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POReturning a void pointer to a constant object in C
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  1. POReturning a void pointer to a constant object in C
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    copied!<p>I'm writing an access function which returns a pointer to an internal buffer and I'd like to hint to users of my function that they shouldn't update the object that's pointed to. A very contrived example would be:</p> <pre><code>void myAccessFunc(bool string, void* p, size_t* len) { static const char* s = "aha!"; static const int i = 123; if (string) { *(char**)p = &amp;s; *len = strlen(s); } else { *(int**)p = &amp;i; *len = sizeof(i); } } char* s; size_t bytes; myAccessFunc(true,&amp;s, &amp;bytes); printf("Got '%s'\n", s); </code></pre> <p>Yes, I know that looks flakey.</p> <p>What I want to prevent is:</p> <pre><code>char* s; size_t bytes; myAccessFunc(true,&amp;s,&amp;bytes); s[4] = '?'; </code></pre> <p>I know I can't completely prevent it but I'd at least like the compiler warning to hint to a user that they shouldn't be doing that. If they cast my pointer, then that's their problem. Is there some combination of const and void and * that will do this? I tried something like:</p> <pre><code>void myAccessFunc(bool string, const void** p, size_t* len); </code></pre> <p>but it seemed do away with the voidness of the pointer so a caller had to do:</p> <pre><code>const void* p; size_t bytes; myAccessFunc(true, &amp;p, &amp;bytes); </code></pre> <p>or</p> <pre><code>const char* s; size_t bytes; myAccessFunc(true, (const void**)&amp;s, &amp;bytes); </code></pre> <p>and couldn't do:</p> <pre><code>const int * ip; const char* s; size_t bytes; myAccessFunc(true, &amp;s, &amp;bytes); myAccessFunc(false, &amp;i, &amp;bytes); </code></pre> <p>I finally came around to:</p> <pre><code>const void* myAccessFunc(bool string, size_t* len); </code></pre> <p>and if the user does:</p> <pre><code>char* p = myAcccessFunc(true,&amp;bytes); </code></pre> <p>the compiler (GCC, at least), does complain about throwing away the qualifier.</p>
 

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