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?
std::integral_constant wraps a static constant of specified type. It is the base class for the C++ type traits. The behavior of a program that adds specializations for integral_constant is undefined.
What is boolean in C++? A boolean data type in C++ is defined using the keyword bool . Usually, 1 ( true ) and 2 ( false ) are assigned to boolean variables as their default numerical values.
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.
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