std::conjunction(3)

okay so you opened std::conjunction(3) from the C++ Standard Libary (that spelling is literally in the header bestie 💀), and she’s a type-trait girlie here to teach you logical AND for types ✨. The manual name is std::conjunction(3) — self-titled, iconic, no notes 💅.

💖 NAME 💖

std::conjunction - std::conjunction

yes bestie that’s the whole name line, she said “I am the moment” and honestly? periodt 💅

✨ Synopsis ✨

Defined in header <type_traits>

template< class... B >           (since C++17)
struct conjunction;

So what’s the vibe? conjunction forms the logical conjunction of the type traits B..., effectively performing a logical AND on the sequence of traits. Translation: it’s the type-trait version of “and,” and she chains those bools together fr fr 🔥.

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.

🚨 If the program adds specializations for std::conjunction or std::conjunction_v, the behavior is undefined.

💅 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.

💫 Helper variable template 💫

she’s got a helper variable template too, because of course she does 🎀:

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

🛠️ Possible implementation 🛠️

if you wanna see how she might be built under the hood, it’s giving this:

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> {};

💭 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.

🌈 Feature-test macro 🌈

the feature-test macro lineup is:

• Feature-test macro: __cpp_lib_logical_traits — Value: 201510L — Std: (C++17) — Feature: Logical operator type traits

🌟 Example 🌟

okay bestie, here’s the code — run this code, she’s demonstrating func enabled if all Ts... have the same type as T, plus the otherwise version 💖:

// 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";
}

template<typename T, typename... Ts>
constexpr bool all_types_are_same = std::conjunction_v<std::is_same<T,
Ts>...>;

static_assert(all_types_are_same<int, int, int>);
static_assert(not all_types_are_same<int, int&, int>);

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

Output:

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

👀 See also 👀

other type-trait besties in the group chat:

• negation — logical NOT metafunction — (C++17) — (class template)
• disjunction — variadic logical OR metafunction — (C++17) — (class template)

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mwah 💋 — source: http://cppreference.com, version/date: 2024.06.10, manual: std::conjunction(3). go build something iconic bestie 🫶