Commit 62b1d047 authored by Eric Niebler's avatar Eric Niebler Committed by Facebook Github Bot 0

rearrange folly::Function so that its template arguments are deducable.

Summary:
`folly::Function` was an alias to a more complex type with template arguments that could not be deduced. For example, the call to `foo` below was failing to compile.

```
template <class R, class... As>
void foo(folly::Function<R(As...)> f) {
}

int main() {
  foo( folly::Function<void(int)>{ [](int i){} } );
}
```

Rearrange the code so that folly::Function is no longer an alias, thus making its template arguments deducable.

Reviewed By: luciang, spacedentist

Differential Revision: D3256130

fb-gh-sync-id: fb403e48d161635b3b7f36e53b1679eb46cbfe7f
fbshipit-source-id: fb403e48d161635b3b7f36e53b1679eb46cbfe7f
parent a6fd9a69
...@@ -227,14 +227,12 @@ ...@@ -227,14 +227,12 @@
namespace folly { namespace folly {
namespace impl { template <typename FunctionType>
template <typename FunctionType, bool Const = false>
class Function; class Function;
template <typename ReturnType, typename... Args> template <typename ReturnType, typename... Args>
Function<ReturnType(Args...), true> constCastFunction( Function<ReturnType(Args...) const> constCastFunction(
Function<ReturnType(Args...), false>&&) noexcept; Function<ReturnType(Args...)>&&) noexcept;
} // impl
namespace detail { namespace detail {
namespace function { namespace function {
...@@ -246,9 +244,6 @@ union Data { ...@@ -246,9 +244,6 @@ union Data {
typename std::aligned_storage<6 * sizeof(void*)>::type small; typename std::aligned_storage<6 * sizeof(void*)>::type small;
}; };
template <bool If, typename T>
using ConstIf = typename std::conditional<If, const T, T>::type;
template <typename Fun, typename FunT = typename std::decay<Fun>::type> template <typename Fun, typename FunT = typename std::decay<Fun>::type>
using IsSmall = std::integral_constant< using IsSmall = std::integral_constant<
bool, bool,
...@@ -259,6 +254,8 @@ using IsSmall = std::integral_constant< ...@@ -259,6 +254,8 @@ using IsSmall = std::integral_constant<
using SmallTag = std::true_type; using SmallTag = std::true_type;
using HeapTag = std::false_type; using HeapTag = std::false_type;
struct CoerceTag {};
template <typename T> template <typename T>
bool isNullPtrFn(T* p) { bool isNullPtrFn(T* p) {
return p == nullptr; return p == nullptr;
...@@ -268,21 +265,135 @@ std::false_type isNullPtrFn(T&&) { ...@@ -268,21 +265,135 @@ std::false_type isNullPtrFn(T&&) {
return {}; return {};
} }
template <typename ReturnType, typename... Args>
ReturnType uninitCall(Data&, Args&&...) {
throw std::bad_function_call();
}
inline bool uninitNoop(Op, Data*, Data*) { inline bool uninitNoop(Op, Data*, Data*) {
return false; return false;
} }
template <typename FunctionType>
struct FunctionTraits;
template <typename ReturnType, typename... Args>
struct FunctionTraits<ReturnType(Args...)> {
using Call = ReturnType (*)(Data&, Args&&...);
using IsConst = std::false_type;
using ConstSignature = ReturnType(Args...) const;
using NonConstSignature = ReturnType(Args...);
using OtherSignature = ConstSignature;
template <typename F, typename G = typename std::decay<F>::type>
using ResultOf = decltype(
static_cast<ReturnType>(std::declval<G&>()(std::declval<Args>()...)));
template <typename Fun>
static ReturnType callSmall(Data& p, Args&&... args) {
return static_cast<ReturnType>((*static_cast<Fun*>(
static_cast<void*>(&p.small)))(static_cast<Args&&>(args)...));
}
template <typename Fun>
static ReturnType callBig(Data& p, Args&&... args) {
return static_cast<ReturnType>(
(*static_cast<Fun*>(p.big))(static_cast<Args&&>(args)...));
}
static ReturnType uninitCall(Data&, Args&&...) {
throw std::bad_function_call();
}
ReturnType operator()(Args... args) {
auto& fn = *static_cast<Function<ReturnType(Args...)>*>(this);
return fn.call_(fn.data_, static_cast<Args&&>(args)...);
}
struct SharedFunctionImpl {
std::shared_ptr<Function<ReturnType(Args...)>> sp_;
ReturnType operator()(Args&&... args) const {
