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POC run time type checking
 

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  1. POC run time type checking
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    <p>I have a C library storing records with a number of fields. Schema are read in from a text file, including the type of each field in the record.</p> <p>To simplify for question purposes, imagine I have</p> <pre><code>typedef enum my_type_enum { INT32, //32-bit integer MYSTRUCT, //some struct I have, details irrelevant ... } my_type_enum; typedef struct my_var { my_type_enum typetag; unsigned char* data; } my_var; my_var myrecord[numfields]; </code></pre> <p>The schema file says whether each field of myrecord should hold an int32_t or a mystruct. My library reads the schema file and for each my_var in myrecord sets the tag and allocates the right amount space for the data.</p> <p>my_var is opaque and client programs basically use, for simple data</p> <pre><code>void set(my_var* record, size_t field, void * src) { memcpy(record[field].data, src, datatypes[record[field].typetag].size); } int32_t x = 5; set(myrecord, 0, &amp;x); </code></pre> <p>to store a value in a record, and a similar get() to take things out.</p> <p>The tagged my_var type allows type checking once data is inside the my_var, but if the schema says record holds three INT32s there is of course nothing to check that src points to an int32_t and not a mystruct when you're trying to set() data into that my_var.</p> <p>Obviously the check needs to take place in something wrapping set(), before the int32_t* or mystruct* is converted to void*. I have seen compile-time checking with typeof() trickery. I feel like what I want probably isn't possible, but you never know all the tricks... </p> <p>Is there anything I can do better than providing facilities that read a schema at client program compilation time and generate a set_CHECKED() wrapper macro that will give a compiler error if someone tries to copy an int32_t into a my_var tagged to hold mystruct? GCC extensions are fine.</p>
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