--- /dev/null
+/* Copyright (C) 2017 Free Software Foundation, Inc.
+
+ This file is part of GDB.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>. */
+
+#ifndef COMMON_FUNCTION_VIEW_H
+#define COMMON_FUNCTION_VIEW_H
+
+/* function_view is a polymorphic type-erasing wrapper class that
+ encapsulates a non-owning reference to arbitrary callable objects.
+
+ A way to put it is that function_view is to std::function like
+ std::string_view is to std::string. While std::function stores a
+ type-erased callable object internally, function_view holds a
+ type-erased reference to an external callable object.
+
+ This is meant to be used as callback type of a function that:
+
+ #1 - Takes a callback as parameter.
+
+ #2 - Wants to support arbitrary callable objects as callback type
+ (e.g., stateful function objects, lambda closures, free
+ functions).
+
+ #3 - Does not store the callback anywhere; instead the function
+ just calls the callback directly or forwards it to some
+ other function that calls it.
+
+ #4 - Can't be, or we don't want it to be, a template function
+ with the callable type as template parameter. For example,
+ when the callback is a parameter of a virtual member
+ function, or when putting the function template in a header
+ would expose too much implementation detail.
+
+ Note that the C-style "function pointer" + "void *data" callback
+ parameter idiom fails requirement #2 above. Please don't add new
+ uses of that idiom. I.e., something like this wouldn't work;
+
+ typedef bool (iterate_over_foos_cb) (foo *f, void *user_data),
+ void iterate_over_foos (iterate_over_foos_cb *callback, void *user_data);
+
+ foo *find_foo_by_type (int type)
+ {
+ foo *found = nullptr;
+
+ iterate_over_foos ([&] (foo *f, void *data)
+ {
+ if (foo->type == type)
+ {
+ found = foo;
+ return true; // stop iterating
+ }
+ return false; // continue iterating
+ }, NULL);
+
+ return found;
+ }
+
+ The above wouldn't compile, because lambdas with captures can't be
+ implicitly converted to a function pointer (because a capture means
+ some context data must be passed to the lambda somehow).
+
+ C++11 gave us std::function as type-erased wrapper around arbitrary
+ callables, however, std::function is not an ideal fit for transient
+ callbacks such as the use case above. For this use case, which is
+ quite pervasive, a function_view is a better choice, because while
+ function_view is light and does not require any heap allocation,
+ std::function is a heavy-weight object with value semantics that
+ generally requires a heap allocation on construction/assignment of
+ the target callable. In addition, while it is possible to use
+ std::function in such a way that avoids most of the overhead by
+ making sure to only construct it with callables of types that fit
+ std::function's small object optimization, such as function
+ pointers and std::reference_wrapper callables, that is quite
+ inconvenient in practice, because restricting to free-function
+ callables would imply no state/capture/closure, which we need in
+ most cases, and std::reference_wrapper implies remembering to use
+ std::ref/std::cref where the callable is constructed, with the
+ added inconvenience that std::ref/std::cref have deleted rvalue-ref
+ overloads, meaning you can't use unnamed/temporary lambdas with
+ them.
+
+ Note that because function_view is a non-owning view of a callable,
+ care must be taken to ensure that the callable outlives the
+ function_view that calls it. This is not really a problem for the
+ use case function_view is intended for, such as passing a temporary
+ function object / lambda to a function that accepts a callback,
+ because in those cases, the temporary is guaranteed to be live
+ until the called function returns.
+
+ Calling a function_view with no associated target is undefined,
+ unlike with std::function, which throws std::bad_function_call.
+ This is by design, to avoid the otherwise necessary NULL check in
+ function_view::operator().
+
+ Since function_view objects are small (a pair of pointers), they
+ should generally be passed around by value.
