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 conjunction (std::conjunction) 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 conjunction metafunction in C++?
In C++, conjunction (std::conjunction) or conjunction_v (std::conjunction_v) is a type trait that is defined in the <type_traits> header that is used to design the logical conjunction between classes (data types, structs, classes). It is a kind of logical template of AND on a variadic pack of values.
Here is the simple definition of std::conjunction.
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template<class... _Bases> struct conjunction; |
Here is the simple definition of std::conjunction_v which is a template of conjunction<...>::value,
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template<class... _Base> _INLINE_VAR _CONST_DATA bool conjunction_v = conjunction<_Base...>::value; |
Is there a simple example of the conjunction metafunction in C++?
Here is a simple example how you can use the conjunction function in C++.
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using Int2 = std::integral_constant<int, 2>; using Int4 = std::integral_constant<int, 4>; std::cout << "Conjunction: " << std::conjunction< Int2, Int4 >::value << std::endl; // C++11 std::cout << "Conjunction: " << std::conjunction_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 below.
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class baseA {}; class baseB {}; class derivedA : public baseA {}; class derivedB : public baseB {}; std::cout << "Conjunction: " << std::conjunction_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, since both derivedA and derivedB inherit from baseA and baseB respectively. The std::conjunction combines both of these results, it should be true.
Is there a full example about the conjunction metafunction in C++?
Here is a full example about std::conjunction 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 << "Conjunction of Integrals: " << std::conjunction< Int2, Int4 >::value << std::endl; // C++11 std::cout << "Conjunction of Integrals: " << std::conjunction_v< Int2, Int4 > << std::endl; std::cout << "Conjunction of Classes: " << std::conjunction_v< std::is_base_of<baseA, derivedA>, // C++17 std::is_base_of<baseB, derivedB> >; 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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