232.md

Info

• Did you know that different overloads can have different specifiers?

  • http://eel.is/c++draft/dcl.spec.general

Example

constexpr auto foo(auto) { return 42; }
consteval auto foo(auto f) requires requires { f.value; } { return f.value; }

int main(int argc, char**) {
  std::cout << foo(argc); // prints 42

  constexpr struct {
    int value{88};
  } v;

  std::cout << foo(v); // prints 88
}

https://godbolt.org/z/a9sKb177j

Puzzle

• Can you implement a function f which for { 0 args: is run-time - returns 42; 1 arg: is run/compile-time, returns arg; >1 args: is compile-time, returns sum of args... }

auto f(...); // TODO

template<auto> auto is_compile_time() {}

int main() {
  using namespace boost::ut;

  "run-time call"_test = [] {
    expect(42_i == f());
    expect(not [](auto... ts) { return requires { is_compile_time<f(ts...)>(); }; }());
  };

  "constexpr call"_test = [] {
    "compile-time call"_test = [] {
      expect(constant<42_i == f(42)>);
      expect(requires { is_compile_time<f(42)>(); });

      constexpr auto i = 43;
      expect(constant<43_i == f(43)>);
    };

    "run-time call"_test = [] {
      auto i = 44;
      expect(44_i == f(i));
    };
  };

  "consteval call"_test = [] {
    expect(constant<3_i == f(1, 2)>);
    expect(constant<6_i == f(1, 2, 3)>);
    auto i = 1;
    //f(42, i); // should not compile
  };
}

https://godbolt.org/z/EWK873qfv

Solutions

auto f() { return 42; }
constexpr auto f(auto v) { return v; }
template<typename ... Args> consteval auto f(Args... args) { return (0 + ... + args); }

https://godbolt.org/z/6hM3nbabv

auto f() { return 42; }
constexpr auto f(auto arg) { return arg; }
consteval auto f(auto... args) { return (... + args); }

https://godbolt.org/z/qdf3z1Khr

auto f() { return 42; }
constexpr auto f(auto value) { return value; }
consteval auto f(auto... values) requires (sizeof...(values) > 1) { return (... + values); }

https://godbolt.org/z/8o3WYG8ze