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       1           0 : \cond NEVER
       2             : Distributed under the MIT License.
       3             : See LICENSE.txt for details.
       4             : \endcond
       5             : # SFINAE {#sfinae}
       6             : 
       7             : \tableofcontents
       8             : 
       9             : SFINAE, Substitution Failure Is Not An Error, means that if a deduced template
      10             : substitution fails, compilation must continue. This can be exploited to make
      11             : decisions at compile time. See [here](http://nilsdeppe.com/posts/tmpl-part1)
      12             : for a discussion using `std::enable_if` to remove certain functions from
      13             : overload resolution or certain template specializations from name lookup.
      14             : Another method of controlling name lookup resolution is using
      15             : `std::void_t`. `void_t` is a metafunction from types to `void`, that is
      16             : 
      17             : ```cpp
      18             : template <typename... Args>
      19             : using void_t = void;
      20             : ```
      21             : 
      22             : `void_t` is useful when used in combination with `decltype` and `std::declval`
      23             : to probe if a type has certain members. For example, we can implement a type
      24             : trait to check if a type `T` is iterable by first have the general definition
      25             : inherit from `std::false_type` as follows,
      26             : 
      27             : ```cpp
      28             : template <typename T, typename = void>
      29             : struct is_iterable : std::false_type {};
      30             : ```
      31             : 
      32             : Next we will have specialization that uses `void_t` to check if the type `T`
      33             : has a `begin()` and `end()` function.
      34             : 
      35             : ```cpp
      36             : template <typename T>
      37             : struct is_iterable<T, std::void_t<decltype(std::declval<T>().begin(),
      38             :                                            std::declval<T>().end())>>
      39             :     : std::true_type {};
      40             : ```
      41             : 
      42             : What is happening here? Well, we use `std::declval` to convert the type `T`
      43             : to a reference type, which allows us to call member functions inside `decltype`
      44             : expressions without having to construct an object. First we try to call the
      45             : member function `begin()` on `std::declval<T>()`, and if that succeeds we
      46             : throw away the result using the comma operator. Next we try to call `end()`
      47             : on `std::declval<T>()`, which, if it succeeds we get the return type of
      48             : using `decltype`. Note that `decltype` is important because we can only call
      49             : member functions on reference types inside of `decltype`, not just anywhere.
      50             : Finally, if all this succeeded use `void_t` to metareturn `void`, otherwise
      51             : the template parameters of `void_t` fail to evaluate and the specialization
      52             : cannot be resolved during name lookup. We could just as well use
      53             : 
      54             : ```cpp
      55             : template <typename T>
      56             : struct is_iterable<T, std::void_t<decltype(std::declval<T>().begin()),
      57             :                                   decltype(std::declval<T>().end())>>
      58             :     : std::true_type {};
      59             : ```
      60             : 
      61             : Which of the two implementations of the `is_iterable` is preferred is simply
      62             : a matter of taste, both behave correctly.
      63             : 
      64             : If you're reading closely you might wonder why the `void_t` is necessary at
      65             : all, why not just `decltype(...)`? Well the reason is that since the default
      66             : template parameter metavalue is `void`, the specialization cannot be resolved
      67             : during name lookup unless the second template parameter in the specialization
      68             : is either `void` as well or is explicitly specified when the class template
      69             : is being invoked. Thus, the clearest implementation probably is
      70             : 
      71             : ```cpp
      72             : template <typename T, typename = std::void_t<>>
      73             : struct is_iterable : std::false_type {};
      74             : 
      75             : template <typename T>
      76             : struct is_iterable<T, std::void_t<decltype(std::declval<T>().begin(),
      77             :                                            std::declval<T>().end())>>
      78             :     : std::true_type {};
      79             : ```
      80             : 
      81             : You could now also define a helper type alias and constexpr boolean
      82             : 
      83             : ```cpp
      84             : template <typename T>
      85             : using is_iterable_t = typename is_iterable<T>::type;
      86             : 
      87             : template <typename T>
      88             : constexpr bool is_iterable_v = is_iterable<T>::value;
      89             : ```