return (*sp_)(static_cast<Args&&>(args)...);
}
};
};
template <typename ReturnType, typename... Args>
struct FunctionTraits<ReturnType(Args...) const> {
using Call = ReturnType (*)(Data&, Args&&...);
using IsConst = std::true_type;
using ConstSignature = ReturnType(Args...) const;
using NonConstSignature = ReturnType(Args...);
using OtherSignature = NonConstSignature;
template <typename F, typename G = typename std::decay<F>::type>
using ResultOf = decltype(static_cast<ReturnType>(
std::declval<const G&>()(std::declval<Args>()...)));
template <typename Fun>
static ReturnType callSmall(Data& p, Args&&... args) {
return static_cast<ReturnType>((*static_cast<const Fun*>(
static_cast<void*>(&p.small)))(static_cast<Args&&>(args)...));
}
template <typename Fun>
static ReturnType callBig(Data& p, Args&&... args) {
return static_cast<ReturnType>(
(*static_cast<const Fun*>(p.big))(static_cast<Args&&>(args)...));
}
static ReturnType uninitCall(Data&, Args&&...) {
throw std::bad_function_call();
}
ReturnType operator()(Args... args) const {
auto& fn = *static_cast<const Function<ReturnType(Args...) const>*>(this);
return fn.call_(fn.data_, static_cast<Args&&>(args)...);
}
struct SharedFunctionImpl {
std::shared_ptr<Function<ReturnType(Args...) const>> sp_;
ReturnType operator()(Args&&... args) const {
return (*sp_)(static_cast<Args&&>(args)...);
}
};
};
template <typename Fun>
bool execSmall(Op o, Data* src, Data* dst) {
switch (o) {
case Op::MOVE:
::new (static_cast<void*>(&dst->small))
Fun(std::move(*static_cast<Fun*>(static_cast<void*>(&src->small))));
FOLLY_FALLTHROUGH;
case Op::NUKE:
static_cast<Fun*>(static_cast<void*>(&src->small))->~Fun();
break;
case Op::FULL:
return true;
case Op::HEAP:
break;
}
return false;
}
template <typename Fun>
bool execBig(Op o, Data* src, Data* dst) {
switch (o) {
case Op::MOVE:
dst->big = src->big;
src->big = nullptr;
break;
case Op::NUKE:
delete static_cast<Fun*>(src->big);
break;
case Op::FULL:
case Op::HEAP:
break;
}
return true;
}
} // namespace function } // namespace function
} // namespace detail } // namespace detail
namespace impl { template <typename FunctionType>
class Function final : private detail::function::FunctionTraits<FunctionType> {
template <typename ReturnType, typename... Args, bool Const>
class Function<ReturnType(Args...), Const> final {
// These utility types are defined outside of the template to reduce // These utility types are defined outside of the template to reduce
// the number of instantiations, and then imported in the class // the number of instantiations, and then imported in the class
// namespace for convenience. // namespace for convenience.
...@@ -290,98 +401,53 @@ class Function<ReturnType(Args...), Const> final { ...@@ -290,98 +401,53 @@ class Function<ReturnType(Args...), Const> final {
using Op = detail::function::Op; using Op = detail::function::Op;
using SmallTag = detail::function::SmallTag; using SmallTag = detail::function::SmallTag;
using HeapTag = detail::function::HeapTag; using HeapTag = detail::function::HeapTag;
using Call = ReturnType (*)(Data&, Args&&...); using CoerceTag = detail::function::CoerceTag;
using Traits = detail::function::FunctionTraits<FunctionType>;
using Call = typename Traits::Call;
using Exec = bool (*)(Op, Data*, Data*); using Exec = bool (*)(Op, Data*, Data*);
template <typename T>
using ConstIf = detail::function::ConstIf<Const, T>;
template <typename Fun> template <typename Fun>
using IsSmall = detail::function::IsSmall<Fun>; using IsSmall = detail::function::IsSmall<Fun>;
/** using OtherSignature = typename Traits::OtherSignature;
* @Function is const-safe:
* - @call_ takes @Data as non-const param to avoid code/data duplication. // The `data_` member is mutable to allow `constCastFunction` to work without
* - @data_ can only be mutated if @constCastFunction is used. // invoking undefined behavior. Const-correctness is only violated when
*/ // `FunctionType` is a const function type (e.g., `int() const`) and `*this`
// is the result of calling `constCastFunction`.
