蓝桥ROS机器人之现代C++学习笔记4.3 元组

Posted 2022-04-09 zhangrelay

学习程序如下：

#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
    

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