///////////////////////////////////////////////////////////////////////////////////////////

實在是沒看出來這樣有什么優勢，為了記錄是否走過某個path？

Query::execute(const SQLString& str)這里看到了遞歸操作，難道是為了輔助遞歸？

總感覺這里越搞越復雜，，，

///////////////////////////////////////////////////////////////////////////////////////////

/// \brief A template for setting a flag on a variable as long as the
/// object that set it is in scope.  Flag resets when object goes
/// out of scope.  Works on anything that looks like bool.

template <class T = bool>
class AutoFlag
{
public:
    /// \brief Constructor: sets ref to true.
    AutoFlag(T& ref) :
    referent_(ref)
    {
        referent_ = true;
    }

    /// \brief Destructor: sets referent passed to ctor to false.
    ~AutoFlag()
    {
        referent_ = false;
    }

private:
    T& referent_;
};

///////////////////////////////////////////////////////////////////////////////////////////

ResNSel
Query::execute(const SQLString& str)
{
    if ((def.size() == 1) && !def.processing_) {
        // .  The
        // auto-reset flag is required because we'll end up back in this
        // function once the query string is built, but we need to take
        // the 'else' path to avoid an infinite loop.
        AutoFlag<> af(def.processing_);
        return execute(SQLQueryParms() << str);
    }
    else {
        // Take str to be the entire query string
        return execute(str.c_str(), str.length());
    }
}

ResNSel
Query::execute(const char* str)
{
    return execute(SQLString(str));
}

ResNSel
Query::execute(const char* str, size_t len)
{
    if (lock()) {
        success_ = false;
        if (throw_exceptions()) {
            throw LockFailed();
        }
        else {
            return ResNSel();
        }
    }

    success_ = !mysql_real_query(&conn_->mysql_, str, len);

    unlock();
    if (success_) {
        return ResNSel(conn_);
    }
    else if (throw_exceptions()) {
        throw BadQuery(error());
    }
    else {
        return ResNSel();
    }
}