#include #include #include using namespace std; class Cat { public: void Speak() { std::cout<<"meow!\n"; } }; class Dog { public: void Speak() { std::cout<<"bow bow!\n"; } }; struct Visitor { auto operator()(Cat c) { c.Speak(); } auto operator()(Dog d) { d.Speak(); } }; void Get (variant anim) { auto f = get(anim); f.Speak(); } int main() { // if animal is not initialized explicitly // it will be initialized based on first // type variant animal = Cat(); // visit the currently held animal, which // is cat cout << "Using visit:\n"; visit(Visitor(), animal); // current animal is now dog animal = Dog(); visit(Visitor(), animal); // retrieve the current animal based on // expicit type using get_if cout << "\nUsing get_if:\n"; if(auto animal_type = get_if(&animal); animal_type) { cout << "Current animal is Cat and it says "; animal_type->Speak(); } else if (auto animal_type = get_if(&animal); animal_type) { cout << "Current animal is Dog and it says "; animal_type->Speak(); } // retrieve current animal using holds_alternative helper cout << "\nUsing holds_alternative:\n"; if(holds_alternative(animal)) { cout << "Current animal is Dog and it says "; visit(Visitor(), animal); } // make current animal Cat animal = Cat(); if(holds_alternative(animal)) { cout << "Current animal is Cat and it says "; visit(Visitor(), animal); } // using get can throw bad_variant_access exception cout << "\nUsing direct type:\n"; auto get_animal = [](auto anim, auto idx){ try { if(idx == 0){ cout << "Getting Cat\n"; auto curr_animal = get(anim); curr_animal.Speak(); } else if(idx == 1) { cout << "Getting Dog\n"; auto curr_animal = get(anim); curr_animal.Speak(); } } catch(bad_variant_access) { cout << "Current animal is not Dog, try again\n"; } }; get_animal(animal, 1); animal = Dog(); get_animal(animal, 1); }

std_variant_example.cpp