av_opt: add av_opt_ptr() to return a pointer to a field of a object based on

an AVClass
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/opt.c

@@ -806,6 +806,14 @@ const AVClass *av_opt_child_class_next(const AVClass *parent, const AVClass *pre
     return NULL;
 }
 
+void *av_opt_ptr(const AVClass *class, void *obj, const char *name)
+{
+    AVOption *opt= av_opt_find2(&class, name, NULL, 0, AV_OPT_SEARCH_FAKE_OBJ, NULL);
+    if(!opt)
+        return NULL;
+    return (uint8_t*)obj + opt->offset;
+}
+
 #ifdef TEST
 
 #undef printf

libavutil/opt.h

@@ -586,6 +586,17 @@ int av_opt_get_double(void *obj, const char *name, int search_flags, double
 int av_opt_get_q     (void *obj, const char *name, int search_flags, AVRational *out_val);
 /**
  * @}
+ */
+/**
+ * Gets a pointer to the requested field in a struct.
+ * This function allows accessing a struct even when its fields are moved or
+ * renamed since the application making the access has been compiled,
+ *
+ * @returns a pointer to the field, it can be cast to the correct type and read
+ *          or written to.
+ */
+void *av_opt_ptr(const AVClass *class, void *obj, const char *name);
+/**
  * @}
  */