C++: What is the difference between std::variant and std::visit?

1#include <variant>
2
3std::variant<int, double, std::string> v;
4
5// Visit with overloaded lambda (C++17)
6std::visit([](auto&& arg) {
7    using T = std::decay_t<decltype(arg)>;
8    if constexpr (std::is_same_v<T, int>) {
9        std::cout << "int: " << arg;
10    } else if constexpr (std::is_same_v<T, double>) {
11        std::cout << "double: " << arg;
12    } else {
13        std::cout << "string: " << arg;
14    }
15}, v);
16
17// Overloaded pattern (C++17)
18template<class... Ts> struct overloaded : Ts... { using Ts::operator()...; };
19template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
20
21std::visit(overloaded{
22    [](int i) { std::cout << i; },
23    [](double d) { std::cout << d; },
24    [](const std::string& s) { std::cout << s; }
25}, v);
26
27// Visit multiple variants
28std::variant<int, double> v1 = 1;
29std::variant<int, std::string> v2 = "hello";
30
31std::visit([](auto&& a, auto&& b) {
32    std::cout << a << ", " << b;
33}, v1, v2);
34
35// Index
36std::cout << v.index();  // Current type index
37
38// Get
39if (auto* p = std::get_if<int>(&v)) {
40    std::cout << *p;
41}