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    <p>Say we had this class</p> <pre><code>struct BigArray{ int operator[](size_t i)const{return m_data[i];} int&amp; operator[](size_t i){return m_data[i];} private: int m_data[10000000]; }; </code></pre> <p>And now say we had two instances:</p> <pre><code>BigArray a; a[0]=1;//initializaation etc BigArray b=a; </code></pre> <p>At this point we want this invariant</p> <pre><code>assert(a[0]==b[0]); </code></pre> <p>The default copy ctor ensures this invariant, however at the expense of deep copying the entire object. We may attempt a speedup like this</p> <pre><code>struct BigArray{ BigArray():m_data(new int[10000000]){} int operator[](size_t i)const{return (*m_data)[i];} int&amp; operator[](size_t i){return (*m_data)[i];} private: shared_ptr&lt;int&gt; m_data; }; </code></pre> <p>This will also meet the invariant, without making the deep copy, so all is good so far. Now using this new implementation we did </p> <pre><code>b[0]=2; </code></pre> <p>Now we want this to work the same as the deep copy case assert(a[0]!=b[0]); But it fails. To solve this we need a slight change:</p> <pre><code>struct BigArray{ BigArray():m_data(new int[10000000]){} int operator[](size_t i)const{return (*m_data)[i];} int&amp; operator[](size_t i){ if(!m_data.unique()){//"detach" shared_ptr&lt;int&gt; _tmp(new int[10000000]); memcpy(_tmp.get(),m_data.get(),10000000); m_data=_tmp; } return (*m_data)[i]; } private: shared_ptr&lt;int&gt; m_data; }; </code></pre> <p>Now we have a class that is shallow copied when only const access is needed, and deep copied when non-const access is needed. This is the idea behind the "shared_data" pointer concept. <strong>const</strong> calls will not deep copy (they call it "detach"), while non-const will deep copy if it is shared. It also adds some semantics on top of operator== so that it is not just comparing the pointer but the data as well, so that this would work:</p> <pre><code>BigArray b=a;//shallow copy assert(a==b);//true b[0]=a[0]+1;//deep copy b[0]=a[0];//put it back assert(a==b);//true </code></pre> <p>This technique is call COW (Copy on Write) and has been around since the dawn of C++. It is also extremely brittle -- the above example seems to work because it is small and has few use cases. In practice it is rarely worth the trouble, and in fact C++0x has deprecated COW strings. So use with caution. </p>
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    1. POC++ smart pointers: sharing pointers vs. sharing data
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    1. USLance Diduck
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    1. COSo you're implying that what he means by sharing the data is COW, and by sharing the pointer is the simple shared_ptr / reference counted pointer idioms?
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      1. USEli Bendersky
    2. COyes. And if you read the QT docs is says as much. http://doc.trolltech.com/4.5/qshareddatapointer.html grep "copy on write"
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