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[libc++] Simplify std::function implementation further #145153

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Merged
merged 1 commit into from
Sep 3, 2025
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[libc++] Simplify std::function implementation further #145153

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126 changes: 54 additions & 72 deletions libcxx/include/__functional/function.h
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Original file line number Diff line number Diff line change
Expand Up @@ -15,16 +15,14 @@
#include <__cstddef/nullptr_t.h>
#include <__exception/exception.h>
#include <__functional/binary_function.h>
#include <__functional/invoke.h>
#include <__functional/unary_function.h>
#include <__memory/addressof.h>
#include <__type_traits/aligned_storage.h>
#include <__type_traits/decay.h>
#include <__type_traits/is_core_convertible.h>
#include <__type_traits/invoke.h>
#include <__type_traits/is_scalar.h>
#include <__type_traits/is_trivially_constructible.h>
#include <__type_traits/is_trivially_destructible.h>
#include <__type_traits/is_void.h>
#include <__type_traits/strip_signature.h>
#include <__utility/forward.h>
#include <__utility/move.h>
Expand Down Expand Up @@ -95,29 +93,29 @@ template <class _Rp, class _A1, class _A2>
struct __maybe_derive_from_binary_function<_Rp(_A1, _A2)> : public __binary_function<_A1, _A2, _Rp> {};

template <class _Fp>
_LIBCPP_HIDE_FROM_ABI bool __not_null(_Fp const&) {
return true;
_LIBCPP_HIDE_FROM_ABI bool __is_null(_Fp const&) {
return false;
}

template <class _Fp>
_LIBCPP_HIDE_FROM_ABI bool __not_null(_Fp* __ptr) {
return __ptr;
_LIBCPP_HIDE_FROM_ABI bool __is_null(_Fp* __ptr) {
return !__ptr;
}

template <class _Ret, class _Class>
_LIBCPP_HIDE_FROM_ABI bool __not_null(_Ret _Class::*__ptr) {
return __ptr;
_LIBCPP_HIDE_FROM_ABI bool __is_null(_Ret _Class::* __ptr) {
return !__ptr;
}

template <class _Fp>
_LIBCPP_HIDE_FROM_ABI bool __not_null(function<_Fp> const& __f) {
return !!__f;
_LIBCPP_HIDE_FROM_ABI bool __is_null(function<_Fp> const& __f) {
return !__f;
}

# if __has_extension(blocks)
template <class _Rp, class... _Args>
_LIBCPP_HIDE_FROM_ABI bool __not_null(_Rp (^__p)(_Args...)) {
return __p;
_LIBCPP_HIDE_FROM_ABI bool __is_null(_Rp (^__p)(_Args...)) {
return !__p;
}
# endif

Expand Down Expand Up @@ -206,12 +204,13 @@ class __value_func<_Rp(_ArgTypes...)> {
_LIBCPP_HIDE_FROM_ABI explicit __value_func(_Fp&& __f) : __f_(nullptr) {
typedef __function::__func<_Fp, _Rp(_ArgTypes...)> _Fun;

if (__function::__not_null(__f)) {
if (sizeof(_Fun) <= sizeof(__buf_) && is_nothrow_copy_constructible<_Fp>::value) {
__f_ = ::new (std::addressof(__buf_)) _Fun(std::move(__f));
} else {
__f_ = new _Fun(std::move(__f));
}
if (__function::__is_null(__f))
return;

if (sizeof(_Fun) <= sizeof(__buf_) && is_nothrow_copy_constructible<_Fp>::value) {
__f_ = ::new (std::addressof(__buf_)) _Fun(std::move(__f));
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I think this needs to be ::new ((void*)std::addressof(__buf_)) _Fun(std::move(__f)); otherwise users can hijack the call to placement new using an overload like

template <class T>
void* operator new(size_t, T*) = delete;

See https://godbolt.org/z/Wfhes8Mff

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_Fun is always a library-internal type, so I don't think users are allowed to hijack it.

