素的类型
        auto t2 = make_tuple(22,44,"nico");
    
        //将t2中的第二个值赋给t1
        get<1>(t1) = get<1>(t2);
    
        //比较和赋值
        // -包含从tuple到tuple的类型转换
        if (t1 < t2) {  // compares value for value
            t1 = t2;    // OK, assigns value for value
        }

#include <iostream>
    #include <tuple>
    #include <complex.h>
    
    using namespace std;
    
    int  main()
    {
    
        char ch = 'x'; short sh = 12; int num = 1234567; double db = 123; const char *p = "abc"; string s;
        tuple <char, short, int, double, const char * , string &>mytuple(ch, sh, num, db, p, s);
    
        // "iterate"遍历元素: tuple不是寻常的元素，不允许迭代元素[数组中唯一的例外]。可以使用成员函数get来处理元素
        // 因此必须在编译期就知道你打算处理的元素的索引
        auto autov0 = get<0>(mytuple); auto autov1 = get<1>(mytuple);
        auto autov2 = get<2>(mytuple); auto autov3 = get<3>(mytuple);
        auto autov4 = get<4>(mytuple); auto autov6 = get<5>(mytuple) = "hello";
    
        cout << autov0 << endl;cout << autov1 << endl; cout << autov2 << endl;
        cout << autov3 << endl;cout << autov4 << endl; cout << autov6 << endl;
    
    
        cin.get();
    }

# tuple与引用 #

#include <iostream>
    #include <tuple>
    #include <complex.h>
    #include <functional>
    
    using namespace std;
    
    int  main()
    {
    
        std::string s;
    
        auto x = std::make_tuple(s);
        std::get<0>(x) = "hello world";
    
        printf("s= %s, x = %s\n", s.c_str(), std::get<0>(x).c_str()) ;  //s= , x = hello world
    
        s = "";
        auto y = std::make_tuple(ref(s));
        std::get<0>(y) = "hello world";
    
        printf("s= %s, x = %s\n", s.c_str(), std::get<0>(y).c_str()) ;  //s= hello world, x = hello world
    
    
    
        cin.get();
    }

# 提取tuple中的元素 #

#include <iostream>
    #include <tuple>
    #include <complex.h>
    #include <functional>
    
    using namespace std;
    
    int  main()
    {
    
        tuple<int,float,string> t1(41,6.3,"nico");
        int i; float f; std::string s;
    
        //method 1:
        std::make_tuple(std::ref(i), std::ref(f), std::ref(s)) = t1;
        printf("%d, %f, %s\n", i, f, s.c_str());
    
        // method 2:
        i = 0; s = "", f = 0.0;
        std::tie(i, std::ignore, s) = t1;
        printf("%d, %f, %s\n", i, f, s.c_str());
    
        cin.get();
    }

![20210207164348380.png][]

# tuple输出到output stream #

tuple最初公开于boost，但是boost中可以将元素写到output stream， 而C++标准库不支持这个功能。除非你有如下头文件：

//printtuple.hpp
    
    #include <tuple>
    #include <iostream>
    
    // helper: print elements with index IDX and higher of tuple t having MAX elements
    template <int IDX, int MAX, typename... Args>
    struct PRINT_TUPLE {
      static void print (std::ostream& strm, const std::tuple<Args...>& t) {
        strm << std::get<IDX>(t) << (IDX+1==MAX ? "" : ",");
        PRINT_TUPLE<IDX+1,MAX,Args...>::print(strm,t);
      }
    };
    
    // partial specialization to end the recursion
    template <int MAX, typename... Args>
    struct PRINT_TUPLE<MAX,MAX,Args...> {
      static void print (std::ostream& strm, const std::tuple<Args...>& t) {
      }
    };
    
    // output operator for tuples
    template <typename... Args>
    std::ostream& operator << (std::ostream& strm,
                               const std::tuple<Args...>& t)
    {
        strm << "[";
        PRINT_TUPLE<0,sizeof...(Args),Args...>::print(strm,t);
        return strm << "]";
    }

使用方法：

#include <tuple>
    #include <iostream>
    #include <string>
    using namespace std;
    
    int main()
    {
        tuple <int,float,string> t(77,1.1,"more light");
        cout << "io: " << t << endl;
    }

![20210207165744709.png][]

# tuple与初值列 #

各个构造函数中，”接受不定个数的实参“被声明为explicti：

![20210207164623463.png][]

这是为了避免单一值被隐式转换为“带着一个元素”的tuple：

![20210207164721964.png][]

# tuple与其他容器使用上的不同： #

![20210207165000961.png][]

![20210207165018700.png][]

但是对pair<>和容器（除了array）是行得通的

![2021020716515522.png][]

对于tuple，正确的做法是明确将初值转换为一个tuple，比如运用make\_tuple

![20210207165239455.png][]

[20210207164348380.png]: /images/20220520/6c12ba88e1fe4c53af98be57ee957cdb.png
[20210207165744709.png]: /images/20220520/697c31e6864c445bbc2e1c5f6a564bf5.png
[20210207164623463.png]: /images/20220520/dac2c1ff5f0c4ec2a318cfe0812a1ca2.png
[20210207164721964.png]: /images/20220520/b2e91d81f5a9408a8ef009528ada7490.png
[20210207165000961.png]: /images/20220520/b984dcae2ad847bc93e1165cfbc97b05.png
[20210207165018700.png]: /images/20220520/def994c1f011418ea5ce393a0467c49b.png
[2021020716515522.png]: /images/20220520/7c516ee3f5b442b681a7fc98383b32cf.png
[20210207165239455.png]: /images/20220520/26558dd37cb34420958dbb2cde46f71d.png