From 1925b2d10f3d9a87432e7756d420059ac3331d0b Mon Sep 17 00:00:00 2001
From: pfeatherstone <45853521+pfeatherstone@users.noreply.github.com>
Date: Sun, 14 Nov 2021 21:18:06 +0000
Subject: [PATCH] [INVOKE] added is_invocable_r, invoke_r, made everything
 constexpr and conditionally noexcept (#2458)

* - added dlib::invoke_r()
- added dlib::is_invocable_r<>
- made everything constexpr and conditionally noexcept !
- added tests
This has required a refactor of dlib::invoke since std::mem_fn is not constexpr in c++11

* - !std::is_function replaced with std::is_object. Let's be a bit more precise

* - made dlib::apply constexpr and conditionally noexcept

Co-authored-by: pf <pf@me>
---
 dlib/invoke.h        | 176 ++++++++++++++++++++++++++++++++++++++++---
 dlib/test/invoke.cpp |  96 +++++++++++++++++++++++
 2 files changed, 260 insertions(+), 12 deletions(-)

diff --git a/dlib/invoke.h b/dlib/invoke.h
index 4abca8a27..f5792c11d 100644
--- a/dlib/invoke.h
+++ b/dlib/invoke.h
@@ -11,32 +11,153 @@ namespace dlib
 {
     // ----------------------------------------------------------------------------------------
     namespace detail {
-        template< typename F, typename ... Args >
-        auto INVOKE(F&& fn, Args&& ... args)
+        template< typename T >
+        struct is_reference_wrapper : std::false_type {};
+        template< typename U >
+        struct is_reference_wrapper<std::reference_wrapper<U>> : std::true_type {};
+
+        template <
+            typename Base,
+            typename T,
+            typename Derived,
+            typename... Args
+        >
+        constexpr auto INVOKE(
+            T Base::*pmf, //pointer to member function
+            Derived&& ref,
+            Args&&... args
+        )
+        noexcept(noexcept((std::forward<Derived>(ref).*pmf)(std::forward<Args>(args)...)))
         -> typename std::enable_if<
-                std::is_member_pointer<typename std::decay<F>::type>::value,
-                decltype(std::mem_fn(fn)(std::forward<Args>(args)...))>::type
+                std::is_function<T>::value &&
+                std::is_base_of<Base, typename std::decay<Derived>::type>::value,
+                decltype((std::forward<Derived>(ref).*pmf)(std::forward<Args>(args)...))
+           >::type
         {
-            return std::mem_fn(fn)(std::forward<Args>(args)...) ;
+            return (std::forward<Derived>(ref).*pmf)(std::forward<Args>(args)...);
         }
 
-        template< typename F, typename ... Args >
-        auto INVOKE(F&& fn, Args&& ... args)
+        template<
+            typename Base,
+            typename T,
+            typename RefWrap,
+            typename... Args
+        >
+        constexpr auto INVOKE(
+            T Base::*pmf, //pointer to member function
+            RefWrap&& ref,
+            Args&&... args
+        )
+        noexcept(noexcept((ref.get().*pmf)(std::forward<Args>(args)...)))
+        -> typename std::enable_if<
+                std::is_function<T>::value &&
+                is_reference_wrapper<typename std::decay<RefWrap>::type>::value,
+                decltype((ref.get().*pmf)(std::forward<Args>(args)...))>::type
+        {
+            return (ref.get().*pmf)(std::forward<Args>(args)...);
+        }
+
+        template<
+            typename Base,
+            typename T,
+            typename Ptr,
+            typename... Args
+        >
+        constexpr auto INVOKE(
+            T Base::*pmf, //pointer to member function
+            Ptr&& ptr,
+            Args&&... args
+        )
+        noexcept(noexcept(((*std::forward<Ptr>(ptr)).*pmf)( std::forward<Args>( args )...)))
+        -> typename std::enable_if<
+                std::is_function<T>::value &&
+                !is_reference_wrapper<typename std::decay<Ptr>::type>::value &&
+                !std::is_base_of<Base, typename std::decay<Ptr>::type>::value,
+                decltype(((*std::forward<Ptr>(ptr)).*pmf)(std::forward<Args>(args)...))>::type
+        {
+            return ((*std::forward<Ptr>(ptr)).*pmf)( std::forward<Args>( args )...);
+        }
+
+        template<
+            typename Base,
+            typename T,
+            typename Derived
+        >
+        constexpr auto INVOKE(
+            T Base::*pmd, //pointer to member data
+            Derived&& ref
+        )
+        noexcept(noexcept(std::forward<Derived>(ref).*pmd))
+        -> typename std::enable_if<
+                std::is_object<T>::value &&
+                std::is_base_of<Base, typename std::decay<Derived>::type>::value,
+                decltype(std::forward<Derived>(ref).*pmd)>::type
+        {
+            return std::forward<Derived>(ref).*pmd;
+        }
+
+        template<
+            typename Base,
+            typename T,
+            typename RefWrap
+        >
+        constexpr auto INVOKE(
+            T Base::*pmd, //pointer to member data
+            RefWrap&& ref
+        )
+        noexcept(noexcept(ref.get().*pmd))
+        -> typename std::enable_if<
+                std::is_object<T>::value &&
+                is_reference_wrapper<typename std::decay<RefWrap>::type>::value,
+                decltype(ref.get().*pmd)>::type
+        {
+            return ref.get().*pmd;
+        }
+
+        template<
+            typename Base,
+            typename T,
+            typename Ptr
+        >
+        constexpr auto INVOKE(
+            T Base::*pmd, //pointer to member data
+            Ptr&& ptr
+        )
+        noexcept(noexcept((*std::forward<Ptr>(ptr)).*pmd))
+        -> typename std::enable_if<
+                std::is_object<T>::value &&
+                !is_reference_wrapper<typename std::decay<Ptr>::type>::value &&
+                !std::is_base_of<Base, typename std::decay<Ptr>::type>::value,
+                decltype((*std::forward<Ptr>(ptr)).*pmd)>::type
+        {
+            return (*std::forward<Ptr>(ptr)).*pmd;
+        }
+
+        template<
+            typename F,
+            typename... Args
+        >
+        constexpr auto INVOKE(
+            F && f,
+            Args&&... args
+        )
+        noexcept(noexcept(std::forward<F>( f )( std::forward<Args>( args )...)))
         -> typename std::enable_if<
                 !std::is_member_pointer<typename std::decay<F>::type>::value,
-                decltype(std::forward<F>(fn)(std::forward<Args>(args)...))>::type
+                decltype(std::forward<F>(f)(std::forward<Args>(args)...))>::type
         {
-            return std::forward<F>(fn)(std::forward<Args>(args)...);
+            return std::forward<F>( f )( std::forward<Args>( args )...);
         }
     }
 
