Accessing results and result rows

Let's say you have a result object. For example, your program may have done:

pqxx::result r = w.exec("SELECT * FROM mytable");

Now, how do you access the data inside r?

Result sets act as standard C++ containers of rows. Rows act as standard C++ containers of fields. So the easiest way to go through them is:

for (auto const &row: r)
{
   for (auto const &field: row) std::cout << field.c_str() << '\t';
   std::cout << std::endl;
}

But results and rows also support other kinds of access. Array-style indexing, for instance, such as r[rownum]:

int const num_rows = r.size();
for (int rownum=0; rownum < num_rows; ++rownum)
{
  pqxx::row const row = r[rownum];
  int const num_cols = row.size();
  for (int colnum=0; colnum < num_cols; ++colnum)
  {
    pqxx::field const field = row[colnum];
    std::cout << field.c_str() << '\t';
  }

  std::cout << std::endl;
}

And of course you can use classic "begin/end" loops:

for (auto row = r.begin(); row != r.end(); row++)
{
  for (auto field = row.begin(); field != row.end(); field++)
    std::cout << field->c_str() << '\t';
    std::cout << std::endl;
}

Result sets are immutable, so all iterators on results and rows are actually const_iterators. There are also const_reverse_iterator types, which iterate backwards from rbegin() to rend() exclusive.

All these iterator types provide one extra bit of convenience that you won't normally find in C++ iterators: referential transparency. You don't need to dereference them to get to the row or field they refer to. That is, instead of row->end() you can also choose to say row.end(). Similarly, you may prefer field.c_str() over field->c_str().

This becomes really helpful with the array-indexing operator. With regular C++ iterators you would need ugly expressions like (*row)[0] or row->operator[](0). With the iterator types defined by the result and row classes you can simply say row[0].