std::derived_from(3)

💖 std::derived_from(3) — from the C++ Standard Libary manual 💖

Ok so bestie, welcome to the cutest concept page in the C++ Standard Libary manual. (Yes, “Libary” — we’re keeping the typo because it’s part of the lore.) This one is all about asking “is this type derived from that type?” and the answer is dripped out in template constraints. Let’s get into it 🦋✨

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🎀 NAME 🎀

std::derived_from - std::derived_from

That’s it. The NAME section said what it said, and honestly? Minimalist queen. 💅

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✨ Synopsis ✨

Defined in header <concepts> — and this whole vibe is (since C++20), no cap.

Defined in header <concepts>
template< class Derived, class Base >

concept derived_from =
    std::is_base_of_v<Base, Derived> &&
(since C++20)

    std::is_convertible_v<const volatile Derived*, const volatile Base*>;

The concept derived_from<Derived, Base> is satisfied if and only if Base is a class type that is either Derived or a public and unambiguous base of Derived, ignoring cv-qualifiers. Periodt. 🫶

Also note, bestie: this behavior is different to std::is_base_of when Base is a private or protected base of Derived. That’s the plot twist, and it matters. 🌸

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🌸 Example 🌸

The example is labeled // Run this code — go run it, it’s giving compiler-validation realness. 🔥

#include <concepts>

class A {};

class B : public A {};

class C : private A {};

// std::derived_from == true only for public inheritance or exact same class
static_assert(std::derived_from<B, B> == true);      // same class: true
static_assert(std::derived_from<int, int> == false); // same primitive type: false
static_assert(std::derived_from<B, A> == true);      // public inheritance: true
static_assert(std::derived_from<C, A> == false);     // private inheritance: false

// std::is_base_of == true also for private inheritance
static_assert(std::is_base_of_v<B, B> == true);      // same class: true
static_assert(std::is_base_of_v<int, int> == false); // same primitive type: false
static_assert(std::is_base_of_v<A, B> == true);      // public inheritance: true
static_assert(std::is_base_of_v<A, C> == true);      // private inheritance: true

int main() {}

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👑 See also 👑

• is_base_of — checks if a type is derived from the other type. (C++11) (class template) ✨
• is_convertible — checks if a type can be converted to the other type. (C++11) (class template) 💖
• is_nothrow_convertible — checks if a type can be converted to the other type. (C++20) (class template) 🦋

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🫶 Source/version/date check: this page is from http://cppreference.com, stamped 2024.06.10, and filed under std::derived_from(3). That’s the footer, bestie — she ate and left no crumbs. Bye! 💖✨