mutable Data data_; mutable Data data_;
Call call_{&detail::function::uninitCall<ReturnType, Args...>}; Call call_{&Traits::uninitCall};
Exec exec_{&detail::function::uninitNoop}; Exec exec_{&detail::function::uninitNoop};
friend Function<ReturnType(Args...), true> constCastFunction<>( friend Traits;
Function<ReturnType(Args...), false>&&) noexcept; friend Function<typename Traits::ConstSignature> folly::constCastFunction<>(
friend class Function<ReturnType(Args...), !Const>; Function<typename Traits::NonConstSignature>&&) noexcept;
friend class Function<OtherSignature>;
template <typename Fun>
struct OpsSmall {
using FunT = typename std::decay<Fun>::type;
static ReturnType call(Data& p, Args&&... args) {
return static_cast<ReturnType>((*static_cast<ConstIf<FunT>*>(
static_cast<void*>(&p.small)))(static_cast<Args&&>(args)...));
}
static bool exec(Op o, Data* src, Data* dst) {
switch (o) {
case Op::MOVE:
::new (static_cast<void*>(&dst->small)) FunT(
std::move(*static_cast<FunT*>(static_cast<void*>(&src->small))));
FOLLY_FALLTHROUGH;
case Op::NUKE:
static_cast<FunT*>(static_cast<void*>(&src->small))->~FunT();
break;
case Op::FULL:
return true;
case Op::HEAP:
break;
}
return false;
}
};
template <typename Fun> template <typename Fun>
Function(Fun&& fun, SmallTag) noexcept { Function(Fun&& fun, SmallTag) noexcept {
using Ops = OpsSmall<Fun>; using FunT = typename std::decay<Fun>::type;
if (!detail::function::isNullPtrFn(fun)) { if (!detail::function::isNullPtrFn(fun)) {
::new (static_cast<void*>(&data_.small)) ::new (static_cast<void*>(&data_.small)) FunT(static_cast<Fun&&>(fun));
typename Ops::FunT(static_cast<Fun&&>(fun)); call_ = &Traits::template callSmall<FunT>;
exec_ = &Ops::exec; exec_ = &detail::function::execSmall<FunT>;
call_ = &Ops::call;
} }
} }
template <typename Fun>
struct OpsHeap {
using FunT = typename std::decay<Fun>::type;
static ReturnType call(Data& p, Args&&... args) {
return static_cast<ReturnType>(
(*static_cast<ConstIf<FunT>*>(p.big))(static_cast<Args&&>(args)...));
}
static bool exec(Op o, Data* src, Data* dst) {
switch (o) {
case Op::MOVE:
dst->big = src->big;
src->big = nullptr;
break;
case Op::NUKE:
delete static_cast<FunT*>(src->big);
break;
case Op::FULL:
case Op::HEAP:
break;
}
return true;
}
};
template <typename Fun> template <typename Fun>
Function(Fun&& fun, HeapTag) { Function(Fun&& fun, HeapTag) {
using Ops = OpsHeap<Fun>; using FunT = typename std::decay<Fun>::type;
data_.big = new typename Ops::FunT(static_cast<Fun&&>(fun)); data_.big = new FunT(static_cast<Fun&&>(fun));
call_ = &Ops::call; call_ = &Traits::template callBig<FunT>;
exec_ = &Ops::exec; exec_ = &detail::function::execBig<FunT>;
} }
template <typename F, typename G = typename std::decay<F>::type> Function(Function<OtherSignature>&& that, CoerceTag) noexcept {
using ResultOf = decltype(static_cast<ReturnType>( that.exec_(Op::MOVE, &that.data_, &data_);
std::declval<ConstIf<G>&>()(std::declval<Args>()...))); std::swap(call_, that.call_);
std::swap(exec_, that.exec_);
}
public: public:
/** /**
...@@ -427,7 +493,7 @@ class Function<ReturnType(Args...), Const> final { ...@@ -427,7 +493,7 @@ class Function<ReturnType(Args...), Const> final {
* \note `typename = ResultOf<Fun>` prevents this overload from being * \note `typename = ResultOf<Fun>` prevents this overload from being
* selected by overload resolution when `fun` is not a compatible function. * selected by overload resolution when `fun` is not a compatible function.
