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Miscellanea fixes for optional #128

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Merged
steve-downey merged 3 commits into from
Jul 8, 2025
Merged

Miscellanea fixes for optional #128

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1d93a02
optional<T>::transform() const &&: move the value into the function
dangelog Jun 19, 2025
75532d4
optional::transform: ensure that it works with non-movable payloads
dangelog Jun 20, 2025
6b5137f
optional<T&>::or_else: copy *this instead of creating a new optional
dangelog Jun 20, 2025
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33 changes: 27 additions & 6 deletions include/beman/optional/optional.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -49,6 +49,12 @@ template <typename T, typename U>
concept optional_ge_rel = requires(const T& t, const U& u) {
{ t >= u } -> std::convertible_to<bool>;
};

struct from_function_t {
explicit from_function_t() = default;
};

inline constexpr from_function_t from_function{};
} // namespace detail

struct in_place_t {
Expand Down Expand Up @@ -403,6 +409,13 @@ class optional {
std::destroy_at(std::addressof(value_));
engaged_ = false;
}

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@steve-downey steve-downey Jul 8, 2025

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Reviewing for creating LWG issues:
I believe this, which is necessary for https://eel.is/c++draft/optional.monadic#note-1, is already covered by the current wording.

template <class U>
friend class optional;

template <class F, class Arg>
constexpr optional(detail::from_function_t, F&& f, Arg&& arg)
: value_(std::invoke(std::forward<F>(f), std::forward<Arg>(arg))), engaged_(true) {}
};

class bad_optional_access : public std::exception {
Expand Down Expand Up @@ -785,7 +798,7 @@ constexpr auto optional<T>::transform(F&& f) & {
static_assert(!std::is_same_v<U, in_place_t>);
static_assert(!std::is_same_v<U, nullopt_t>);
static_assert(std::is_object_v<U> || std::is_reference_v<U>); /// References now allowed
return (has_value()) ? optional<U>{std::invoke(std::forward<F>(f), value_)} : optional<U>{};
return (has_value()) ? optional<U>{detail::from_function, std::forward<F>(f), value_} : optional<U>{};
}

template <class T>
Expand All @@ -796,7 +809,7 @@ constexpr auto optional<T>::transform(F&& f) && {
static_assert(!std::is_same_v<U, in_place_t>);
static_assert(!std::is_same_v<U, nullopt_t>);
static_assert(std::is_object_v<U> || std::is_reference_v<U>); /// References now allowed
return (has_value()) ? optional<U>{std::invoke(std::forward<F>(f), std::move(value_))} : optional<U>{};
return (has_value()) ? optional<U>{detail::from_function, std::forward<F>(f), std::move(value_)} : optional<U>{};
}

template <class T>
Expand All @@ -807,7 +820,7 @@ constexpr auto optional<T>::transform(F&& f) const& {
static_assert(!std::is_same_v<U, in_place_t>);
static_assert(!std::is_same_v<U, nullopt_t>);
static_assert(std::is_object_v<U> || std::is_reference_v<U>); /// References now allowed
return (has_value()) ? optional<U>{std::invoke(std::forward<F>(f), value_)} : optional<U>{};
return (has_value()) ? optional<U>{detail::from_function, std::forward<F>(f), value_} : optional<U>{};
}

template <class T>
Expand All @@ -818,7 +831,7 @@ constexpr auto optional<T>::transform(F&& f) const&& {
static_assert(!std::is_same_v<U, in_place_t>);
static_assert(!std::is_same_v<U, nullopt_t>);
static_assert(std::is_object_v<U> || std::is_reference_v<U>); /// References now allowed
return (has_value()) ? optional<U>{std::invoke(std::forward<F>(f), value_)} : optional<U>{};
return (has_value()) ? optional<U>{detail::from_function, std::forward<F>(f), std::move(value_)} : optional<U>{};
}

/// Calls `f` if the optional is empty
Expand Down Expand Up @@ -1207,6 +1220,14 @@ class optional<T&> {
T& r(std::forward<U>(u));
value_ = std::addressof(r);
}

template <class U>
friend class optional;

template <class F, class Arg>
constexpr optional(detail::from_function_t, F&& f, Arg&& arg) {
convert_ref_init_val(std::invoke(std::forward<F>(f), std::forward<Arg>(arg)));
}
};

// \rSec3[optionalref.ctor]{Constructors}
Expand Down Expand Up @@ -1356,7 +1377,7 @@ constexpr optional<std::invoke_result_t<F, T&>> optional<T&>::transform(F&& f) c
static_assert((std::is_object_v<U> && !std::is_array_v<U>) || std::is_lvalue_reference_v<U>,
"Result must be an non-array object or an lvalue reference");
if (has_value()) {
return optional<U>{std::invoke(std::forward<F>(f), *value_)};
return optional<U>{detail::from_function, std::forward<F>(f), *value_};
} else {
return optional<U>{};
}
Expand All @@ -1368,7 +1389,7 @@ constexpr optional<T&> optional<T&>::or_else(F&& f) const {
using U = std::invoke_result_t<F>;
static_assert(std::is_same_v<std::remove_cvref_t<U>, optional>, "Result must be an optional");
if (has_value()) {
return *value_;
return *this;
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@steve-downey steve-downey Jul 8, 2025

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This is the LWG issue fodder. We wish to change from constructing a new optional over the deferenced val, to using the converting constructor for optional from optional, in order to avoid the deref of a possibly dangling pointer and instead copy the value of the pointer. This may not be safe on all platforms, but is safer and defined on many.

