243.md

Info

• Did you know about C++2X Pattern matching using is and as proposal?

  • http://wg21.link/p2392

Example

void f(auto const& x) {
  inspect (x) {
    i as int           => std::cout << "int " << i;
    [_,y] is [0,even]  => std::cout << "point on y-axis and even y " << y;
    [a,b] is [int,int] => std::cout << "2-int tuple " << a << " " << b;
    s as std::string   => std::cout << "string \"" + s + "\"";
    is _               => std::cout << "((no matching value))";
  }
}

int main() {
  f(42);
  f(std::pair{0, 2});
  f(std::tuple{1, 2});
  f("str");
  struct {} foo;
  f(foo);
}

int 42
point on y-axis and even y 2
2-int tuple 1 2
istring "str"
((no matching value))

https://circle.godbolt.org/z/64E99s3qq

Puzzle

• Can you implement inspect function which will applly a pattern matching proposal to output values/types into a given ostream?

void inspect(const auto& value, std::ostream& os); // TODO

int main() {
  "inspect"_test = [] {
    "empty"_test = [] {
      std::stringstream str{};
      inspect(std::tuple{}, str);
      expect("(...)"sv == str.str());
    };

    "int, double"_test = [] {
      std::stringstream str{};
      inspect(std::tuple{1, 2.}, str);
      expect("(1, 2)"sv == str.str());
    };

    "int, double, string"_test = [] {
      std::stringstream str{};
      inspect(std::tuple{1, 2., "str"}, str);
      expect("(int, double, _)"sv == str.str());
    };

    "int, double, long"_test = [] {
      std::stringstream str{};
      inspect(std::tuple{1, 2., 3l}, str);
      expect("(int, double, _)"sv == str.str());
    };

    "int, int, int"_test = [] {
      std::stringstream str{};
      inspect(std::tuple{4, 2, 3}, str);
      expect("(int, ...)"sv == str.str());
    };

    "int"_test = [] {
      std::stringstream str{};
      inspect(std::tuple{42}, str);
      expect("(int, ...)"sv == str.str());
    };

    "double"_test = [] {
      std::stringstream str{};
      inspect(std::tuple{42.0}, str);
      expect("(...)"sv == str.str());
    };
  };
}

https://circle.godbolt.org/z/bf8M7461K

Solutions