std::is_cor...ding_member(3)

💖 std::is_corresponding_member 💖

okay so bestie, we're in the C++ Standard Library today, specifically the <type_traits> corner of it, and this one's a std::is_corresponding_member situation. it's giving "quiet little trait that goes feral in aggregate-initialization discourse," and honestly? i'm obsessed.

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✨ Synopsis ✨

first things first, it lives in this header, periodt:

#include <type_traits>

and here's the signature, character for character, no cap:

template< class S1, class S2, class M1, class M2 >
constexpr bool is_corresponding_member( M1 S1::* mp, M2 S2::* mq )

so yes, it's constexpr and it's been around (since C++20) — and it's noexcept; too, she doesn't throw, she simply tells you the truth.

what does she do? 💅 she determines whether mp and mq refer to corresponding members in the common initial sequence of S1 and S2. like, are these two pointer-to-members pointing at the same slot in the shared layout prefix of those two types. that's the whole gig.

🫶 bestie heads up: the program is ill-formed if either S1 or S2 is an incomplete type. so don't be out here with incomplete types, that's not the vibe.

also, the boring-but-important escape hatch: if either S1 or S2 is not a StandardLayoutType, or either M1 or M2 is not an object type, or either mp or mq is equal to nullptr — the result is always false. always. no exceptions. it doesn't get mad, it just says no. 😭

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🌸 Parameters 🌸

• mp, mq — pointers-to-member to detect. that's it, those are the two besties under inspection 🔍

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🎀 Return value 🎀

true if mp and mq refer to corresponding members in the common initial sequence of S1 and S2 — otherwise false. simple. iconic. slay.

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🦋 Notes 🦋

okay this part is genuinely the tea, so pay attention 🍓:

the type of a pointer-to-member expression &S::m is not always M S::*, where m is of type M, because m may be a member inherited from a base class of S. sneaky little inheritance moment. 🙈

so the template arguments can be specified explicitly in order to avoid potentially surprising results. iykyk — sometimes you gotta spell it out.

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🔥 Example 🔥

run this code, bestie, it speaks for itself:

// Run this code

#include <type_traits>

struct Foo
{
    int x;
    double d;
};

struct Bar
{
    int y;
    double z;
};

struct Baz : Foo, Bar {}; // not standard-layout

static_assert(
    std::is_same_v<decltype(&Baz::x), int Foo::*> == true &&
    std::is_same_v<decltype(&Baz::y), int Bar::*> == true &&
    std::is_corresponding_member(&Foo::x, &Bar::y) == true &&
    std::is_corresponding_member(&Foo::d, &Bar::z) == true &&
    std::is_corresponding_member(&Baz::x, &Baz::y) == true &&
    std::is_corresponding_member<Baz, Baz, int, int>(&Baz::x, &Baz::y)
       == false
);

int main() {}

look at that last line. 👑 std::is_corresponding_member<Baz, Baz, int, int>(&Baz::x, &Baz::y) is false when you spell the template args out — and the default dedecution version above it was true. that's literally the whole Notes section living its best life in real time. main character energy, fr fr.

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💜 See also 💜

• is_standard_layout (C++11) — checks if a type is a standard-layout type (class template) ✨
• is_layout_compatible (C++20) — checks if two types are layout-compatible (class template) 💫
• is_member_object_pointer (C++11) — checks if a type is a pointer to a non-static member object (class template) ⭐

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aaaand that's the page, bestie! 🧚 source/version and date check, so you know this roundup is legit: it's from cppreference.com, version 2024.06.10, and your girl std::is_corresponding_member(3) says goodnight. go forth and check your common initial sequences responsibly. 💖🌈