member-sfinae-1.cpp

#include <iostream>
#include <cstdlib>
#include <utility>

template <typename T>
struct Hoge {
  template <typename U=T>
  auto func(void)
  -> std::enable_if_t<std::is_same<U, void>::value, void> {
    std::cout << "void" << std::endl;
  }

  template <typename U=T>
  auto func(void)
  -> std::enable_if_t<!std::is_same<U, void>::value, void> {
    std::cout << "not void" << std::endl;
  }
};

int main(int argc, char* argv[]) {
  Hoge<int>().func();
  Hoge<void>().func();
  
  Hoge<void>().func<int>();  // not void

  return EXIT_SUCCESS;
}

member-sfinae-2.cpp

#include <iostream>
#include <cstdlib>
#include <utility>

template <typename T>
struct Hoge {
  template <bool AlwaysTrue = true>
  auto func(void)
  -> std::enable_if_t<std::is_same<T, void>::value && AlwaysTrue, void> {
    std::cout << "void" << std::endl;
  }

  template <bool AlwaysTrue = true>
  auto func(void)
  -> std::enable_if_t<!std::is_same<T, void>::value && AlwaysTrue, void> {
    std::cout << "not void" << std::endl;
  }
};

int main(int argc, char* argv[]) {
  Hoge<int>().func();
  Hoge<void>().func();
  
  // error!
  // Hoge<void>().func<false>();

  return EXIT_SUCCESS;
}

member-sfinae-3.cpp

#include <iostream>
#include <cstdlib>
#include <utility>

template <typename T>
struct Hoge {
  template <std::nullptr_t Dummy = nullptr>
  auto func(void)
  -> std::enable_if_t<std::is_same<T, void>::value && Dummy == nullptr, void> {
    std::cout << "void" << std::endl;
  }

  template <std::nullptr_t Dummy = nullptr>
  auto func(void)
  -> std::enable_if_t<!std::is_same<T, void>::value && Dummy == nullptr, void> {
    std::cout << "not void" << std::endl;
  }
};

int main(int argc, char* argv[]) {
  Hoge<int>().func();
  Hoge<void>().func();

  return EXIT_SUCCESS;
}