+
+ Usage:
+
+ Given this function that accepts a callback:
+
+ void
+ iterate_over_foos (gdb::function_view<void (foo *)> callback)
+ {
+ for (auto &foo : foos)
+ callback (&foo);
+ }
+
+ you can call it like this, passing a lambda as callback:
+
+ iterate_over_foos ([&] (foo *f)
+ {
+ process_one_foo (f);
+ });
+
+ or like this, passing a function object as callback:
+
+ struct function_object
+ {
+ void operator() (foo *f)
+ {
+ if (s->check ())
+ process_one_foo (f);
+ }
+
+ // some state
+ state *s;
+ };
+
+ state mystate;
+ function_object matcher {&mystate};
+ iterate_over_foos (matcher);
+
+ or like this, passing a function pointer as callback:
+
+ iterate_over_foos (process_one_foo);
+
+ You can find unit tests covering the whole API in
+ unittests/function-view-selftests.c. */
+
+namespace gdb {
+
+namespace traits {
+ /* A few trait helpers. */
+ template<typename Predicate>
+ struct Not : public std::integral_constant<bool, !Predicate::value>
+ {};
+
+ template<typename...>
+ struct Or;
+
+ template<>
+ struct Or<> : public std::false_type
+ {};
+
+ template<typename B1>
+ struct Or<B1> : public B1
+ {};
+
+ template<typename B1, typename B2>
+ struct Or<B1, B2>
+ : public std::conditional<B1::value, B1, B2>::type
+ {};
+
+ template<typename B1,typename B2,typename B3, typename... Bn>
+ struct Or<B1, B2, B3, Bn...>
+ : public std::conditional<B1::value, B1, Or<B2, B3, Bn...>>::type
+ {};
+} /* namespace traits */
+
+namespace fv_detail {
+/* Bits shared by all function_view instantiations that do not depend
+ on the template parameters. */
+
+/* Storage for the erased callable. This is a union in order to be
+ able to save both a function object (data) pointer or a function
+ pointer without triggering undefined behavior. */
+union erased_callable
+{
+ /* For function objects. */
+ void *data;
+
+ /* For function pointers. */
+ void (*fn) ();
+};
+
+} /* namespace fv_detail */
+
+/* Use partial specialization to get access to the callable's
+ signature. */
+template<class Signature>
+struct function_view;
+
+template<typename Res, typename... Args>
+class function_view<Res (Args...)>
+{
+ template<typename From, typename To>
+ using CompatibleReturnType
+ = traits::Or<std::is_void<To>,
+ std::is_same<From, To>,
+ std::is_convertible<From, To>>;
+
+ /* True if Func can be called with Args, and either the result is
+ Res, convertible to Res or Res is void. */
+ template<typename Callable,
+ typename Res2 = typename std::result_of<Callable &(Args...)>::type>
+ struct IsCompatibleCallable : CompatibleReturnType<Res2, Res>
+ {};
+
+ /* True if Callable is a function_view. Used to avoid hijacking the
+ copy ctor. */
+ template <typename Callable>
+ struct IsFunctionView
+ : std::is_same<function_view, typename std::decay<Callable>::type>
+ {};
+
+ /* Helper to make SFINAE logic easier to read. */
+ template<typename Condition>
+ using Requires = typename std::enable_if<Condition::value, void>::type;
+
+ public:
+
+ /* NULL by default. */
+ constexpr function_view () noexcept
+ : m_erased_callable {},
+ m_invoker {}
+ {}
+
+ /* Default copy/assignment is fine. */
+ function_view (const function_view &) = default;
+ function_view &operator= (const function_view &) = default;
+
+ /* This is the main entry point. Use SFINAE to avoid hijacking the
+ copy constructor and to ensure that the target type is
+ compatible. */
+ template
+ <typename Callable,
+ typename = Requires<traits::Not<IsFunctionView<Callable>>>,
+ typename = Requires<IsCompatibleCallable<Callable>>>
+ function_view (Callable &&callable) noexcept
+ {
+ bind (callable);
+ }
+
+ /* Construct a NULL function_view. */
+ constexpr function_view (std::nullptr_t) noexcept
+ : m_erased_callable {},
+ m_invoker {}
+ {}
+
+ /* Clear a function_view. */
+ function_view &operator= (std::nullptr_t) noexcept
+ {
+ m_invoker = nullptr;
+ return *this;
+ }
+
+ /* Return true if the wrapper has a target, false otherwise. Note
+ we check M_INVOKER instead of M_ERASED_CALLABLE because we don't
+ know which member of the union is active right now. */
+ constexpr explicit operator bool () const noexcept
+ { return m_invoker != nullptr; }
+
+ /* Call the callable. */
+ Res operator () (Args... args) const
+ { return m_invoker (m_erased_callable, std::forward<Args> (args)...); }
+
+ private:
+
+ /* Bind this function_view to a compatible function object
+ reference. */
+ template <typename Callable>
+ void bind (Callable &callable) noexcept
+ {
+ m_erased_callable.data = (void *) std::addressof (callable);
+ m_invoker = [] (fv_detail::erased_callable ecall, Args... args)
+ noexcept (noexcept (callable (std::forward<Args> (args)...))) -> Res
+ {
+ auto &restored_callable = *static_cast<Callable *> (ecall.data);
+ /* The explicit cast to Res avoids a compile error when Res is
+ void and the callable returns non-void. */
+ return (Res) restored_callable (std::forward<Args> (args)...);
+ };
+ }
+
+ /* Bind this function_view to a compatible function pointer.
