315.md

Info

• Did you know about C++20 is_layout_compatible_v type_trait?

  • https://eel.is/c++draft/meta.type.synop

Example

struct foo {
    int a{};
    int b{};
};

struct bar {
    const int x{};
    int y{};
};

struct baz : bar { };

struct other {
    int a{};
    char b[4]{};
};

static_assert(not std::is_layout_compatible_v<void, int>);
static_assert(not std::is_layout_compatible_v<const int*, const int&>);

static_assert(std::is_layout_compatible_v<const int, int const volatile>);
static_assert(std::is_layout_compatible_v<foo, bar>);
static_assert(std::is_layout_compatible_v<foo, baz>);
static_assert(std::is_layout_compatible_v<bar, baz>);
static_assert(not std::is_layout_compatible_v<bar, other>);
static_assert(not std::is_layout_compatible_v<bar, void>);

https://godbolt.org/z/v9KM34qMW

Puzzle

Can you implemtn count_compatible which returns the number of compatible types to the given one?

template<class T, class... Ts>
constexpr auto count_compatible = 0; // TODO

static_assert(0 == count_compatible<int, const int*, const int&, volatile int*, int()>);
static_assert(0 == count_compatible<void*, volatile void*, const volatile void*, volatile const void* const>);
static_assert(0 == count_compatible<void, int>);

static_assert(1 == count_compatible<int, int>);
static_assert(1 == count_compatible<int, float, int>);
static_assert(2 == count_compatible<int, short, signed int, unsigned int, int const>);
static_assert(4 == count_compatible<int, int, const int, int const, unsigned const, signed const int>);

https://godbolt.org/z/7cjqdxGxE

Solutions

template<class T, class... Ts>
constexpr auto count_compatible = (std::is_layout_compatible_v<T, Ts> + ... + std::size_t{});

https://godbolt.org/z/8v741635E

template<class T, class... Ts>
constexpr auto count_compatible = []{
    return (std::is_layout_compatible_v<T, Ts> + ...);
}();

https://godbolt.org/z/GEcr44h49

template<class T, class... Ts>
constexpr auto count_compatible = (... + std::is_layout_compatible_v<T, Ts>);

https://godbolt.org/z/fxqYeben5