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POConfused when I should and shouldn't use "const" in C
 

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  1. POConfused when I should and shouldn't use "const" in C
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    <p>I have a dictionary that goes like this:</p> <pre><code>typedef struct dictNode { int key; char *value; struct dictNode *next; } Dict; </code></pre> <p>And a <strong>get()</strong> function that goes like this:</p> <pre><code>char *get(const Dict *dict, int key) { if(!dict) return NULL; Dict *currPtr = dict; while(currPtr) { if(currPtr-&gt;key == key) { return currPtr-&gt;value; } currPtr = currPtr-&gt;next; } } </code></pre> <p>Compiling this code produces the following error:<br> <strong>dict.c:85: warning: initialization discards qualifiers from pointer target type</strong></p> <p>This warning refers to the line:</p> <pre><code>Dict *currPtr = dict; </code></pre> <p>If I add a "const" before that line like this:</p> <pre><code>const Dict *currPtr = dict; </code></pre> <p>The warning goes away...</p> <p><strong>1)</strong> First thing I don't understand is this: I added "const" to the <strong>dict argument</strong> in <strong>get()</strong> so that the compiler warns me if I try to change the address dict is pointing to, otherwise I won't be able to access the dictionary in the main program cause I lost the address I was pointing too. Now, I'm creating a new pointer, <strong>currPtr</strong>, that points to the same place as <strong>dict</strong>. This way, I use this pointer instead, to traverse the dictionary and keep <strong>dict</strong> pointer intact. Why do I also need to have <strong>const</strong> for <strong>currPtr</strong>?</p> <p><strong>2)</strong> Second thing I don't understand is this: The line <strong>currPtr = currPtr->next;</strong> is changing the <strong>currPtr</strong> pointer, so, why doesn't the compile warn me about that if I added a <strong>const</strong> to <strong>currPtr</strong>?</p> <p>Then I have a <strong>del()</strong> function that goes like this:</p> <pre><code>Dict *del(const Dict *dict, int key) { if(!dict) return NULL; Dict *currPtr = dict; Dict *prevPtr = dict; while(currPtr) { if(currPtr-&gt;key == key) { prevPtr-&gt;next = currPtr-&gt;next; free(currPtr); } prevPtr = currPtr; currPtr = currPtr-&gt;next; } return dict; } </code></pre> <p><em>Please note that I'm aware that this delete function code is not complete, it does not correctly work if I want to delete the first element for instance. It doesn't matter, I'll finish later, it suffices to demonstrate my problem...</em></p> <p><strong>3)</strong> So, in the <strong>get()</strong> function I needed to add <strong>const</strong> to <strong>currPtr</strong> but in this <strong>del()</strong> function, I don't need to add <strong>const</strong> to <strong>currPtr</strong> nor <strong>prevPtr</strong>? In both functions I am changing the <strong>currPtr</strong> pointer and in the case of the <strong>del()</strong> function I'm changing the <strong>prevPtr</strong> pointer too. Why does the <strong>get()</strong> function require me to add a <strong>const</strong> before <strong>currPtr</strong> and the <strong>del()</strong> function does not require me to add a <strong>const</strong> before <strong>currPtr</strong> and <strong>prevPtr</strong>?</p> <p>I can basically resume this whole post to: When and where exactly in my <strong>get()</strong> and <strong>del()</strong> functions should I use <strong>const</strong> and why, and, when and where I shouldn't?</p>
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