     template< typename F, typename... Args>
-    auto invoke(F && f, Args &&... args)
+    constexpr auto invoke(F && f, Args &&... args)
     /*!
         ensures
             - identical to std::invoke(std::forward<F>(f), std::forward<Args>(args)...)
             - works with C++11 onwards
     !*/
+    noexcept(noexcept(detail::INVOKE(std::forward<F>( f ), std::forward<Args>( args )...)))
     -> decltype(detail::INVOKE(std::forward<F>( f ), std::forward<Args>( args )...))
     {
         return detail::INVOKE(std::forward<F>( f ), std::forward<Args>( args )...);
@@ -88,10 +209,38 @@ namespace dlib
 
     // ----------------------------------------------------------------------------------------
 
+    template <typename R, typename F, typename... Args>
+    struct is_invocable_r : std::integral_constant<bool, dlib::is_invocable<F, Args...>::value &&
+                                                         std::is_convertible<invoke_result_t<F, Args...>, R>::value> {};
+    /*!
+        ensures
+            - identical to std::is_invocable_r<R, F, Args..>
+            - works with C++11 onwards
+    !*/
+
+    // ----------------------------------------------------------------------------------------
+
+    template< typename R, typename F, typename... Args>
+    constexpr typename std::enable_if<dlib::is_invocable_r<R, F, Args...>::value, R>::type
+    invoke_r(F && f, Args &&... args)
+    /*!
+        ensures
+            - identical to std::invoke_r<R>(std::forward<F>(f), std::forward<Args>(args)...)
+            - works with C++11 onwards
+    !*/
+    noexcept(noexcept(dlib::invoke(std::forward<F>( f ), std::forward<Args>( args )...)))
+    {
+        return dlib::invoke(std::forward<F>( f ), std::forward<Args>( args )...);
+    }
+
+    // ----------------------------------------------------------------------------------------
+
     namespace detail
     {
         template<typename F, typename Tuple, std::size_t... I>
-        auto apply_impl(F&& fn, Tuple&& tpl, index_sequence<I...>)
+        constexpr auto apply_impl(F&& fn, Tuple&& tpl, index_sequence<I...>)
+        noexcept(noexcept(dlib::invoke(std::forward<F>(fn),
+                                       std::get<I>(std::forward<Tuple>(tpl))...)))
         -> decltype(dlib::invoke(std::forward<F>(fn),
                                  std::get<I>(std::forward<Tuple>(tpl))...))
         {
@@ -101,12 +250,15 @@ namespace dlib
     }
 
     template<typename F, typename Tuple>
-    auto apply(F&& fn, Tuple&& tpl)
+    constexpr auto apply(F&& fn, Tuple&& tpl)
     /*!
         ensures
             - identical to std::apply(std::forward<F>(f), std::forward<Tuple>(tpl))
             - works with C++11 onwards
     !*/
+    noexcept(noexcept(detail::apply_impl(std::forward<F>(fn),
+                                         std::forward<Tuple>(tpl),
+                                         make_index_sequence<std::tuple_size<typename std::remove_reference<Tuple>::type >::value>{})))
     -> decltype(detail::apply_impl(std::forward<F>(fn),
                                    std::forward<Tuple>(tpl),
                                    make_index_sequence<std::tuple_size<typename std::remove_reference<Tuple>::type >::value>{}))
diff --git a/dlib/test/invoke.cpp b/dlib/test/invoke.cpp
index 4d555c5a7..a23b6dd15 100644
--- a/dlib/test/invoke.cpp
+++ b/dlib/test/invoke.cpp
@@ -234,6 +234,100 @@ namespace
 
