Wrapping ellipsis-based function with variadic template function

Question

I am trying to wrap a function 'prepared(.)' from an external library (libpqxx). Said function accepts a variable number of arguments. This is achieved by using multiple operator() in succession. One operator() pr. argument, like so:

pqxx::connection connection(connection_str);
connection.prepare("update_person", "UPDATE person SET name = $1 WHERE id = $2 AND version = $3");
pqxx::work w(connection);

// The following executes prepared statement.
// The number of arguments is variable. Notice
// the syntax with multiple () in succession...
w.prepared("update_person")("Jack")(1)(0).exec();

I am trying to wrap the last function by using a variadic template function, like so:

template<typename... T>
pqxx::result exec_prepared(const std::string& name, const T&... args) const {
    return w.prepared(name)(args...).exec();
}

...but it does not work. The code compiles, but I get a runtime error saying that the number of arguments do not match the expected number of arguments given the prepared-sql-statement.

Could someone please clarify how wrapping of this type of function is done using variadic template? Thanks!

Answer 1

If the pack args represents three parameters, then

return w.prepared(name)(args...).exec();

expands to

return w.prepared(name)(args0, args1, args2).exec();

which is different from the

return w.prepared(name)(args0)(args1)(args2).exec();

which you need.

It looks like you'll need a helper function which recursively applies one argument at a time:

private:
    template <typename E>
    static E&& apply_prepared_args(E&& expr) {
        return std::forward<E>(expr);
    }
    template <typename E, typename First, typename... Rest>
    static decltype(auto) apply_prepared_args(
        E&& expr, const First& first, const Rest& ...rest) {
        return apply_prepared_args(std::forward<E>(expr)(first), rest...);
    }

public:
    template<typename... T>
    pqxx::result exec_prepared(
        const std::string& name, const T&... args) const {
        return apply_prepared_args(w.prepared(name), args...).exec();
    }