variant.cpp

// Guideline 17: Consider std::variant for Implementing Visitors

#include <cstdlib>
#include <iostream>
#include <string>
#include <variant>

struct Print
{
   void operator()(int value) const
   {
      std::cout << "int: " << value << '\n';
   }
   void operator()(double value) const
   {
      std::cout << "double: " << value << '\n';
   }
   void operator()(std::string const &value) const
   {
      std::cout << "string: " << value << '\n';
   }
};

int main()
{
   // Creates a default variant that contains an 'int' initialized to 0
   std::variant<int, double, std::string> v{};

   v = 42;       // Assigns the 'int' 42 to the variant
   v = 3.14;     // Assigns the 'double' 3.14 to the variant
   v = 2.71F;    // Assigns a 'float', which is promoted to 'double'
   v = "Bjarne"; // Assigns the string literal 'Bjarne' to the variant
   v = 43;       // Assigns the 'int' 43 to the variant

   int const i = std::get<int>(v); // Direct access to the value

   int *const pi = std::get_if<int>(&v); // Direct access to the value

   std::visit(Print{}, v); // Applying the Print visitor

   return EXIT_SUCCESS;
}