C++ try not to add footguns challenge (impossible)

Zangoose@lemmy.world to Programmer Humor@programming.dev – 120 points –

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::: spoiler Alt text: A screenshot from the linked article titled "Reflection in C++26", showing reflection as one of the bullet points listed in the "Core Language" section :::

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I can see the footguns, but I can also see the huge QoL improvement - no more std::enable_if spam to check if a class type has a member, if you can just check for them.

... at least I hope it would be less ugly than std::enable_if.

There's a pretty big difference though. To my understanding enable_if happens at compile time, while reflection typically happens at runtime. Using the latter would cause a pretty big performance impact over a (large) list of data.

Wouldn't compilers be able to optimize runtime things out? I know that GCC does so for some basic RTTI things, when types are known at compile time.

For runtime reflection, no, you'd specifically be able to do things that would be impossible to optimize out.

But the proposal is actually for static (i.e. compile-time) reflection anyway, so the original performance claim is wrong.

Yeah, that's what I was thinking of. I don't know how C++ could reasonably have Java-like reflections anyway...

C++26 reflection is compiletime

You already can do that with C++20 concepts and the requires expression

template 
concept has_member_foo = requires(T t) {
    t.foo();
};

// Will fail to instantiate (with nice error 
// message) if t.foo() is ill-formed
template 
void bar(T t) {
    // ...
}

// abbreviated form of above
void baz(has_member_foo auto t) {
    // ...
}

// verbose form of above
template  requires
    has_member_foo
void biz(T t) {
    // ...
}

// same as above but with anonymous concept
template  requires
    requires(T t) { t.foo(); }
void bom(T t) {
    // ...
}

// If already inside a function
if constexpr (has_member_foo) {
    // ...
}

// Same but with anonymous concept
if constexpr (requires(T t) { t.foo(); }) {
    // ...
}

I imagine reflections would make the process more straightforward, requires expressions are powerful but either somewhat verbose or possibly incomplete.

For instance, in your example foo could have any of the following declarations in a class:

  • void foo();
  • int foo() const;
  • template foo(T = { }) &&;
  • decltype([]() { }) foo;

I'm not sure if there's anything enable_if can do that concepts can't do somewhat better but yeah there's definitely a lot of subtleties that reflection is going to make nicer or possible in the first place

Oh, std::enable_if is straight up worse, they're unreadable and don't work when two function overloads (idk about variables) have the same signature.

I'm not even sure enable_if can do something that constraints can't at all...