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# std::conjunction

From cppreference.com
< cpp‎ | types

Defined in header `<type_traits>` |  |
template<class... B> struct conjunction; | (1) | (since C++17)

Forms the logical conjunction of the type traits `B...`, effectively performing a logical AND on the sequence of traits.

The specialization std::conjunction<B1, ..., BN> has a public and unambiguous base that is

- if sizeof...(B) == 0, std::true_type; otherwise
- the first type `Bi` in `B1, ..., BN` for which bool(Bi::value) == false, or `BN` if there is no such type.

The member names of the base class, other than `conjunction` and `operator=`, are not hidden and are unambiguously available in `conjunction`.

Conjunction is short-circuiting: if there is a template type argument `Bi` with bool(Bi::value) == false, then instantiating conjunction<B1, ..., BN>::value does not require the instantiation of Bj::value for `j > i`.

### [edit] Template parameters

B... | - | every template argument `Bi` for which Bi::value is instantiated must be usable as a base class and define member `value` that is convertible to bool

### [edit] Helper variable template

template<class... B> inline constexpr bool conjunction_v = conjunction<B...>::value; |  | (since C++17)

### [edit] Possible implementation

    template<class...> struct conjunction : std::true_type { };
    template<class B1> struct conjunction<B1> : B1 { };
    template<class B1, class... Bn>
    struct conjunction<B1, Bn...>
        : std::conditional_t<bool(B1::value), conjunction<Bn...>, B1> {};

### [edit] Notes

A specialization of `conjunction` does not necessarily inherit from either std::true_type or std::false_type: it simply inherits from the first `B` whose `::value`, explicitly converted to bool, is false, or from the very last `B` when all of them convert to true. For example, std::conjunction<std::integral_constant<int, 2>, std::integral_constant<int, 4>>::value is 4.

The short-circuit instantiation differentiates `conjunction` from fold expressions: a fold expression like (... && Bs::value) instantiates every `B` in `Bs`, while std::conjunction_v<Bs...> stops instantiation once the value can be determined. This is particularly useful if the later type is expensive to instantiate or can cause a hard error when instantiated with the wrong type.

### [edit] Example

Run this code

    #include <iostream>
    #include <type_traits>

    // func is enabled if all Ts... have the same type as T
    template<typename T, typename... Ts>
    std::enable_if_t<std::conjunction_v<std::is_same<T, Ts>...>>
    func(T, Ts...) {
        std::cout << "all types in pack are T\n";
    }

    // otherwise
    template<typename T, typename... Ts>
    std::enable_if_t<!std::conjunction_v<std::is_same<T, Ts>...>>
    func(T, Ts...) {
        std::cout << "not all types in pack are T\n";
    }

    int main() {
        func(1, 2, 3);
        func(1, 2, "hello!");
    }

Output:

    all types in pack are T
    not all types in pack are T

### [edit] See also

negation (C++17) | logical NOT metafunction (class template) [edit]
disjunction (C++17) | variadic logical OR metafunction (class template) [edit]

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