2411C++,学习C++提示4

结构绑定

  auto [first, ...ts] = std::tuple{1, 2 ,3};
  assert(1 == first);

浮点作为非类型模板参数

template<double Value> constexpr auto value = Value;

int main() {
    std::cout << value<4.2>; // prints 4.2
}

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) {
    constexpr auto mul = (Vl2s * ...);
    return ((Vl1s * mul) + ...);
}

template <double... List1, double... List2>
[[nodiscard]] consteval auto calc(values<List1...>, values<List2...>) {
    return (0 + ... + (List1 * (1 * ... * List2)));
}

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) {
    return ((Vl1s * (Vl2s * ...)) + ...);
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) {
    constexpr auto c = (Vl2s * ...);
    return ((Vl1s * c) + ...);
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>) {
  return (Vl1s + ...) * (Vl2s * ...);
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

template<double... Values> struct values{};

template<double Vl1, double... Vl2s>
[[nodiscard]] consteval auto mult(){
  return  (Vl1 * ... * Vl2s);
}

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>){
  return (... + mult<Vl1s, Vl2s...>());
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

template<double... Values> struct values {};

template<double... Vl1s, double... Vl2s>
[[nodiscard]] consteval auto calc(values<Vl1s...>, values<Vl2s...>){
    return (0.0 + ... + (Vl1s * (Vl2s * ... )));
}

template<double Epsilon = 0.1>
[[nodiscard]] consteval auto eq(double a, double b) {
    return std::fabs(a - b) < Epsilon;
}

不可达

#include <utility>

int main() {
    std::unreachable();
    return 42; // invokes undefined behavior
}

抑制ADL

namespace adl {
    struct foo {};
    void bar(foo) {}
}

int main() {
    adl::foo foo;
    bar(foo);   // OK,    ADL
    (bar)(foo); // error: no ADL
}

交换字节

#include <bit>

int main() {
   constexpr auto value = std::uint16_t(0xCAFE);
   std::cout << std::hex << value; // pritns cafe
   std::cout << std::hex << std::byteswap(value); // prints feca
}

行多态

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

struct bar {
  int a{};
};

struct missing_a {
  int b{};
};

struct row_with_member_a {
  constexpr explicit(false) row_with_member_a(const auto& t)
    : a{t.a}
  { }

  int a{};
};

auto shrink(row_with_member_a r) {
  std::cout << r.a;
}

int main() {
  //shrink(missing_a{.b = 42}); // error
  shrink(foo{.a = 4, .b = 2}); // prints 4
  shrink(bar{.a = 42}); // prints 42
}

多维数组

template <class T, auto Dimensions> class mdarray2 {
public:
  template <class I1, class I2> constexpr T &operator[](I1 i1, I2 i2) {
    return vs_[i1][i2];
  }

private:
  std::array<std::array<T, 2>, Dimensions> vs_{};
};

int main() {
  mdarray2<int, 2> a{};
  a[1, 1] = 42;
  assert(0 == (a[0, 0]));
  assert(42 == (a[1, 1]));
}

基于策略的设计

template<class TPolicy>
struct foo : TPolicy {
  static constexpr auto bar() {
    return TPolicy::bar();
  }
};

template<auto N>
struct policy { static constexpr auto bar() { return N; } };

static_assert(0 == foo<policy<0>>::bar());
static_assert(42 == foo<policy<42>>::bar());