*/ */
template <class Fun, typename = ResultOf<Fun>> template <class Fun, typename = typename Traits::template ResultOf<Fun>>
/* implicit */ Function(Fun&& fun) noexcept(IsSmall<Fun>::value) /* implicit */ Function(Fun&& fun) noexcept(IsSmall<Fun>::value)
: Function(static_cast<Fun&&>(fun), IsSmall<Fun>{}) {} : Function(static_cast<Fun&&>(fun), IsSmall<Fun>{}) {}
...@@ -435,13 +501,10 @@ class Function<ReturnType(Args...), Const> final { ...@@ -435,13 +501,10 @@ class Function<ReturnType(Args...), Const> final {
* For moving a `Function<X(Ys..) const>` into a `Function<X(Ys...)>`. * For moving a `Function<X(Ys..) const>` into a `Function<X(Ys...)>`.
*/ */
template < template <
bool OtherConst, bool Const = Traits::IsConst::value,
typename std::enable_if<!Const && OtherConst, int>::type = 0> typename std::enable_if<!Const, int>::type = 0>
Function(Function<ReturnType(Args...), OtherConst>&& that) noexcept { Function(Function<OtherSignature>&& that) noexcept
that.exec_(Op::MOVE, &that.data_, &data_); : Function(std::move(that), CoerceTag{}) {}
std::swap(call_, that.call_);
std::swap(exec_, that.exec_);
}
/** /**
* If `ptr` is null, constructs an empty `Function`. Otherwise, * If `ptr` is null, constructs an empty `Function`. Otherwise,
...@@ -489,7 +552,7 @@ class Function<ReturnType(Args...), Const> final { ...@@ -489,7 +552,7 @@ class Function<ReturnType(Args...), Const> final {
* \note `typename = ResultOf<Fun>` prevents this overload from being * \note `typename = ResultOf<Fun>` prevents this overload from being
* selected by overload resolution when `fun` is not a compatible function. * selected by overload resolution when `fun` is not a compatible function.
*/ */
template <class Fun, typename = ResultOf<Fun>> template <class Fun, typename = typename Traits::template ResultOf<Fun>>
Function& operator=(Fun&& fun) noexcept( Function& operator=(Fun&& fun) noexcept(
noexcept(/* implicit */ Function(std::declval<Fun>()))) { noexcept(/* implicit */ Function(std::declval<Fun>()))) {
// Doing this in place is more efficient when we can do so safely. // Doing this in place is more efficient when we can do so safely.
...@@ -526,31 +589,8 @@ class Function<ReturnType(Args...), Const> final { ...@@ -526,31 +589,8 @@ class Function<ReturnType(Args...), Const> final {
/** /**
* Call the wrapped callable object with the specified arguments. * Call the wrapped callable object with the specified arguments.
* If this `Function` object is a const `folly::Function` object,
* this overload shall not participate in overload resolution.
*/ */
template < using Traits::operator();
// `True` makes `operator()` a template so we can SFINAE on `Const`,
// which is non-deduced here.
bool True = true,
typename std::enable_if<True && !Const, int>::type = 0>
ReturnType operator()(Args... args) {
return call_(data_, static_cast<Args&&>(args)...);
}
/**
* Call the wrapped callable object with the specified arguments.
* If this `Function` object is not a const `folly::Function` object,
* this overload shall not participate in overload resolution.
*/
template <
// `True` makes `operator()` a template so we can SFINAE on `Const`,
// which is non-deduced here.
bool True = true,
typename std::enable_if<True && Const, int>::type = 0>
ReturnType operator()(Args... args) const {
return call_(data_, static_cast<Args&&>(args)...);
}
/** /**
* Exchanges the callable objects of `*this` and `that`. * Exchanges the callable objects of `*this` and `that`.
...@@ -582,80 +622,51 @@ class Function<ReturnType(Args...), Const> final { ...@@ -582,80 +622,51 @@ class Function<ReturnType(Args...), Const> final {
* Note that the returned `std::function` will share its state (i.e. captured * Note that the returned `std::function` will share its state (i.e. captured
* data) across all copies you make of it, so be very careful when copying. * data) across all copies you make of it, so be very careful when copying.
