I saw its usage as below
template <typename T> struct DependentFalse : std::false_type {}; Then, it is used here
template <typename T> class RadarSensor { static_assert(DependentFalse<T>::value, "RadarSensor must be created using Identifier template"); }; I do not have idea what is it used for?
What is DependentFalse structure?
42 Answers
std::false_type is used as a building block in type traits and is defined as std::integral_constant<bool, false> (which I will skip over here). It's definition boils down to something like this (simplified):
struct false_type { static constexpr bool value = false; constexpr operator bool() const noexcept { return value; } // There is more here, but it doesn't really matter for your question }; Similarly:
struct true_type { static constexpr bool value = true; constexpr operator bool() const noexcept { return value; } // There is more here, but it doesn't really matter for your question }; It is used to represent the values false and true as types. This is useful in type traits where you let a class template inherit from either std::false_type or std::true_type for different (partial) specializations, depending on some condition met by the template argument. Doing so allows one to test whether a given type satisfies the condition of the type trait and to obtain a compile time constant value indicating the result through access to the static value member which is inherited from either std::false_type or std::true_type or alternative through conversion of an instance of the type trait using the conversion operator.
What you are showing here is a simple type trait which always (for all T) evaluates to std::false_type. It is used in static_asserts that should always fail when the template they are located in is instantiated. This is necessary, because a static_assert that does not dependent on a template parameter is triggered already at the point of definition, rather than the point of instantiation, therefore making every program containing something like static_assert(false); ill-formed.
There is likely a specialisation of DependentFalse for Identifier that might look like this:
template<class ... Args> class DependentFalse<Identifier<Args...>> : public std::true_type {} This makes sure that you can not compile a RadarSensor, as long as the template parameter does not fullfill whatever is needed for the specialisation (in this case being of type Identifier).