+
+ Making this a separate function allows avoiding one indirection,
+ by storing the function pointer directly in the storage, instead
+ of a pointer to pointer. erased_callable is then a union in
+ order to avoid storing a function pointer as a data pointer here,
+ which would be undefined. */
+ template<class Res2, typename... Args2>
+ void bind (Res2 (*fn) (Args2...)) noexcept
+ {
+ m_erased_callable.fn = reinterpret_cast<void (*) ()> (fn);
+ m_invoker = [] (fv_detail::erased_callable ecall, Args... args)
+ noexcept (noexcept (fn (std::forward<Args> (args)...))) -> Res
+ {
+ auto restored_fn = reinterpret_cast<Res2 (*) (Args2...)> (ecall.fn);
+ /* The explicit cast to Res avoids a compile error when Res is
+ void and the callable returns non-void. */
+ return (Res) restored_fn (std::forward<Args> (args)...);
+ };
+ }
+
+ /* Storage for the erased callable. */
+ fv_detail::erased_callable m_erased_callable;
+
+ /* The invoker. This is set to a capture-less lambda by one of the
+ 'bind' overloads. The lambda restores the right type of the
+ callable (which is passed as first argument), and forwards the
+ args. */
+ Res (*m_invoker) (fv_detail::erased_callable, Args...);
+};
+
+/* Allow comparison with NULL. Defer the work to the in-class
+ operator bool implementation. */
+
+template<typename Res, typename... Args>
+constexpr inline bool
+operator== (const function_view<Res (Args...)> &f, std::nullptr_t) noexcept
+{ return !static_cast<bool> (f); }
+
+template<typename Res, typename... Args>
+constexpr inline bool
+operator== (std::nullptr_t, const function_view<Res (Args...)> &f) noexcept
+{ return !static_cast<bool> (f); }
+
+template<typename Res, typename... Args>
+constexpr inline bool
+operator!= (const function_view<Res (Args...)> &f, std::nullptr_t) noexcept
+{ return static_cast<bool> (f); }
+
+template<typename Res, typename... Args>
+constexpr inline bool
+operator!= (std::nullptr_t, const function_view<Res (Args...)> &f) noexcept
+{ return static_cast<bool> (f); }
+
+} /* namespace gdb */
+
+#endif
--- /dev/null
+/* Self tests for function_view for GDB, the GNU debugger.
+
+ Copyright (C) 2017 Free Software Foundation, Inc.
+
+ This file is part of GDB.
+
+ This program is free software; you can redistribute it and/or modify
+ it under the terms of the GNU General Public License as published by
+ the Free Software Foundation; either version 3 of the License, or
+ (at your option) any later version.
+
+ This program is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ GNU General Public License for more details.
