negation of std::integral_constant<bool>

Question

Sorry for asking so simple question, but I cannot find the answer easily. Google says nothing interesting about "C++ negation integral_constant" and similar queries.

Is there in C++11 any trait that make std::true_type from std::false_type and vice versa? In other words, I'd like some more readeble version of

std::is_same<my_static_bool, std::false_type>

I know of course I can write it myself, but I'd like to use the existing one if there is such.

Complainant

What is the problem with doing !my_static_bool? – Goddamned

@DavidRodríguez-dribeas: I think my_static_bool is an integral constant, so he would have to write std::integral_constant<bool, !my_static_bool::value> (intentionally without ::value) to make it right. – Amaze

@Amaze constexpr operator! might make it possible, haven't tried though. – Aerodontia

@MaximYegorushkin: You can't apply operators on types. – Amaze

@ipc: From the test above, it seems that my_static_bool can be std::false_type (or possibly std::true_type I imagine). Other than that, I don't quite understand your comment... std::integral_constant has a constexpr conversion operator operator value_type() that in the case of std::false_type yields false, which can be used with the not to yield a true constant expression – Goddamned

@DavidRodríguez-dribeas: You can't write !std::true_type. – Amaze

@ipc: Right... my bad, you can only apply the operator to an object, not to the type :) The alternatives are either !my_static_bool::value or !my_static_bool{} – Goddamned

Answer 1

There is not, because it's essentially a one-liner and the <type_traits> should be as small as possible.

template <typename T> using static_not = std::integral_constant<bool, !T::value>;

Usage:

static_not<my_static_bool>

This is the correct way because the standard always says "false_type or derived from such", so you can't depend on being equal to std::false_type. I usually relax that to "having a constexpr boolean ::value property" because I don't use tag dispatching.

Amaze

Answer 2

Yet another way to do it:

template <typename T>
using static_not = typename std::conditional<
    T::value,
    std::false_type,
    std::true_type
>::type;

Theolatheologian

Answer 3

The following code uses template metafunction forwarding (i.e. it inherits from std::integral_constant with a negated boolean value, this is of course inspired by the Boost.MPL that heavily uses this pattern)

#include <type_traits>

template<typename T>
struct logical_not
:
    std::integral_constant<bool, !T::value>
{};

int main()
{
   typedef logical_not<std::false_type>::type T;
   typedef logical_not<std::true_type>::type F;

   static_assert((std::is_same<T, std::true_type>::value), "");
   static_assert((std::is_same<F, std::false_type>::value), "");   
}

Output on LiveWorkSpace

Pasho

Answer 4

The types true_type and false_type have a nested typedef that refers to themselves, so you can write:

std::is_same<my_static_bool::type,std::false_type>::value

Depending on the context it might be simpler to just do !my_static_bool{}, which is a constexpr of value true if your type is indeed std::false_type.

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