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    <p>Let me repeat your question: "My question, how cyclic data structures makes reference count above zero, kindly request to show with example in C++ program. How the problem is solved by <code>weak_ptrs</code> again with example please."</p> <p>The problem occurs with C++ code like this (conceptually):</p> <pre><code>class A { shared_ptr&lt;B&gt; b; ... }; class B { shared_ptr&lt;A&gt; a; ... }; shared_ptr&lt;A&gt; x(new A); // +1 x-&gt;b = new B; // +1 x-&gt;b-&gt;a = x; // +1 // Ref count of 'x' is 2. // Ref count of 'x-&gt;b' is 1. // When 'x' leaves the scope, there will be a memory leak: // 2 is decremented to 1, and so both ref counts will be 1. // (Memory is deallocated only when ref count drops to 0) </code></pre> <p>To answer the second part of your question: It is mathematically impossible for reference counting to deal with cycles. Therefore, a <code>weak_ptr</code> (which is basically just a stripped down version of <code>shared_ptr</code>) <strong>cannot</strong> be used to solve the cycle problem - the programmer is solving the cycle problem.</p> <p>To solve it, the programmer needs to be aware of the <strong>ownership</strong> relationship among the objects, or needs to invent an ownership relationship if no such ownership exists naturally.</p> <p>The above C++ code can be changed so that A owns B:</p> <pre><code>class A { shared_ptr&lt;B&gt; b; ... }; class B { weak_ptr&lt;A&gt; a; ... }; shared_ptr&lt;A&gt; x(new A); // +1 x-&gt;b = new B; // +1 x-&gt;b-&gt;a = x; // No +1 here // Ref count of 'x' is 1. // Ref count of 'x-&gt;b' is 1. // When 'x' leaves the scope, its ref count will drop to 0. // While destroying it, ref count of 'x-&gt;b' will drop to 0. // So both A and B will be deallocated. </code></pre> <p>A crucial question is: Can <code>weak_ptr</code> be used in case the programmer cannot tell the ownership relationship and cannot establish any static ownership because of lack of privilege or lack of information?</p> <p>The answer is: If ownership among objects is unclear, <code>weak_ptr</code> <strong>cannot</strong> help. If there is a cycle, the programmer has to find it and break it. An alternative remedy is to use a programming language with full garbage collection (such as: Java, C#, Go, Haskell), or to use a conservative (=imperfect) garbage collector which works with C/C++ (such as: Boehm GC).</p>
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    1. POshared_ptr and weak_ptr differences
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    1. COBut all uses of `B::a` must now be prepared the weak reference to be dead. If this must not be the case, then it means `weak_ptr` is not an adequat tool.
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    2. COIf B::a is a weak_ptr, then nothing should depend on it's existence - as it does not own a. A::b is the dependable guy here.
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    3. COgood examples !
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