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POFunction pointers in Objective C
 

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    <p>I came across the following code while debugging. I was able to get the result correctly but I didn't not understand the function pointers defined in here.</p> <p>Here <code>pNewMsgeFunc</code> is the alias name of the function pointer <code>tNewMsg</code> which is created inside the structure <code>stRsStruct</code>.</p> <p>RsMsg.h</p> <pre><code>typedef RsMsg* (*tNewMsg)(void); tNewMsg pNewMsgFunc; typedef struct { int nMsgId; NSString* sAsciiName; tNewMsg pNewMsgFunc; // calls new for the particular message } stRsStruct; </code></pre> <p>RsMsg.cpp</p> <pre><code>RsMsg(int uMessageId,tNewMsg pNewMsg,const char* szAsciiName,void* pData, size_t uDataSize) { //intialisations } </code></pre> <p>RsMsgDerived.h</p> <pre><code>#define DECLARE_NEWMSG(CssName,CssID) \ static CssName* FromMsg(RsMsg* pMsg) \ { \ return dynamic_cast&lt;CssName*&gt;(pMsg); \ } \ static RsMsg* NewMsg() \ { \ return new CssName; \ } \ enum {ID = CssID}; \ </code></pre> <p>Its said that in the structure pNewMsgFunc will point to the NewMsg() function.</p> <p>But I could not get it how its possible without intialising tNewMsg with the address of </p> <p>the NewMsg() function.But this code is running fine.There is no other constructors used to </p> <p>intialise the function pointer with the address of the function NewMsg().</p> <p>Event.cpp</p> <pre><code>#import "RsMsg.h" #import "RsMsgDerived.h" RsMsg* RsMsg::CreateMessage(REMOTE_MESSAGE_ID nMessageNumber) { RsMsg* pMsg = NULL; stRsStruct* pMsgStruct; pMsg = pMsgStruct-&gt;pNewMsgFunc(); //Invoking the function pointer } </code></pre> <p>Here by invoking the function pointer I'm calling the static <code>NewMsg()</code> function.</p> <p>But how this function gets called as <code>pNewMsgFunc()</code> is not assigned the address of <code>NewMsg</code>.</p> <p>I need to call the <code>NewMsg()</code> function through <code>pNewMsgFunc()</code>. Is there any change to be made in the above code?</p> <p>EDITED:</p> <p>How to implement the same code in Objective C. This function pointer calls a function whose return type is a class.So though the function pointers can be implemented in C as its calling a function whose return type is a class cannot be implemented in Objective C as c function.</p>
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    1. COWhat are those `.m` files?
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    2. COMost likely objective-c module files.
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      1. USMario
    3. COCould you clean up some things? You don't tell us what platform (Objective-C++?), and that may be significant in telling why things appear to behave. The code samples you list look odd: where do you use `DECLARE_NEWMSG`? Also, the title means almost nothing.
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      1. USDavid Thornley
 

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