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Subject: Re: [std-proposals] proposal: new const-able feature ( use "const?" , or new "constable" keyword) for method/function
From: Barry Revzin (barry.revzin_at_[hidden])
Date: 2019-12-15 17:41:49


On Fri, Dec 13, 2019 at 12:58 AM jianping z via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> // example-1b.hpp : much more concise, no more duplicated code (less
> error prone)
> class elements
> {
> int elem[10];
> const? int& operator[](size_t n) const? { return elem[n]; }
> const? int* data() const? { return elem; }
> const? int* begin() const? { return elem; }
> const? int* end() const? { return elem+10; }
> }
>

I think this is pretty nice, and I appreciate the examples clearly
illustrating how this is intended to work.

const or not const isn't the only axis we have to deal with though - we
have one more: lvalue or rvalue. Today, optional::operator*() looks
something like this (reduced for sanity):

template <typename T>
class optional {
    union { char _; T value; };
    bool has_value;
public:
    auto operator*() & -> T& { return value; }
    auto operator*() const& -> T const& { return value; }
    auto operator*() && -> T&& { return std::move(value); }
    auto operator*() const&& -> T const&& { return std::move(value); }
};

Now, with the const? feature, I could reduce these four overloads to two:

template <typename T>
class optional {
    union { char _; T value; };
    bool has_value;
public:
    auto operator*() const?& -> T const?& { return value; }
    auto operator*() const?&& -> T const?&& { return std::move(value); }
};

It'd really be nice to just have the one overload here. But with &/&&, it
doesn't work as nicely since the question is no longer "to token or not to
token", but rather a choice of two tokens. Maybe that's... fine and we
could just say that &&? means & or && while const? means const or nothing.
So that this spits out four functions for us:

template <typename T>
class optional {
    union { char _; T value; };
    bool has_value;
public:
    auto operator*() const?&&? -> T const?&&? { return static_cast<T
const?&&?>(value); }
};

Maybe this is too ugly, maybe we need a different way to pick references.
But regardless one problem is that we'd need these two to unpack
independently (that is, get the full cartesian product of {const, } x {&,
&&}). But this goes against one of your examples where you illustrated:

template<typename T>
const? T& max(const? T& a, const? T& b)

as become two function templates (rather than four).

This 4x thing comes up a lot. std::get for std::tuple for instance might
become:

template <size_t I, typename... Ts>
auto get(tuple<Ts...> const?&&?) -> tuple_element_t<I,
tuple<Ts...>>const?&&?;

Something to think about.

Barry



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