bson_t* bson_new_from_json(in char* data, long len, bson_error_t*
error);
string str_utf8 = "{\"a\":1}";
bson_error_t error;
auto bson = bson_new_from_json(str_utf8.ptr, str_utf8.length,
&error);
You have a wrong declaration for bson_error_t too.