*/ */
std::function<ReturnType(Args...)> asStdFunction() && { std::function<typename Traits::NonConstSignature> asStdFunction() && {
struct Impl { using Impl = typename Traits::SharedFunctionImpl;
std::shared_ptr<Function> sp_;
ReturnType operator()(Args&&... args) const {
return (*sp_)(static_cast<Args&&>(args)...);
}
};
return Impl{std::make_shared<Function>(std::move(*this))}; return Impl{std::make_shared<Function>(std::move(*this))};
} }
}; };
template <typename FunctionType, bool Const> template <typename FunctionType>
void swap( void swap(Function<FunctionType>& lhs, Function<FunctionType>& rhs) noexcept {
Function<FunctionType, Const>& lhs,
Function<FunctionType, Const>& rhs) noexcept {
lhs.swap(rhs); lhs.swap(rhs);
} }
template <typename FunctionType, bool Const> template <typename FunctionType>
bool operator==(const Function<FunctionType, Const>& fn, std::nullptr_t) { bool operator==(const Function<FunctionType>& fn, std::nullptr_t) {
return !fn; return !fn;
} }
template <typename FunctionType, bool Const> template <typename FunctionType>
bool operator==(std::nullptr_t, const Function<FunctionType, Const>& fn) { bool operator==(std::nullptr_t, const Function<FunctionType>& fn) {
return !fn; return !fn;
} }
template <typename FunctionType, bool Const> template <typename FunctionType>
bool operator!=(const Function<FunctionType, Const>& fn, std::nullptr_t) { bool operator!=(const Function<FunctionType>& fn, std::nullptr_t) {
return !(fn == nullptr); return !(fn == nullptr);
} }
template <typename FunctionType, bool Const> template <typename FunctionType>
bool operator!=(std::nullptr_t, const Function<FunctionType, Const>& fn) { bool operator!=(std::nullptr_t, const Function<FunctionType>& fn) {
return !(nullptr == fn); return !(nullptr == fn);
} }
/** /**
* NOTE: See detailed note about @constCastFunction at the top of the file. * NOTE: See detailed note about `constCastFunction` at the top of the file.
* This is potentially dangerous and requires the equivalent of a `const_cast`. * This is potentially dangerous and requires the equivalent of a `const_cast`.
*/ */
template <typename ReturnType, typename... Args> template <typename ReturnType, typename... Args>
Function<ReturnType(Args...), true> constCastFunction( Function<ReturnType(Args...) const> constCastFunction(
Function<ReturnType(Args...), false>&& that) noexcept { Function<ReturnType(Args...)>&& that) noexcept {
Function<ReturnType(Args...), true> fn{}; return Function<ReturnType(Args...) const>{std::move(that),
that.exec_(detail::function::Op::MOVE, &that.data_, &fn.data_); detail::function::CoerceTag{}};
std::swap(fn.call_, that.call_);
std::swap(fn.exec_, that.exec_);
return fn;
} }
template <typename FunctionType>
Function<FunctionType, true> constCastFunction(
Function<FunctionType, true>&& that) noexcept {
return std::move(that);
}
template <typename FunctionType>
struct MakeFunction {};
template <typename ReturnType, typename... Args>
struct MakeFunction<ReturnType(Args...)> {
using type = Function<ReturnType(Args...), false>;
};
template <typename ReturnType, typename... Args> template <typename ReturnType, typename... Args>
struct MakeFunction<ReturnType(Args...) const> { Function<ReturnType(Args...) const> constCastFunction(
using type = Function<ReturnType(Args...), true>; Function<ReturnType(Args...) const>&& that) noexcept {
}; return std::move(that);
} // namespace impl
/* using override */ using impl::constCastFunction;
template <typename FunctionType>
using Function = typename impl::MakeFunction<FunctionType>::type;
} }
} // namespace folly
...@@ -49,6 +49,10 @@ struct Functor { ...@@ -49,6 +49,10 @@ struct Functor {
return oldvalue; return oldvalue;
} }
}; };
template <typename Ret, typename... Args>
void deduceArgs(Function<Ret(Args...)>) {}
} // namespace } // namespace
// TEST ===================================================================== // TEST =====================================================================
...@@ -849,3 +853,9 @@ TEST(Function, SelfMoveAssign) { ...@@ -849,3 +853,9 @@ TEST(Function, SelfMoveAssign) {
f = std::move(g); f = std::move(g);
EXPECT_TRUE(f); EXPECT_TRUE(f);
} }
TEST(Function, DeducableArguments) {
deduceArgs(Function<void()>{[] {}});
deduceArgs(Function<void(int, float)>{[](int, float) {}});
deduceArgs(Function<int(int, float)>{[](int i, float) { return i; }});
}
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