c++ 17 demo

// Cpp.cpp : 此文件包含 "main" 函数。程序执行将在此处开始并结束。
//

#include <iostream>
#include <variant>
#include <any>
#include <string>
#include <optional>

using namespace std;
struct Visitor
{
    void operator()(int m) {
        cout << m << endl;
    }
    void operator()(string m) {
        cout << m.data() << endl;
    }
    void operator()(float m) {
        cout << m << endl;
    }
};
std::optional<int> asInt(const std::string& s)
{
    try
    {
        return std::stoi(s);
    }
    catch (...)
    {
        return std::nullopt;
    }
}
int mains()
{
    //variant 的好处是可以作为函数返回值
    using va = std::variant<std::string, int, float>;
    va v = 23;
    cout << std::get<int>(v);
    va v2 = "this is hello";
    cout << std::get<string>(v2).data();
    //std::visit 访问variant
    any data = "this is any data 12";
    v = 23;
    std::visit(Visitor(), v);        //23
    v = "this is visitor";
    std::visit(Visitor(), v);    //"this is visitor"
    v = 3.14157f;
    std::visit(Visitor(), v);    //3.14159
    //any
    std::any a = 2;
    cout << endl << a.type().name() << endl;
    a = "string";
    cout << endl << a.type().name() << endl;
    a = nullptr;
    cout << endl << a.type().name() << endl;
    if (nullptr == std::any_cast<std::nullptr_t>(a))
    {
        cout << "nullptr" << endl;
    }

    string tstr = "this is my str";
    //string_view 不会创建tstr的副本
    std::string_view view = tstr;
    cout << view.data() << endl;    //this is my str
    tstr = "i changed it";
    cout << view.data() << endl;    //i changed it
    //条件内声明变量
    int m = 12;
    if (int j = 23; j < 24)
    {
        cout << j;
    }
    //
    // convert string to int if possible:
    /*std::optional<int> asInt(const std::string& s)
    {
        try
        {
            return std::stoi(s);
        }
        catch (...)
        {
            return std::nullopt;
        }
    }*/
    for (auto s : { "42", " 077", "hello", "0x33" })
    {
        std::optional<int> oi = asInt(s);
        if (oi.emplace()) {    //.has_value() 
            std::cout << "convert '" << s << "' to int: " << *oi << "\n";
        }
        else {
            std::cout << "can't convert '" << s << "' to int\n";
        }
    }
    std::optional<int> opt1 = std::make_optional(23);
    std::optional<int> opt2 = 22;
    //访问值*opt2 或者opt2.value()
    cout << *opt2 << endl;        //opt2
    cout << opt2.value()<<endl;    //opt2
    opt2.emplace(34);    //重新赋值
    cout << *opt2 << endl;    //34
    cout << opt2.value()<<endl;    //34
    opt2.reset();    //清空
    //opt2.value();    //为nullopt时，调用value报异常
    cout << opt2.value()<<endl;

}