类模板 std::variant 表示一个类型安全的联合体。 std::variant 的一个实例在任意时刻要么保有其一个可选类型之一的值，要么在错误情况下无值。

std::visit应用观览器 vis 到 variant 变量组 vars 。

std::visit参数:

例子1

• 观察器vis针对variant每个可选项必须有之对应的函数，不能缺。
• 返回值是观察器共同返回的类型，不能不一致。

#include <iostream>
#include <variant>

struct Apple { };
struct Orange { };
struct Banana { };
struct Pear { };

template<class... Ts> struct overload : Ts... { using Ts::operator()...; };
template<class... Ts> overload(Ts...) -> overload<Ts...>; // C++20不在需要

int main() {
    std::variant<Apple, Orange, Banana, Pear> package =  { Banana() };

    auto result =  std::visit(overload{
        [](Apple& )      { std::cout << "Apple\n"; return 1;},
        [](Orange& )   { std::cout << "Orange\n"; return 2;},
        [](Banana& )   { std::cout << "Banana\n"; return 3;},
        [](Pear& ) { std::cout << "Pear\n"; return 4;}
    }, package);
    
    std::cout << result << std::endl;    
}

例子2

观察器的是多个variant选项笛卡尔积

#include <iostream>
#include <variant>

struct Apple { };
struct Orange { };
struct Banana { };
struct Pear { };

struct Light { };
struct Heavy { };

template<class... Ts> struct overload : Ts... { using Ts::operator()...; };
template<class... Ts> overload(Ts...) -> overload<Ts...>; // C++20不在需要

int main() {
    std::variant<Apple, Orange> package1 =  { Orange() };
    std::variant<Light, Heavy> package2 =  { Light() };

    auto result =  std::visit(overload{
        [](Apple&, Light&)      { std::cout << "Light Apple\n"; return 1;},
        [](Orange&,  Light&)  { std::cout << "Light Orange\n"; return 2;},
        [](Apple&, Heavy&)    { std::cout << "Heavy Apple\n"; return 3;},
        [](Orange&, Heavy&) { std::cout << "Heavy Orange\n"; return 4;}
    }, package1, package2);
    
    std::cout << result << std::endl;    
}

https://wandbox.org/nojs/gcc-head
https://wandbox.org/nojs/clang-head
https://wandbox.org/permlink/wyeJ1TNulMwEM6QJ
https://wandbox.org/permlink/qpn9gVjb4vzKja0I