ScopeGuard: Add new heavyweight ErasedScopeGuard type

modules/juce_core/misc/juce_Functional.h

@@ -86,35 +86,6 @@ template <typename Object, typename OtherObject, typename Member, typename Other
     return copy;
 }
 
-/** An easy way to ensure that a function is called at the end of the current
-    scope.
-
-    Usage:
-    @code
-    {
-        if (flag == true)
-            return;
-
-        // While this code executes, flag is true e.g. to prevent reentrancy
-        flag = true;
-        // When we exit this scope, flag must be false
-        const ScopeGuard scope { [&] { flag = false; } };
-
-        if (checkInitialCondition())
-            return; // Scope's lambda will fire here...
-
-        if (checkCriticalCondition())
-            throw std::runtime_error{}; // ...or here...
-
-        doWorkHavingEstablishedPreconditions();
-    } // ...or here!
-    @endcode
-
-    @tags{Core}
-*/
-template <typename Fn> struct ScopeGuard : Fn { ~ScopeGuard() { Fn::operator()(); } };
-template <typename Fn> ScopeGuard (Fn) -> ScopeGuard<Fn>;
-
 #ifndef DOXYGEN
 namespace detail
 {

modules/juce_core/misc/juce_ScopeGuard.cpp

@@ -0,0 +1,55 @@
+/*
+  ==============================================================================
+
+   This file is part of the JUCE library.
+   Copyright (c) 2022 - Raw Material Software Limited
+
+   JUCE is an open source library subject to commercial or open-source
+   licensing.
+
+   The code included in this file is provided under the terms of the ISC license
+   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
+   To use, copy, modify, and/or distribute this software for any purpose with or
+   without fee is hereby granted provided that the above copyright notice and
+   this permission notice appear in all copies.
+
+   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+namespace juce
+{
+
+ErasedScopeGuard::ErasedScopeGuard (std::function<void()> d)
+    : detach (std::move (d)) {}
+
+ErasedScopeGuard::ErasedScopeGuard (ErasedScopeGuard&& other) noexcept
+    : detach (std::exchange (other.detach, nullptr)) {}
+
+ErasedScopeGuard& ErasedScopeGuard::operator= (ErasedScopeGuard&& other) noexcept
+{
+    ErasedScopeGuard token { std::move (other) };
+    std::swap (token.detach, detach);
+    return *this;
+}
+
+ErasedScopeGuard::~ErasedScopeGuard() noexcept
+{
+    reset();
+}
+
+void ErasedScopeGuard::reset()
+{
+    if (auto d = std::exchange (detach, nullptr))
+        d();
+}
+
+void ErasedScopeGuard::release()
+{
+    detach = nullptr;
+}
+
+} // namespace juce

modules/juce_core/misc/juce_ScopeGuard.h

@@ -0,0 +1,113 @@
+/*
+  ==============================================================================
+
+   This file is part of the JUCE library.
+   Copyright (c) 2022 - Raw Material Software Limited
+
+   JUCE is an open source library subject to commercial or open-source
+   licensing.
+
+   The code included in this file is provided under the terms of the ISC license
+   http://www.isc.org/downloads/software-support-policy/isc-license. Permission
+   To use, copy, modify, and/or distribute this software for any purpose with or
+   without fee is hereby granted provided that the above copyright notice and
+   this permission notice appear in all copies.
+
+   JUCE IS PROVIDED "AS IS" WITHOUT ANY WARRANTY, AND ALL WARRANTIES, WHETHER
+   EXPRESSED OR IMPLIED, INCLUDING MERCHANTABILITY AND FITNESS FOR PURPOSE, ARE
+   DISCLAIMED.
+
+  ==============================================================================
+*/
+
+namespace juce
+{
+
+/** An easy way to ensure that a function is called at the end of the current
+    scope.
+
+    Usage:
+    @code
+    {
+        if (flag == true)
+            return;
+
+        // While this code executes, flag is true e.g. to prevent reentrancy
+        flag = true;
+        // When we exit this scope, flag must be false
+        const ScopeGuard scope { [&] { flag = false; } };
+
+        if (checkInitialCondition())
+            return; // Scope's lambda will fire here...
+
+        if (checkCriticalCondition())
+            throw std::runtime_error{}; // ...or here...
+
+        doWorkHavingEstablishedPreconditions();
+    } // ...or here!
+    @endcode
+
+    @tags{Core}
+*/
+template <typename Fn> struct ScopeGuard : Fn { ~ScopeGuard() { Fn::operator()(); } };
+template <typename Fn> ScopeGuard (Fn) -> ScopeGuard<Fn>;
+
+/**
+    A ScopeGuard that uses a std::function internally to allow type erasure.
+    This can be handy; it allows lots of ErasedScopeGuards, all with different
+    callbacks, to be stored in a homogeneous container.
+
+    An instance of this type will automatically call its callback when it is destroyed.
+
+    ErasedScopeGuard has a few similarities with std::unique_ptr:
+    - Calling reset() on a unique_ptr destroys the object if it hasn't been destroyed yet
+      and puts the unique_ptr back into a default/null state; calling reset() on an
+      ErasedScopeGuard calls the callback if it hasn't been called yet and puts the Guard
+      back into a default/null state.
+    - Calling release() on a unique_ptr returns the unique_ptr back to a default state
+      without destroying the managed object; calling release() on an ErasedScopeGuard
+      returns the Guard back to a default state without calling the callback.
+    - Moving a unique_ptr transfers the responsibility of destroying the managed object
+      to another unique_ptr instance; moving an ErasedScopeGuard transfers the
+      responsibility of calling the callback to another Guard instance.
+*/
+class [[nodiscard]] ErasedScopeGuard
+{
+public:
+    /** Constructs an ErasedScopeGuard with no callback. */
+    ErasedScopeGuard() = default;
+
+    /** Constructs an ErasedScopeGuard that will call the provided callback
+        when the Guard is destroyed.
+    */
+    explicit ErasedScopeGuard (std::function<void()> d);
+
+    /** Constructs an instance that assumes responsibility for calling other's callback. */
+    ErasedScopeGuard (ErasedScopeGuard&& other) noexcept;
+
+    /** Calls the stored callback, if any, then assumes responsibility for calling
+        other's callback. After this call, other will be reset to its default state.
+    */
+    ErasedScopeGuard& operator= (ErasedScopeGuard&& other) noexcept;
+
+    /** Destructor, calls the callback assigned to this ScopeGuard.
+    */
+    ~ErasedScopeGuard() noexcept;
+
+    /** Calls the stored callback, if any, then resets this instance to its
+        default state.
+    */
+    void reset();
+
+    /** Resets this instance to its default state without calling the stored
+        callback.
+    */
+    void release();
+
+    JUCE_DECLARE_NON_COPYABLE (ErasedScopeGuard)
+
+private:
+    std::function<void()> detach;
+};
+
+} // namespace juce