SFINAE —— Substitution Failure Is Not An Error

IsConvertible

#include <iostream>
#include <string>

template <typename FROM, typename TO> struct IsConvetible {
  // FROM should be converted to TO implicitly
  static void aux(TO);

  // Should use another typename F, otherwise it will cause an compilation error
  // immediately
  template <typename F, typename = decltype(aux(std::declval<F>()))>
  static std::true_type test(void *);

  // Will failed with: No viable conversion from 'double' to 'std::string'
  //   template <typename = decltype(aux(std::declval<FROM>()))>
  //   static std::true_type test(void *);

  // If the above function failed, it will fallback here
  template <typename> static std::false_type test(...);

  static constexpr bool value = decltype(test<FROM>(nullptr))::value;
};

int main() {
  std::cout << std::boolalpha;
  std::cout << IsConvetible<double, std::string>::value << std::endl;
  std::cout << IsConvetible<double, int>::value << std::endl;
  std::cout << IsConvetible<const char *, std::string>::value << std::endl;
  std::cout << IsConvetible<const char[10], std::string>::value << std::endl;
  std::cout << IsConvetible<char[10], std::string>::value << std::endl;
}

输出：

false
true
true
true
true

IsDefaultConstructible

#include <iostream>
#include <string>

template <typename T> struct IsDefaultConstructibleHelper {

  // Should use another typename U, otherwise it will cause an compilation error
  // immediately
  template <typename U, typename = decltype(U())>
  static std::true_type test(void *);

  // If the above function failed, it will fallback here.
  template <typename> static std::false_type test(...);

  static constexpr bool value = decltype(test<T>(nullptr))::value;
};

template <typename T>
constexpr bool IsDefaultConstructible = IsDefaultConstructibleHelper<T>::value;

class WithDefaultConstructor {};

class WithoutDefaultConstructor {
  WithoutDefaultConstructor() = delete;
};

int main() {
  std::cout << std::boolalpha;
  std::cout << IsDefaultConstructible<int> << std::endl;
  std::cout << IsDefaultConstructible<WithDefaultConstructor> << std::endl;
  std::cout << IsDefaultConstructible<WithoutDefaultConstructor> << std::endl;
}

输出：

true
true
false