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maybe_t: make maybe_t<T> trivially copyable if T is

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When passing a value of type maybe_t<size_t>, clangd complains:

    Parameter 'cursor' is passed by value and only copied once; consider
    moving it to avoid unnecessary copies (fix available)

We get this warning because maybe_t<size_t> is not trivially copyable
because it has a user-defined destructor and copy-constructor.  Let's remove
them if the contained type is trivially copyable, to avoid such warnings.
No functional change.
This commit is contained in:
Johannes Altmanninger 2022-10-26 14:20:38 +02:00
parent 1ce2961561
commit 9d303a74e3
1 changed files with 36 additions and 16 deletions
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52
src/maybe.h
View file

@ -7,10 +7,11 @@
#include <utility>
namespace maybe_detail {
// Template magic to make maybe_t<T> copyable iff T is copyable.
// maybe_impl_t is the "too aggressive" implementation: it is always copyable.
// Template magic to make maybe_t<T> (trivially) copyable iff T is.
// This is an unsafe implementation: it is always trivially copyable.
template <typename T>
struct maybe_impl_t {
struct maybe_impl_trivially_copyable_t {
alignas(T) char storage[sizeof(T)];
bool filled = false;
@ -38,11 +39,13 @@ struct maybe_impl_t {
return res;
}
maybe_impl_t() = default;
maybe_impl_trivially_copyable_t() = default;
// Move construction/assignment from a T.
explicit maybe_impl_t(T &&v) : filled(true) { new (storage) T(std::forward<T>(v)); }
maybe_impl_t &operator=(T &&v) {
explicit maybe_impl_trivially_copyable_t(T &&v) : filled(true) {
new (storage) T(std::forward<T>(v));
}
maybe_impl_trivially_copyable_t &operator=(T &&v) {
if (filled) {
value() = std::move(v);
} else {
@ -53,8 +56,8 @@ struct maybe_impl_t {
}
// Copy construction/assignment from a T.
explicit maybe_impl_t(const T &v) : filled(true) { new (storage) T(v); }
maybe_impl_t &operator=(const T &v) {
explicit maybe_impl_trivially_copyable_t(const T &v) : filled(true) { new (storage) T(v); }
maybe_impl_trivially_copyable_t &operator=(const T &v) {
if (filled) {
value() = v;
} else {
@ -63,14 +66,26 @@ struct maybe_impl_t {
}
return *this;
}
};
// Move construction/assignment from a maybe_impl.
maybe_impl_t(maybe_impl_t &&v) : filled(v.filled) {
// This is an unsafe implementation: it is always copyable.
template <typename T>
struct maybe_impl_not_trivially_copyable_t : public maybe_impl_trivially_copyable_t<T> {
using base_t = maybe_impl_trivially_copyable_t<T>;
using base_t::maybe_impl_trivially_copyable_t;
using base_t::operator=;
using base_t::filled;
using base_t::reset;
using base_t::storage;
// Move construction/assignment from another instance.
maybe_impl_not_trivially_copyable_t(maybe_impl_not_trivially_copyable_t &&v) {
filled = v.filled;
if (filled) {
new (storage) T(std::move(v.value()));
}
}
maybe_impl_t &operator=(maybe_impl_t &&v) {
maybe_impl_not_trivially_copyable_t &operator=(maybe_impl_not_trivially_copyable_t &&v) {
if (!v.filled) {
reset();
} else {
@ -79,13 +94,14 @@ struct maybe_impl_t {
return *this;
}
// Copy construction/assignment from a maybe_impl.
maybe_impl_t(const maybe_impl_t &v) : filled(v.filled) {
// Copy construction/assignment from another instance.
maybe_impl_not_trivially_copyable_t(const maybe_impl_not_trivially_copyable_t &v) : base_t() {
filled = v.filled;
if (v.filled) {
new (storage) T(v.value());
}
}
maybe_impl_t &operator=(const maybe_impl_t &v) {
maybe_impl_not_trivially_copyable_t &operator=(const maybe_impl_not_trivially_copyable_t &v) {
if (&v == this) return *this;
if (!v.filled) {
reset();
@ -95,7 +111,7 @@ struct maybe_impl_t {
return *this;
}
~maybe_impl_t() { reset(); }
~maybe_impl_not_trivially_copyable_t() { reset(); }
};
struct copyable_t {};
@ -128,7 +144,11 @@ inline constexpr none_t none() { return none_t::none; }
// This is a value-type class that stores a value of T in aligned storage.
template <typename T>
class maybe_t : private maybe_detail::conditionally_copyable_t<T> {
maybe_detail::maybe_impl_t<T> impl_;
using maybe_impl_t =
typename std::conditional<std::is_trivially_copyable<T>::value,
maybe_detail::maybe_impl_trivially_copyable_t<T>,
maybe_detail::maybe_impl_not_trivially_copyable_t<T>>::type;
maybe_impl_t impl_;
public:
// return whether the receiver contains a value.