c++ 17 demo

  1 // Cpp.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。
  2 //
  3 
  4 #include <iostream>
  5 #include <variant>
  6 #include <any>
  7 #include <string>
  8 #include <optional>
  9 
 10 using namespace std;
 11 struct Visitor
 12 {
 13     void operator()(int m) {
 14         cout << m << endl;
 15     }
 16     void operator()(string m) {
 17         cout << m.data() << endl;
 18     }
 19     void operator()(float m) {
 20         cout << m << endl;
 21     }
 22 };
 23 std::optional<int> asInt(const std::string& s)
 24 {
 25     try
 26     {
 27         return std::stoi(s);
 28     }
 29     catch (...)
 30     {
 31         return std::nullopt;
 32     }
 33 }
 34 int mains()
 35 {
 36     //variant 的好处是可以作为函数返回值
 37     using va = std::variant<std::string, int, float>;
 38     va v = 23;
 39     cout << std::get<int>(v);
 40     va v2 = "this is hello";
 41     cout << std::get<string>(v2).data();
 42     //std::visit 访问variant
 43     any data = "this is any data 12";
 44     v = 23;
 45     std::visit(Visitor(), v);        //23
 46     v = "this is visitor";
 47     std::visit(Visitor(), v);    //"this is visitor"
 48     v = 3.14157f;
 49     std::visit(Visitor(), v);    //3.14159
 50     //any
 51     std::any a = 2;
 52     cout << endl << a.type().name() << endl;
 53     a = "string";
 54     cout << endl << a.type().name() << endl;
 55     a = nullptr;
 56     cout << endl << a.type().name() << endl;
 57     if (nullptr == std::any_cast<std::nullptr_t>(a))
 58     {
 59         cout << "nullptr" << endl;
 60     }
 61 
 62     string tstr = "this is my str";
 63     //string_view 不会创建tstr的副本
 64     std::string_view view = tstr;
 65     cout << view.data() << endl;    //this is my str
 66     tstr = "i changed it";
 67     cout << view.data() << endl;    //i changed it
 68     //条件内声明变量
 69     int m = 12;
 70     if (int j = 23; j < 24)
 71     {
 72         cout << j;
 73     }
 74     //
 75     // convert string to int if possible:
 76     /*std::optional<int> asInt(const std::string& s)
 77     {
 78         try
 79         {
 80             return std::stoi(s);
 81         }
 82         catch (...)
 83         {
 84             return std::nullopt;
 85         }
 86     }*/
 87     for (auto s : { "42", " 077", "hello", "0x33" })
 88     {
 89         std::optional<int> oi = asInt(s);
 90         if (oi.emplace()) {    //.has_value() 
 91             std::cout << "convert '" << s << "' to int: " << *oi << "n";
 92         }
 93         else {
 94             std::cout << "can't convert '" << s << "' to intn";
 95         }
 96     }
 97     std::optional<int> opt1 = std::make_optional(23);
 98     std::optional<int> opt2 = 22;
 99     //访问值*opt2 或者opt2.value()
100     cout << *opt2 << endl;        //opt2
101     cout << opt2.value()<<endl;    //opt2
102     opt2.emplace(34);    //重新赋值
103     cout << *opt2 << endl;    //34
104     cout << opt2.value()<<endl;    //34
105     opt2.reset();    //清空
106     //opt2.value();    //为nullopt时，调用value报异常
107     cout << opt2.value()<<endl;
108 
109 }