Metaprogramming is another great feature of modern C++ that allows programs to redesign themselves during compilation or run time. In C++17, another new feature of metaprogramming is introduced, logical operation metafunctions. These are variadic metafunctions that are conjunction, disjunction, and negation which can be used for metaprogramming features of applications. In this post, we explain what the disjunction (std::disjunction) metafunction is.
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What are the logical operation metafunctions in C++?
Modern C++ has metaprogramming abilities which is a programming technique that a program can be compiled to read, create, analyze, or transfer other program codes or it can compile itself, it can change the way of running codes while running.
In C++17, new variadic metafunctions are released for metaprogramming, these are conjunction, conjunction_v, disjunction, disjunction_v, and negation, negation_v. These traits short-circuit in the metaprogramming sense: template specializations that are not required to determine the result are not instantiated.
What is the disjunction (disjunction) metafunction in C++ ?
In C++, disjunction (std::disjunction) or disjunction_v (std::disjunction_v) is a type trait that is defined in the <type_traits> header that is used to design the logical disjunction between classes (data types, structs, classes). It is a kind of logical template of OR on a variadic p
Here is the simple definition of std::disjunction:
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template<class... _Bases> struct disjunction; |
Here is the simple definition of std::disjunction_v which is a template of disjunction<...>::value,
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template<class... _Base> _INLINE_VAR _CONST_DATA bool disjunction_v = disjunction<_Base...>::value; |
Is there a simple example of how to use disjunction in C++?
Here is a simple example how you can use disjunction in C++.
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using Int2 = std::integral_constant<int, 2>; using Int4 = std::integral_constant<int, 4>; std::cout << "Disjunction: " << std::disjunction< Int2, Int4 >::value << std::endl; // C++11 std::cout << "Disjunction: " << std::disjunction_v< Int2, Int4 > << std::endl; // C++17 |
Here above, std::conjunction<Int2, Int4>::value evaluates to true, which corresponds to the logical AND of Int2::value (2) and Int4::value (4) . The short-circuit behavior of std::conjunction ensures that it stops evaluating as soon as it encounters a false value, similar to how logical AND works.
We can use it to check class relations logically as shown below.
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class baseA {}; class baseB {}; class derivedA : public baseA {}; class derivedB : public baseB {}; std::cout << "Disjunction: " << std::disjunction_v< std::is_base_of<baseA, derivedA>, // C++17 std::is_base_of<baseB, derivedB> >; |
Here, above std::is_base_of<BaseA, DerivedA> and std::is_base_of<baseB, derivedB> both evaluate to true.
Is there a full example about disjunction in C++?
Here is a full example about the std::disjunction metafunction in C++.
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#include <iostream> #include <type_traits> class baseA {}; class baseB {}; class derivedA : public baseA {}; class derivedB : public baseB {}; int main() { // Let's define two integral constants using Int2 = std::integral_constant<int, 2>; using Int4 = std::integral_constant<int, 4>; // Let's print the results std::cout << "Disjunction of Integrals: " << std::disjunction< Int2, Int4 >::value << std::endl; std::cout << "Disjunction of Integrals: " << std::disjunction_v< Int2, Int4 > << std::endl; std::cout << "Disjunction of Classes: " << std::conjunction_v< std::is_base_of<baseA, derivedA>, // C++17 std::is_base_of<baseB, derivedB> >; system("pause"); return 0; } |
For more new details about std::set, you can check this paper P0013R1
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