ConfigurationElement(const char* key, const char* desc, xbt_cfg_cb_t cb)
: key(key ? key : ""), desc(desc ? desc : ""), old_callback(cb) {}
ConfigurationElement(const char* key, const char* desc, xbt_cfg_cb_t cb)
: key(key ? key : ""), desc(desc ? desc : ""), old_callback(cb) {}