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POoperators on Iterators in C++
 

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  1. POoperators on Iterators in C++
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    <p>I am porting an old code base to OSX.</p> <p>I have the following snippet of code:</p> <pre><code>FxLayerList::iterator lastVisible = NULL; for (FxLayerList::iterator iter = mBranch.begin(); iter != mBranch.end(); iter++) { if ( (*iter)-&gt;IsVisible() &amp;&amp; !(*iter)-&gt;IsBypass()) { lastVisible = iter; } } if (lastVisible != NULL &amp;&amp; (*lastVisible)-&gt;GetGeneratedImage()) { </code></pre> <p>I get an error that says: <code>error: no match for 'operator!=' in 'lastVisible != 0'</code></p> <p>I dont follow, I thought operations like != and ==, etc were standard operations. Why the complaint from the compiler?</p> <p>UPDATE: I am trying to understand the comparison of objects. What if the code is like this:</p> <pre><code>FxBool FxLayerList::Contains(FxLayer *layer) const { for (FxLayerList::const_iterator iter=this-&gt;begin(); iter != this-&gt;end(); iter++) { if ((*iter) == layer) { return true; } } return false; } </code></pre> <p>with errors like: <code>error: ISO C++ says that these are ambiguous, even though the worst conversion for the first is better than the worst conversion for the second:</code></p> <p>What is the core concept I am missing?</p> <p>Update 2: </p> <pre><code>// FxSmartPtr is a smart pointer that is also typed for each class, avoiding the need for any casting. // Setting an FxSmartPtr to NULL actually kills the memory that it's pointing to as well. template &lt;class eachClass&gt; class FxSmartPtr { public: // Construction // Default constructor makes an empty container. FxSmartPtr(void) : mPtr(NULL) {} // Construction with a ptr adds one reference to it. FxSmartPtr(eachClass *ptr) : mPtr(ptr) { this-&gt;Reference(); } // Copy construction means that both smart pointers end up with a reference to the object. FxSmartPtr(const FxSmartPtr &amp; inPtr) :mPtr(NULL) { FrAssignRef(mPtr,(eachClass *)inPtr.mPtr); } // Default construction FxSmartPtr(FxConstructArg cons) { if (cons == FcNew) mPtr = new eachClass(); } FxSmartPtr(FxConstructArg cons,eachClass *ptr) { if (cons == FcNew) mPtr = ptr; } // Destructor removes only the one reference that we own. ~FxSmartPtr() { this-&gt;Dispose(); } // Most important and common use is via assignment. References are always safely balanced. // AssignReference safely replaces one reference counted ptr with another. static inline eachClass * FrAssignRef(eachClass *&amp; to, eachClass * from) { if (from) from-&gt;AddReference(); if (to) to-&gt;RemoveReference(); to = from; return to; } // If you assign a pointer to this object we add one reference count to it. const FxSmartPtr&lt;eachClass&gt; &amp; operator = (const eachClass *ptr) { FrAssignRef(mPtr,(eachClass *)ptr); return *this; } // Replace our referenced object with a reference added to the incoming one. const FxSmartPtr&lt;eachClass&gt; &amp; operator = (const FxSmartPtr &amp; inPtr) { FrAssignRef(mPtr,(eachClass *)inPtr.mPtr); return *this; } // Assignment to a dumb pointer takes/gives no references. operator eachClass * (void) const { return mPtr; } eachClass * operator-&gt;(void) { if (mPtr != NULL) if (mPtr-&gt;GetRefCount() &lt; 1 || mPtr-&gt;GetRefCount() &gt; 10000) ASSERT(0); return mPtr; } const eachClass * operator-&gt;(void) const { if (mPtr != NULL) if (mPtr-&gt;GetRefCount() &lt; 1 || mPtr-&gt;GetRefCount() &gt; 10000) ASSERT(0); return mPtr; } // Explicit assignment and object transfers // Get() - return ptr with no reference eachClass * Get(void) const { return mPtr; } eachClass * GetPtr(void) { return mPtr; } // Own() - return ownership with ptr eachClass * Own(void) { if (mPtr) mPtr-&gt;AddReference(); return mPtr; } // Set() - we take our own reference on your object FxSmartPtr&lt;eachClass&gt; &amp; Set(eachClass * ptr) { FrAssignRef(mPtr, ptr); return *this; } // Take() - you give us your reference FxSmartPtr&lt;eachClass&gt; &amp; Take(eachClass * ptr) { FrDispose(mPtr); mPtr = ptr; return *this; } // Comparison operators compare the pointers contained in each FxBool operator == (const FxSmartPtr &amp; inPtr) const { return (mPtr == inPtr.mPtr); } FxBool operator == (const eachClass * inPtr) const { return (mPtr == inPtr); } FxBool operator != (const FxSmartPtr &amp; inPtr) const { return (mPtr != inPtr.mPtr); } FxBool operator != (const eachClass * inPtr) const { return (mPtr != inPtr); } // Reference() and Dispose() change the normal reference count. If you use these then // you end up having to count references externally. // Safely take a reference if the ptr is not nil void Reference(void) { if (mPtr != NULL) mPtr-&gt;AddReference(); } // Safely dispose one reference count. void Dispose(void) { if (mPtr != NULL) // JASON/INDIE - SLACKMOEHRLE@GMAIL.COM // { ULONG refs = mPtr-&gt;GetRefCount(); mPtr-&gt;RemoveReference(); if (refs &lt;= 1) mPtr = NULL; } } { FxUInt32 refs = mPtr-&gt;GetRefCount(); mPtr-&gt;RemoveReference(); if (refs &lt;= 1) mPtr = NULL; } } protected: eachClass *mPtr; }; </code></pre>
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    1. COHow ist FxLayerList defined?
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    2. COtry something like *lastVisible != NULL
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    3. COIf FxLayerList is a vector, the iterator might be a plain pointer in some implementations. The proper way is to initialize it and test against end().
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