README.md

Contents

1. constexpr
2. consteval

⬅️ Back to Contents

constexpr

• C++20 allows virtual functions to be declared constexpr

  struct Base {
    virtual ~Base() noexcept = default;
    virtual int foo() const noexcept = 0;
  };
  
  struct Derived : public Base {
    constexpr int foo() const noexcept override { return 2; };
  };
  
  int main() {
    {
      constexpr Derived derived;
      constexpr auto value = derived.foo();
      static_assert(value == 2);
    }
    {
      std::unique_ptr<Base> base = std::make_unique<Derived>();
      const auto value = base->foo();
    }
  }

  Note: A constexpr virtual function can override a non-constexpr function and vice versa.

• string, string_view, vector, and tuple are now constexpr as well as sort, all_of, ... algorithms

• Use dynamic_cast and typeid

• Do dynamic memory allocations and deallocations

• Change the active member of a union

• Contain try/catch blocks

  • But throw statements are still not allowed

  • try/catch blocks are no-ops when evaluated in a constant expression

⬆️ Back to Contents

consteval

consteval declares immediate functions that must produce a compile time constant expression, a non-constant result should be a compilation error.

consteval auto Square(int number) {
  return number * number;
}

auto number = 10;
auto value = Square(number); // error: the value of 'number' is not usable in a constant expression

const auto number = 10;
auto value = Square(number); // ok: everything is constant

constexpr auto number = 10;
constexpr auto value = Square(number); // ok

📎 As a comparison, constexpr function may be evaluated at compile time and runtime, and need not produce a constant in all cases.

⬆️ Back to Contents