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POOpaque Data Type in C without opaque pointer
 

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    <p>I'd like to create an opaque type while still allowing users of the type to instantiate it e.g. on the stack by typing</p> <pre><code>struct Foo obj; /*...*/ Foo_init(&amp;obj); Foo_do_something(&amp;obj, param); /*...*/ </code></pre> <p>The customary opaque pointer approach doesn't allow the first line of this example. As a workaround I am placing a public but opaque "data" array inside the public header file with a fixed size. This seems to work for a few examples but I'm a bit unsure about two points:</p> <ol> <li><p>Is it safe to cast the address of the data array as is done in Foo_get_bar and Foo_set_bar? This works okay in my tests but it looks questionable.</p></li> <li><p>If FOO_DATA_SIZE remains fixed is it reasonable to expect ABI compatibility in user code?</p></li> </ol> <hr> <h2>main.c (Example user code)</h2> <pre><code>#include &lt;stdio.h&gt; #include &lt;limits.h&gt; #include "foo.h" int main() { struct Foo foo; Foo_init(&amp;foo); Foo_set_bar(&amp;foo, INT_MAX); int bar = Foo_get_bar(&amp;foo); printf("Got bar: %d\n", bar); } </code></pre> <hr> <h2>foo.h (Public header)</h2> <pre><code>#pragma once #include &lt;inttypes.h&gt; #define FOO_DATA_SIZE (64) struct Foo { uint8_t data[FOO_DATA_SIZE]; }; void Foo_init(struct Foo *f); void Foo_set_bar(struct Foo *f, int barval); int Foo_get_bar(struct Foo *f); </code></pre> <hr> <h2>foo.c (Implementation)</h2> <pre><code>#include "foo.h" #include &lt;stdio.h&gt; #include &lt;string.h&gt; #include &lt;inttypes.h&gt; typedef int64_t bar_t; struct Foo_private { bar_t bar; }; _Static_assert(sizeof(struct Foo_private) &lt;= FOO_DATA_SIZE, "FOO_DATA_SIZE is insufficient for struct Foo_private"); void Foo_init(struct Foo *foo) { struct Foo_private foodata; foodata.bar = (bar_t)0; memcpy(foo-&gt;data, &amp;foodata, sizeof(struct Foo_private)); } void Foo_set_bar(struct Foo *foo, int barval) { struct Foo_private *foodata = (void*)&amp;(foo-&gt;data); foodata-&gt;bar = (bar_t)barval; int stored = (int)foodata-&gt;bar; if (stored != barval) { fprintf(stderr, "Foo_set_bar(%"PRId64"): warning: bar rounded to %"PRId64"\n", (int64_t)barval, (int64_t)stored); } } int Foo_get_bar(struct Foo *foo) { struct Foo_private *foodata = (void*)&amp;(foo-&gt;data); bar_t bar = foodata-&gt;bar; return (int)bar; } </code></pre>
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    1. USBrandin
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    1. COThis is nonsensical. By definition, if it's opaque the compiler doesn't know how big it is, so it can't allocate it.
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      1. USKevin
    2. COThis was asked literally a few days ago. And no, it's not safe because of the different alignment requirements of `int8_t` and `int64_t`. Also, you got the size wrong - it should be `8` instead of `64`.
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    3. CO@Kevin I think you misunderstood the question. That's not what OP is asking about, read the first sentence of the second paragraph.
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