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

void inspect(const auto& value, std::ostream& os) {
  inspect(value) {
    [x, y] is [int, double] => os << '(' << x << ", " << y << ')';
    is [int, double, _]     => os << "(int, double, _)";
    is [int, ...]           => os << "(int, ...)";
    is _                    => os << "(...)";
  }
}

https://circle.godbolt.org/z/ea8zMEres

void inspect(const auto& value, std::ostream& os) {
    inspect(value) {
        [a, b] as [int, double] => os << "(" << a << ", " << b << ")";
        is [int, double, _] => os << "(int, double, _)";
        is [int, ...] => os << "(int, ...)" ;
        is [...] => os << "(...)";
    }
}

https://circle.godbolt.org/z/qExzx3ddr

void inspect(const auto& value, std::ostream& os) {
    inspect(value) {
        [a, b] => os << "(" << a << ", " << b << ")";
        is [int, double, _] => os << "(int, double, _)";
        is [int, ...] => os << "(int, ...)";
        _ => os << "(...)";
    }
}

https://circle.godbolt.org/z/x48qxdWo5

void inspect(const auto& value, std::ostream& os) {
    inspect (value) {
        is [1, 2.] => os << "(1, 2)";
        is [1, 2., _] => os << "(int, double, _)";
        is [int, ...] => os << "(int, ...)";
        is _ => os << "(...)";
    }
}

https://circle.godbolt.org/z/5Tqq3qahh

void inspect(const auto& value, std::ostream& os) {
    inspect(value) {
        [a,b] is [int, double]       => os << "(" << a << ", " << b << ")" ;
        [a,b,c] is [int, double, _]  => os << "(int, double, _)";
        [a,b,c] is [int, int, int]   => os << "(int, ...)";
        [i] is [int]                 => os << "(int, ...)";
        is _                         => os << "(...)";
    }
}

https://circle.godbolt.org/z/KG51jjTaE