} else {
__f_ = new _Fun(std::move(__f));
}
}

Expand Down Expand Up @@ -356,7 +355,31 @@ struct __policy {
// type.
template <typename _Fun>
_LIBCPP_HIDE_FROM_ABI static const __policy* __create() {
return __choose_policy<_Fun>(__use_small_storage<_Fun>());
if constexpr (__use_small_storage<_Fun>::value) {
static constexpr __policy __policy = {
nullptr,
nullptr,
false,
# if _LIBCPP_HAS_RTTI
&typeid(_Fun)
# else
nullptr
# endif
};
return &__policy;
} else {
static constexpr __policy __policy = {
std::addressof(__large_clone<_Fun>),
std::addressof(__large_destroy<_Fun>),
false,
# if _LIBCPP_HAS_RTTI
&typeid(_Fun)
# else
nullptr
# endif
};
return &__policy;
}
}

_LIBCPP_HIDE_FROM_ABI static const __policy* __create_empty() {
Expand Down Expand Up @@ -384,36 +407,6 @@ struct __policy {
_LIBCPP_HIDE_FROM_ABI static void __large_destroy(void* __s) {
delete static_cast<_Fun*>(__s);
}

template <typename _Fun>
_LIBCPP_HIDE_FROM_ABI static const __policy* __choose_policy(/* is_small = */ false_type) {
static constexpr __policy __policy = {
std::addressof(__large_clone<_Fun>),
std::addressof(__large_destroy<_Fun>),
false,
# if _LIBCPP_HAS_RTTI
&typeid(_Fun)
# else
nullptr
# endif
};
return &__policy;
}

template <typename _Fun>
_LIBCPP_HIDE_FROM_ABI static const __policy* __choose_policy(/* is_small = */ true_type) {
static constexpr __policy __policy = {
nullptr,
nullptr,
false,
# if _LIBCPP_HAS_RTTI
&typeid(_Fun)
# else
nullptr
# endif
};
return &__policy;
}
};

// Used to choose between perfect forwarding or pass-by-value. Pass-by-value is
Expand Down Expand Up @@ -455,14 +448,15 @@ class __policy_func<_Rp(_ArgTypes...)> {

template <class _Fp, __enable_if_t<!is_same<__decay_t<_Fp>, __policy_func>::value, int> = 0>
_LIBCPP_HIDE_FROM_ABI explicit __policy_func(_Fp&& __f) : __policy_(__policy::__create_empty()) {
if (__function::__not_null(__f)) {
__func_ = __call_func<_Fp>;
__policy_ = __policy::__create<_Fp>();
if (__use_small_storage<_Fp>()) {
::new ((void*)&__buf_.__small) _Fp(std::move(__f));
} else {
__buf_.__large = ::new _Fp(std::move(__f));
}
if (__function::__is_null(__f))
return;

__func_ = __call_func<_Fp>;
__policy_ = __policy::__create<_Fp>();
if (__use_small_storage<_Fp>()) {
::new ((void*)&__buf_.__small) _Fp(std::move(__f));
} else {
__buf_.__large = ::new _Fp(std::move(__f));
}
}

Expand Down Expand Up @@ -615,21 +609,9 @@ class function<_Rp(_ArgTypes...)>

__func __f_;

template <class _Fp,
bool = _And<_IsNotSame<__remove_cvref_t<_Fp>, function>, __is_invocable<_Fp, _ArgTypes...> >::value>
struct __callable;
template <class _Fp>
struct __callable<_Fp, true> {
static const bool value =
is_void<_Rp>::value || __is_core_convertible<__invoke_result_t<_Fp, _ArgTypes...>, _Rp>::value;
};
template <class _Fp>
struct __callable<_Fp, false> {
static const bool value = false;
};

template <class _Fp>
using _EnableIfLValueCallable _LIBCPP_NODEBUG = __enable_if_t<__callable<_Fp&>::value>;
using _EnableIfLValueCallable _LIBCPP_NODEBUG = __enable_if_t<
_And<_IsNotSame<__remove_cvref_t<_Fp>, function>, __is_invocable_r<_Rp, _Fp&, _ArgTypes...>>::value>;

public:
typedef _Rp result_type;
Expand Down
6 changes: 6 additions & 0 deletions libcxx/include/__type_traits/invoke.h
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Original file line number Diff line number Diff line change
Expand Up @@ -62,6 +62,9 @@
//
// template <class Func, class... Args>
// using __invoke_result_t = invoke_result_t<Func, Args...>;
//
// template <class Ret, class Func, class... Args>
// struct __is_invocable_r : is_invocable_r<Ret, Func, Args...> {};

_LIBCPP_BEGIN_NAMESPACE_STD

Expand Down Expand Up @@ -329,6 +332,9 @@ using __invoke_result_t _LIBCPP_NODEBUG = typename __invoke_result<_Func, _Args.

#endif // __has_builtin(__builtin_invoke_r)

template <class _Ret, class _Func, class... _Args>
struct __is_invocable_r : integral_constant<bool, __is_invocable_r_v<_Ret, _Func, _Args...> > {};

template <class _Ret, bool = is_void<_Ret>::value>
struct __invoke_void_return_wrapper {
template <class... _Args>
Expand Down
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