Add WeakPtr/Weakable templates.

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/3e9a45d7f47

AK/Retainable.h

@@ -7,8 +7,6 @@ namespace AK {
 template<typename T>
 class Retainable {
 public:
-    Retainable() { }
-
     void retain()
     {
         ASSERT(m_retainCount);
@@ -28,6 +26,7 @@ public:
     }
 
 protected:
+    Retainable() { }
     ~Retainable()
     {
         ASSERT(!m_retainCount);

AK/WeakPtr.h

@@ -0,0 +1,37 @@
+#pragma once
+
+#include "Weakable.h"
+
+namespace AK {
+
+template<typename T>
+class WeakPtr {
+    friend class Weakable<T>;
+public:
+    WeakPtr() { }
+    WeakPtr(std::nullptr_t) { }
+
+    operator bool() const { return ptr(); }
+
+    T* ptr() { return m_link ? m_link->ptr() : nullptr; }
+    const T* ptr() const { return m_link ? m_link->ptr() : nullptr; }
+    bool isNull() const { return !m_link || !m_link->ptr(); }
+
+private:
+    WeakPtr(RetainPtr<WeakLink<T>>&& link) : m_link(std::move(link)) { }
+
+    RetainPtr<WeakLink<T>> m_link;
+};
+
+template<typename T>
+inline WeakPtr<T> Weakable<T>::makeWeakPtr()
+{
+    if (!m_link)
+        m_link = adopt(*new WeakLink<T>(*this));
+    return WeakPtr<T>(m_link.copyRef());
+}
+
+}
+
+using AK::WeakPtr;
+

AK/Weakable.h

@@ -0,0 +1,45 @@
+#pragma once
+
+#include "Assertions.h"
+#include "Retainable.h"
+
+namespace AK {
+
+template<typename T> class Weakable;
+template<typename T> class WeakPtr;
+
+template<typename T>
+class WeakLink : public Retainable<WeakLink<T>> {
+    friend class Weakable<T>;
+public:
+    T* ptr() { return static_cast<T*>(m_ptr); }
+    const T* ptr() const { return static_cast<const T*>(m_ptr); }
+
+private:
+    explicit WeakLink(Weakable<T>& weakable) : m_ptr(&weakable) { }
+    Weakable<T>* m_ptr;
+};
+
+template<typename T>
+class Weakable {
+private:
+    class Link;
+public:
+    WeakPtr<T> makeWeakPtr();
+
+protected:
+    Weakable() { }
+
+    ~Weakable()
+    {
+        if (m_link)
+            m_link->m_ptr = nullptr;
+    }
+
+private:
+    RetainPtr<WeakLink<T>> m_link;
+};
+
+}
+
+using AK::Weakable;

AK/test.cpp

@@ -7,6 +7,10 @@
 #include "HashMap.h"
 #include "TemporaryFile.h"
 #include "Buffer.h"
+#include "Weakable.h"
+#include "WeakPtr.h"
+
+static void testWeakPtr();
 
 int main(int, char**)
 {
@@ -183,5 +187,27 @@ int main(int, char**)
         printInts(h);
     }
 
+    testWeakPtr();
+
     return 0;
 }
+
+class TestWeakable : public Weakable<TestWeakable> {
+public:
+    TestWeakable() { }
+    ~TestWeakable() { }
+};
+
+void testWeakPtr()
+{
+    auto* weakable = new TestWeakable;
+
+    auto weakPtr = weakable->makeWeakPtr();
+    ASSERT(weakPtr);
+    ASSERT(weakPtr.ptr() == weakable);
+
+    delete weakable;
+
+    ASSERT(!weakPtr);
+    ASSERT(weakPtr.ptr() == nullptr);
+}