C++ try not to add footguns challenge (impossible)
C++ try not to add footguns challenge (impossible)
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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_ifspam 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 <typename T> concept has_member_foo = requires(T t) { t.foo(); }; // Will fail to instantiate (with nice error // message) if t.foo() is ill-formed template <has_member_foo T> void bar(T t) { // ... } // abbreviated form of above void baz(has_member_foo auto t) { // ... } // verbose form of above template <typename T> requires has_member_foo<T> void biz(T t) { // ... } // same as above but with anonymous concept template <typename T> requires requires(T t) { t.foo(); } void bom(T t) { // ... } // If already inside a function if constexpr (has_member_foo<T>) { // ... } // 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
foocould have any of the following declarations in a class:void foo();int foo() const;template <typename T> foo(T = { }) &&;decltype([]() { }) foo;
I'm not sure if there's anything
enable_ifcan 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 placeOh,
std::enable_ifis 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...