} else {
return std::forward<F>(f)();
}
Expand Down
45 changes: 45 additions & 0 deletions tests/beman/optional/optional_monadic.t.cpp
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Expand Up @@ -3,6 +3,8 @@

#include <beman/optional/optional.hpp>

#include <beman/optional/test_types.hpp>

#include <gtest/gtest.h>

constexpr int get_int(int) { return 42; }
Expand Down Expand Up @@ -81,6 +83,24 @@ TEST(OptionalMonadicTest, Transform) {
beman::optional::optional<const int&> o38r = o38.transform([](int& i) -> const int& { return i; });
EXPECT_TRUE(o38r);
EXPECT_TRUE(*o38r == 42);

// transform and return a non-movable class
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@steve-downey steve-downey Jul 1, 2025

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These are the tests I missed for transform and the reason for the from_function tag?

Do existing implementations get this right? Do we have a spec bug, or is silence enough to require it to work?

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@dangelog dangelog Jul 1, 2025

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Well, the spec says "[Note 1: There is no requirement that U is movable ([dcl.init.general]).
— end note]" in https://eel.is/c++draft/optional#monadic-note-1

Implementations support such non-movable payloads: https://gcc.godbolt.org/z/Wh6q7EGsj

using immovable = beman::optional::tests::immovable;
beman::optional::optional<int> o39 = 42;
auto o39r = o39.transform([](int) { return immovable(); });
EXPECT_TRUE(o39r);

beman::optional::optional<int> o40 = 42;
auto o40r = std::move(o40).transform([](int) { return immovable(); });
EXPECT_TRUE(o40r);

const beman::optional::optional<int> o41 = 42;
auto o41r = o41.transform([](int) { return immovable(); });
EXPECT_TRUE(o41r);

const beman::optional::optional<int> o42 = 42;
auto o42r = std::move(o42).transform([](int) { return immovable(); });
EXPECT_TRUE(o42r);
}

TEST(OptionalMonadicTest, TransformConstexpr) {
Expand Down Expand Up @@ -323,3 +343,28 @@ TEST(OptionalMonadicTest, Constexpr_or_else) {
constexpr auto test4 = *(std::move(o2).or_else([] { return beman::optional::make_optional(13); })) == 13;
EXPECT_TRUE(test4);
}

constexpr bool optional_or_else_do_not_dereference_impl() {
// create a dangling optional<T&>
int* ptr = new int(42);
beman::optional::optional<int&> iref = *ptr;
delete ptr;

if (!iref)
return false;

const auto or_else_fun = []() { return beman::optional::optional<int&>(); };

// This must not dereference the pointer inside `iref`
beman::optional::optional<int&> iref2 = iref.or_else(or_else_fun);
if (!iref2)
return false;

return true;
}

static_assert(optional_or_else_do_not_dereference_impl());

TEST(OptionalMonadicTest, optional_or_else_do_not_derefence) {
EXPECT_TRUE(optional_or_else_do_not_dereference_impl());
}
21 changes: 21 additions & 0 deletions tests/beman/optional/optional_ref_monadic.t.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,8 @@

#include <beman/optional/optional.hpp>

#include <beman/optional/test_types.hpp>

#include <gtest/gtest.h>

namespace {
Expand Down Expand Up @@ -89,6 +91,25 @@ TEST(OptionalRefMonadicTest, Transform) {
auto o38r = o38.transform([](int& i) -> const int& { return i; });
EXPECT_TRUE(o38r);
EXPECT_TRUE(*o38r == 42);

// transform and return a non-movable class
using immovable = beman::optional::tests::immovable;

beman::optional::optional<int&> o39 = fortytwo;
auto o39r = o39.transform([](int&) { return immovable(); });
EXPECT_TRUE(o39r);

beman::optional::optional<int&> o40 = fortytwo;
auto o40r = std::move(o40).transform([](int&) { return immovable(); });
EXPECT_TRUE(o40r);

const beman::optional::optional<int&> o41 = fortytwo;
auto o41r = o41.transform([](int&) { return immovable(); });
EXPECT_TRUE(o41r);

const beman::optional::optional<int&> o42 = fortytwo;
auto o42r = std::move(o42).transform([](int&) { return immovable(); });
EXPECT_TRUE(o42r);
}

TEST(OptionalRefMonadicTest, TransformConstexpr) {
Expand Down