C++ 用于泛型类型上泛型操作的SFINAE

我有

• 可以用不同类型（Foo、Bar、int等）构造的类值

• 类值应该具有常见的操作，如可以替换的

  std::ostream &operator<<(std::ostream &os, const Value &value)
  {
      // ...
  }
  

  ---

  您可以添加包装以仅强制一次转换：

  template <typename T>
  struct OneConversion
  {
      OneConversion(const T& t) : t(t) {}
  
      operator const T&() const {return t;}
  
      const T& t;  
  };
  
  template <typename T>
  struct isOneConversion : std::false_type {};
  
  template <typename T>
  struct isOneConversion<OneConversion<T>> : std::true_type {};
  
  struct Value {
      template<typename T, std::enable_if_t<!isOneConversion<T>::value>* = nullptr>
      Value(T) {}
  };
  
  std::ostream &operator<<(std::ostream &os, const Value &value) {
  
      auto visitor = [&](auto a) -> decltype(os << OneConversion<decltype(a)>(a)) {
          return os << OneConversion<decltype(a)>(a);
      };
  
      // Works
      visitor(Bar{});
  
      visitor(Foo{}); // Error as expected.
  
      return os;
  }
  

  模板
  结构转换
  {
  OneConversion（const T&T）：T（T）{}
  运算符常量T&（）常量{return T；}
  康斯特T&T；
  };
  模板
  结构isOneConversion:std:：false_type{}；
  模板
  结构isOneConversion:std:：true_type{}；
  结构值{
  模板*=nullptr>
  值（T）{}
  };
  std:：ostream&operator不修改
  std:：ostream&operator的签名
  std::ostream &operator<<(std::ostream &os, const Value &value)
  {
      // ...
  }
  
  template <typename T, typename = std::enable_if_t<std::is_same_v<T, Value>>>
  std::ostream &operator<<(std::ostream &os, const T &value)
  {
      // ...
  }
  
  ...
  main.cpp:29:12: error: no match for call to '(operator<<(std::ostream&, const T&) [with T = Value; <template-parameter-1-2> = void; std::ostream = std::basic_ostream<char>]::<lambda(auto:1)>) (Foo)'
   visitor(Foo{});
   ~~~~~~~^~~~~~~
  ...
  
  template <typename T>
  struct OneConversion
  {
      OneConversion(const T& t) : t(t) {}
  
      operator const T&() const {return t;}
  
      const T& t;  
  };
  
  template <typename T>
  struct isOneConversion : std::false_type {};
  
  template <typename T>
  struct isOneConversion<OneConversion<T>> : std::true_type {};
  
  struct Value {
      template<typename T, std::enable_if_t<!isOneConversion<T>::value>* = nullptr>
      Value(T) {}
  };
  
  std::ostream &operator<<(std::ostream &os, const Value &value) {
  
      auto visitor = [&](auto a) -> decltype(os << OneConversion<decltype(a)>(a)) {
          return os << OneConversion<decltype(a)>(a);
      };
  
      // Works
      visitor(Bar{});
  
      visitor(Foo{}); // Error as expected.
  
      return os;
  }
  
  auto visitor = [&](auto a) -> decltype(
                                static_cast<std::ostream&(*)(std::ostream&, const decltype(a)&)>(&operator<<)
                               (os, a)
                               )
  {
      return os << a;
  };
  
  error: invalid static_cast from type '<unresolved overloaded function type>' to type 'std::ostream& (*)(std::ostream&, const Foo&