+
+ You should have received a copy of the GNU General Public License
+ along with this program. If not, see <http://www.gnu.org/licenses/>. */
+
+#include "defs.h"
+#include "selftest.h"
+#include "common/function-view.h"
+
+namespace selftests {
+namespace function_view {
+
+static int
+plus_one_fn_int (int val)
+{
+ return ++val;
+}
+
+static short
+plus_one_fn_short (short val)
+{
+ return ++val;
+}
+
+static int
+call_callback_int (int val, gdb::function_view <int (int)> callback)
+{
+ return callback (val);
+}
+
+static void
+call_callback_void (int val, gdb::function_view <void (int)> callback)
+{
+ callback (val);
+}
+
+struct plus_one_int_func_obj
+{
+ int operator () (int val)
+ {
+ ++call_count;
+ return ++val;
+ }
+
+ /* Number of times called. */
+ int call_count = 0;
+};
+
+static void
+run_tests ()
+{
+ /* A simple lambda. */
+ auto plus_one_lambda = [] (int val) { return ++val; };
+
+ /* A function_view that references the lambda. */
+ gdb::function_view<int (int)> plus_one_func_view (plus_one_lambda);
+
+ /* Check calling the lambda directly. */
+ SELF_CHECK (plus_one_lambda (0) == 1);
+ SELF_CHECK (plus_one_lambda (1) == 2);
+
+ /* Check calling lambda via the view. */
+ SELF_CHECK (plus_one_func_view (2) == 3);
+ SELF_CHECK (plus_one_func_view (3) == 4);
+
+ /* Check calling a function that takes a function_view as argument,
+ by value. Pass a lambda, making sure a function_view is properly
+ constructed implicitly. */
+ SELF_CHECK (call_callback_int (1, [] (int val)
+ {
+ return val + 2;
+ }) == 3);
+
+ /* Same, passing a named/lvalue lambda. */
+ SELF_CHECK (call_callback_int (1, plus_one_lambda) == 2);
+ /* Same, passing a named/lvalue function_view (should copy). */
+ SELF_CHECK (call_callback_int (1, plus_one_func_view) == 2);
+
+ /* Check constructing a function view over a function-object
+ callable, and calling it. */
+ plus_one_int_func_obj func_obj;
+ SELF_CHECK (func_obj (0) == 1);
+ SELF_CHECK (call_callback_int (1, func_obj) == 2);
+ /* Check that the callable was referenced, not copied. */
+ SELF_CHECK (func_obj.call_count == 2);
+
+ /* Check constructing a function_view over a free-function callable,
+ and calling it. */
+ SELF_CHECK (call_callback_int (1, plus_one_fn_int) == 2);
+
+ /* Check calling a function with a
+ compatible-but-not-exactly-the-same prototype. */
+ SELF_CHECK (call_callback_int (1, [] (short val) -> short
+ {
+ return val + 2;
+ }) == 3);
+ /* Same, but passing a function pointer. */
+ SELF_CHECK (call_callback_int (1, plus_one_fn_short) == 2);
+
+ /* Like std::function, a function_view that expects a void return
+ can reference callables with non-void return type. The result is
+ simply discarded. Check a lambda, function object and a function
+ pointer. */
+ call_callback_void (1, [] (int val) -> int
+ {
+ return val + 2;
+ });
+ call_callback_void (1, func_obj);
+ call_callback_void (1, plus_one_fn_int);
+
+ /* Check that the main ctor doesn't hijack the copy ctor. */
+ auto plus_one_func_view2 (plus_one_func_view);
+ auto plus_one_func_view3 (plus_one_func_view2);
+ static_assert (std::is_same<decltype (plus_one_func_view),
+ decltype (plus_one_func_view2)>::value, "");
+ static_assert (std::is_same<decltype (plus_one_func_view),
+ decltype (plus_one_func_view3)>::value, "");
+
+ SELF_CHECK (plus_one_func_view3 (1) == 2);
+
+ /* Likewise, but propagate a NULL callable. If this calls the main
+ function_view ctor instead of the copy ctor by mistake, then
+ null_func_2 ends up non-NULL (because it'd instead reference
+ null_func_1 as just another callable). */
+ constexpr gdb::function_view<int (int)> null_func_view_1 = nullptr;
+ constexpr auto null_func_view_2 (null_func_view_1);
+
+ /* While at it, check whether the function_view is bound using
+ various forms, op==, op!= and op bool. */
+
+ /* op== */
+ static_assert (null_func_view_2 == nullptr, "");
+ static_assert (nullptr == null_func_view_2, "");
+ static_assert (null_func_view_2 == NULL, "");
+ static_assert (NULL == null_func_view_2, "");
+
+ /* op!= */
+ static_assert (!(null_func_view_2 != nullptr), "");
+ static_assert (!(nullptr != null_func_view_2), "");
+ static_assert (!(null_func_view_2 != NULL), "");
+ static_assert (!(NULL != null_func_view_2), "");
+
+ /* op bool */
+ static_assert (!null_func_view_2, "");
+
+ /* Check the nullptr_t ctor. */
+ constexpr gdb::function_view<int (int)> check_ctor_nullptr (nullptr);
+ static_assert (!check_ctor_nullptr, "");
+
+ /* Check the nullptr_t op= */
+ gdb::function_view<int (int)> check_op_eq_null (plus_one_fn_int);
+ SELF_CHECK (check_op_eq_null);
+ check_op_eq_null = nullptr;
+ SELF_CHECK (!check_op_eq_null);
+}
+
+} /* namespace function_view */
+} /* namespace selftests */
+
+void
+_initialize_function_view_selftests ()
+{
+ register_self_test (selftests::function_view::run_tests);
+}
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