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POHow to disable integer assignment (0) to an opaque variable?
 

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  1. POHow to disable integer assignment (0) to an opaque variable?
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    <p>I have a module whose implementation I want to hide from its clients.</p> <p>I chose to declare an opaque type which is actually a pointer to structure to be defined only in the implementation.</p> <p>It all is working fine, except that I can assign the zero value to a variable of this type, which I want to avoid.</p> <p>Here's an example in C.</p> <p>header file foo.h</p> <pre><code>/* foo.h */ typedef struct foo *foo_t; /* &lt;- sorry this was obviously flawed, the '*' was missing */ extern void foo_create( foo_t *t ); extern void foo_destroy( foo_t *t ); extern void foo_tile( foo_t x ); </code></pre> <p>implementation file foo.c</p> <pre><code>/* foo.c */ #include &lt;stdlib.h&gt; #include "foo.h" struct foo { int some_member; }; void foo_create( foo_t *t ) { if ( *t==0 ) { *t = malloc( sizeof(struct foo) ); } } void foo_destroy( foo_t *t ) { if ( *t!=0 ) { free(*t); *t = 0; } } void foo_tile( foo_t t ) { t-&gt;some_member++; } </code></pre> <p>And now here is an example client that uses the module: bar.c:</p> <pre><code>#include "foo.h" int main( int argc , char **argv ) { foo_t toe; foo_create( &amp;toe ); toe = 0; /* &lt;-- How to make the compiler (gcc) refuse this? */ toe = 1; /* &lt;--- the compiler rejects this YAY!! */ } </code></pre> <p>The opaque type is actually a pointer to a dynamically allocated structure; If I assign the value 0 to it I incur a memory leak which could be avoided If the compiler denied the assignment of 0 to this opaque pointer.</p> <p>Assigning non-null values to the pointer isn't accepted by the compiler, so I supose that with a little more effort the same could be achieved for zero value.</p> <p>Is it possible to disable this assignment? How can I achieve that? If using some bit of C++ or gcc specific constructs is needed, I'll be ok with that, although a pure C solution would be nice.</p> <p>Thanks in advance.</p>
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      1. POHow to disable integer assignment (0) to an opaque variable?
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    1. COC++ or C? They're not the same, you probably should pick just one.
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    2. COAlso 0 is the only integer that can be implicitly converted to any pointer type. You can't stop that if `foo_t` really is a pointer. It's like trying to stop `5` from being assigned to an `int`.
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    3. COthe huge difference between 0 and 1 is that 0 in this case is 'the null pointer' which is a perfectly valid value for any pointer. Also, since this is tagged C++, why don't use new and delete instead of free and malloc? Then you can take advantage of constructors and destructors.
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