     // ----------------------------------------------------------------------------------------
 
+    const char* func_return_c_string()
+    {
+        return "hello darkness my old friend";
+    }
+
+    struct obj_return_c_string
+    {
+        obj_return_c_string() = default;
+        obj_return_c_string(const obj_return_c_string& rhs) = delete;
+        obj_return_c_string(obj_return_c_string&& rhs)      = delete;
+
+        const char* run()
+        {
+            return "i've come to talk with you again";
+        }
+    };
+
+    void test_invoke_r()
+    {
+        {
+            static_assert(dlib::is_invocable_r<std::string, decltype(func_return_c_string)>::value, "should be invocable");
+            auto str = dlib::invoke_r<std::string>(func_return_c_string);
+            static_assert(std::is_same<decltype(str), std::string>::value, "bad return type");
+            DLIB_TEST(str == "hello darkness my old friend");
+        }
+
+        {
+            obj_return_c_string obj;
+            static_assert(dlib::is_invocable_r<std::string, decltype(&obj_return_c_string::run), decltype(obj)>::value, "should be invocable");
+            auto str = dlib::invoke_r<std::string>(&obj_return_c_string::run, obj);
+            static_assert(std::is_same<decltype(str), std::string>::value, "bad return type");
+            DLIB_TEST(str == "i've come to talk with you again");
+        }
+
+        {
+            obj_return_c_string obj;
+            static_assert(dlib::is_invocable_r<std::string, decltype(&obj_return_c_string::run), decltype(&obj)>::value, "should be invocable");
+            auto str = dlib::invoke_r<std::string>(&obj_return_c_string::run, &obj);
+            static_assert(std::is_same<decltype(str), std::string>::value, "bad return type");
+            DLIB_TEST(str == "i've come to talk with you again");
+        }
+
+        {
+            auto obj = std::make_shared<obj_return_c_string>();
+            static_assert(dlib::is_invocable_r<std::string, decltype(&obj_return_c_string::run), decltype(obj)>::value, "should be invocable");
+            auto str = dlib::invoke_r<std::string>(&obj_return_c_string::run, obj);
+            static_assert(std::is_same<decltype(str), std::string>::value, "bad return type");
+            DLIB_TEST(str == "i've come to talk with you again");
+        }
+
+        {
+            std::unique_ptr<obj_return_c_string> obj(new obj_return_c_string());
+            static_assert(dlib::is_invocable_r<std::string, decltype(&obj_return_c_string::run), decltype(obj)>::value, "should be invocable");
+            auto str = dlib::invoke_r<std::string>(&obj_return_c_string::run, obj);
+            static_assert(std::is_same<decltype(str), std::string>::value, "bad return type");
+            DLIB_TEST(str == "i've come to talk with you again");
+        }
+
+        {
+            auto lambda_return_c_string = [] {
+                return "because a vision softly creeping";
+            };
+            static_assert(dlib::is_invocable_r<std::string, decltype(lambda_return_c_string)>::value, "should be invocable");
+            auto str = dlib::invoke_r<std::string>(lambda_return_c_string);
+            static_assert(std::is_same<decltype(str), std::string>::value, "bad return type");
+            DLIB_TEST(str == "because a vision softly creeping");
+        }
+    }
+
+    // ----------------------------------------------------------------------------------------
+
+    constexpr int multiply_ints(int i, int j)
+    {
+        return i*j;
+    }
+
+    struct constexpr_object
+    {
+        constexpr int multiply_ints(int i, int j) const
+        {
+            return i*j;
+        }
+    };
+
+    void test_constexpr()
+    {
+        static_assert(dlib::invoke(multiply_ints, 2, 5) == 10, "this should be constexpr");
+        static_assert(dlib::invoke_r<long>(multiply_ints, 2, 5) == 10, "this should be constexpr");
+        constexpr constexpr_object constexpr_obj;
+        static_assert(dlib::invoke(&constexpr_object::multiply_ints, constexpr_obj, 2, 5) == 10);
+        static_assert(dlib::invoke_r<long>(&constexpr_object::multiply_ints, constexpr_obj, 2, 5) == 10);
+    }
+
+    // ----------------------------------------------------------------------------------------
 
     class invoke_tester : public tester
     {
@@ -251,6 +345,8 @@ namespace
             test_member_functions_and_data();
             test_return_types();
             test_make_from_tuple();
+            test_invoke_r();
+            test_constexpr();
         }
     } a;
 }