C++的std::variant如何访问_C++17中std::visit与std::get的使用方法

#include <variant>
#include <iostream>

int main() {
    std::variant<int, double, std::string> v = 3.14;

    // 按类型获取
    if (v.index() == 1) {
        double val = std::get<double>(v);
        std::cout << "Value: " << val << "\n";
    }

    // 按索引获取（double 是第1个索引）
    double val2 = std::get<1>(v);
    std::cout << "By index: " << val2 << "\n";

    // 错误访问会抛异常
    try {
        std::string s = std::get<std::string>(v); // 当前不是 string
    } catch (const std::bad_variant_access&) {
        std::cout << "Not a string!\n";
    }
}

if (std::holds_alternative<double>(v)) {
    std::cout << std::get<double>(v);
}

std::visit(可调用对象, variant...);

#include <variant>
#include <vector>
#include <iostream>

int main() {
    std::vector<std::variant<int, double, std::string>> vec = {10, 3.14, "hello"};

    for (auto& v : vec) {
        std::visit([](auto& arg) {
            std::cout << arg << " (" << typeid(arg).name() << ")\n";
        }, v);
    }
}

struct MyVisitor {
    void operator()(int i) const {
        std::cout << "Integer: " << i << "\n";
    }
    void operator()(double d) const {
        std::cout << "Double: " << d << "\n";
    }
    void operator()(const std::string& s) const {
        std::cout << "String: " << s << "\n";
    }
};

// 使用
std::variant<int, double, std::string> v = "world";
std::visit(MyVisitor{}, v);

std::variant<int, double> v1 = 5;
std::variant<int, double> v2 = 3.14;

std::visit([](auto& a, auto& b) {
    std::cout << a + b << "\n";
}, v1, v2);