pqxx::icursorstream Class Reference

Simple read-only cursor represented as a stream of results. More...

#include <cursor.hxx>

Public Types
using	size_type = cursor_base::size_type
using	difference_type = cursor_base::difference_type

Public Member Functions
	icursorstream (transaction_base &context, std::string_view query, std::string_view basename, difference_type sstride=1)
	Set up a read-only, forward-only cursor. More...
	icursorstream (transaction_base &context, field const &cname, difference_type sstride=1, cursor_base::ownership_policy op=cursor_base::owned)
	Adopt existing SQL cursor. Use with care. More...
constexpr	operator bool () const &noexcept
	Return true if this stream may still return more data. More...
icursorstream &	get (result &res)
	Read new value into given result object; same as operator >>. More...
icursorstream &	operator>> (result &res)
	Read new value into given result object; same as get(result&). More...
icursorstream &	ignore (std::streamsize n=1) &
	Move given number of rows forward without reading data. More...
void	set_stride (difference_type stride) &
	Change stride, i.e. the number of rows to fetch per read operation. More...
constexpr difference_type	stride () const noexcept

Friends
class	internal::gate::icursorstream_icursor_iterator

Detailed Description

Simple read-only cursor represented as a stream of results.

SQL cursors can be tricky, especially in C++ since the two languages seem to have been designed on different planets. An SQL cursor has two singular positions akin to end() on either side of the underlying result set.

These cultural differences are hidden from view somewhat by libpqxx, which tries to make SQL cursors behave more like familiar C++ entities such as iterators, sequences, streams, and containers.

Data is fetched from the cursor as a sequence of result objects. Each of these will contain the number of rows defined as the stream's stride, except of course the last block of data which may contain fewer rows.

This class can create or adopt cursors that live outside any backend transaction, which your backend version may not support.

Member Typedef Documentation

difference_type

using pqxx::icursorstream::difference_type = cursor_base::difference_type

size_type

using pqxx::icursorstream::size_type = cursor_base::size_type

Constructor & Destructor Documentation

icursorstream() [1/2]

pqxx::icursorstream::icursorstream	(	transaction_base &	context,
		std::string_view	query,
		std::string_view	basename,
		difference_type	sstride = 1
	)

Set up a read-only, forward-only cursor.

Roughly equivalent to a C++ Standard Library istream, this cursor type supports only two operations: reading a block of rows while moving forward, and moving forward without reading any data.

Parameters

context	Transaction context in which this cursor will be active.
query	SQL query whose results this cursor shall iterate.
basename	Suggested name for the SQL cursor; the library will append a unique code to ensure its uniqueness.
sstride	Number of rows to fetch per read operation; must be a positive number.

References pqxx::read_only, and set_stride().

icursorstream() [2/2]

pqxx::icursorstream::icursorstream	(	transaction_base &	context,
		field const &	cname,
		difference_type	sstride = 1,
		cursor_base::ownership_policy	op = cursor_base::owned
	)

Adopt existing SQL cursor. Use with care.

Forms a cursor stream around an existing SQL cursor, as returned by e.g. a server-side function. The SQL cursor will be cleaned up by the stream's destructor as if it had been created by the stream; cleaning it up by hand or adopting the same cursor twice is an error.

Passing the name of the cursor as a string is not allowed, both to avoid confusion with the other constructor and to discourage unnecessary use of adopted cursors.

Warning

It is technically possible to adopt a "WITH HOLD" cursor, i.e. a cursor that stays alive outside its creating transaction. However, any cursor stream (including the underlying SQL cursor, naturally) must be destroyed before its transaction context object is destroyed. Therefore the only way to use SQL's WITH HOLD feature is to adopt the cursor, but defer doing so until after entering the transaction context that will eventually destroy it.

Parameters

context	Transaction context in which this cursor will be active.
cname	Result field containing the name of the SQL cursor to adopt.
sstride	Number of rows to fetch per read operation; must be a positive number.
op	Ownership policy. Determines whether the cursor underlying this stream will be destroyed when the stream is closed.

References set_stride().

Member Function Documentation

get()

icursorstream& pqxx::icursorstream::get

(

result &

res

)

Read new value into given result object; same as operator >>.

The result set may continue any number of rows from zero to the chosen stride, inclusive. An empty result will only be returned if there are no more rows to retrieve.

Parameters

res	Write the retrieved data into this result object.

Returns

Reference to this very stream, to facilitate "chained" invocations ("C.get(r1).get(r2);")

ignore()

pqxx::icursorstream & pqxx::icursorstream::ignore

(

std::streamsize

n = 1

)

&

Move given number of rows forward without reading data.

Ignores any stride that you may have set. It moves by a given number of rows, not a number of strides.

Returns

Reference to this stream itself, to facilitate "chained" invocations.

operator bool()

constexpr pqxx::icursorstream::operator bool ( ) const &

constexprnoexcept

Return true if this stream may still return more data.

operator>>()

icursorstream& pqxx::icursorstream::operator>>

(

result &

res

)

Read new value into given result object; same as get(result&).

The result set may continue any number of rows from zero to the chosen stride, inclusive. An empty result will only be returned if there are no more rows to retrieve.

Parameters

res	Write the retrieved data into this result object.

Returns

Reference to this very stream, to facilitate "chained" invocations ("C >> r1 >> r2;")

set_stride()

void pqxx::icursorstream::set_stride

(

difference_type

stride

)

&

Change stride, i.e. the number of rows to fetch per read operation.

Parameters

stride

Must be a positive number.

Referenced by icursorstream().

stride()

constexpr difference_type pqxx::icursorstream::stride ( ) const

constexprnoexcept

Friends And Related Function Documentation

internal::gate::icursorstream_icursor_iterator

friend class internal::gate::icursorstream_icursor_iterator

friend

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The documentation for this class was generated from the following files:

• cursor.hxx
• cursor.cxx