libpqxx  7.0.1
pqxx::field Class Reference

Reference to a field in a result set. More...

#include <field.hxx>

Inheritance diagram for pqxx::field:

Public Types

using size_type = field_size_type
 

Public Member Functions

 field (row const &r, row_size_type c) noexcept
 Constructor. More...
 
template<>
bool to (char const *&obj) const
 Specialization: to(char const *&). More...
 
Comparison
bool operator== (field const &) const
 Byte-by-byte comparison of two fields (all nulls are considered equal) More...
 
bool operator!= (field const &rhs) const
 Byte-by-byte comparison (all nulls are considered equal) More...
 
Column information
char const * name () const
 Column name. More...
 
oid type () const
 Column type. More...
 
oid table () const
 What table did this column come from? More...
 
row_size_type num () const
 
row_size_type table_column () const
 What column number in its originating table did this column come from? More...
 
Content access
std::string_view view () const
 Read as string_view. More...
 
char const * c_str () const
 Read as plain C string. More...
 
bool is_null () const noexcept
 Is this field's value null? More...
 
size_type size () const noexcept
 Return number of bytes taken up by the field's value. More...
 
template<typename T >
auto to (T &obj) const -> typename std::enable_if<(not std::is_pointer< T >::value or std::is_same< T, char const *>::value), bool >::type
 Read value into obj; or if null, leave obj untouched and return false. More...
 
template<typename T >
bool operator>> (T &obj) const
 Read value into obj; or leave obj untouched and return false if null. More...
 
template<typename T >
auto to (T &obj, T const &default_value) const -> typename std::enable_if<(not std::is_pointer< T >::value or std::is_same< T, char const *>::value), bool >::type
 Read value into obj; or if null, use default value and return false. More...
 
template<typename T >
as (T const &default_value) const
 Return value as object of given type, or default value if null. More...
 
template<typename T >
as () const
 Return value as object of given type, or throw exception if null. More...
 
template<typename T , template< typename > class O = std::optional>
constexpr O< T > get () const
 Return value wrapped in some optional type (empty for nulls). More...
 
array_parser as_array () const
 Parse the field as an SQL array. More...
 

Protected Member Functions

result const & home () const noexcept
 
result::size_type idx () const noexcept
 
row_size_type col () const noexcept
 

Protected Attributes

row_size_type m_col
 

Detailed Description

Reference to a field in a result set.

A field represents one entry in a row. It represents an actual value in the result set, and can be converted to various types.

Member Typedef Documentation

◆ size_type

Constructor & Destructor Documentation

◆ field()

pqxx::field::field ( pqxx::row const &  r,
row_size_type  c 
)
noexcept

Constructor.

Create field as reference to a field in a result set.

Parameters
rRow that this field is part of.
cColumn number of this field.

Implementation of the pqxx::field class.

pqxx::field refers to a field in a query result.

Copyright (c) 2000-2020, Jeroen T. Vermeulen.

See COPYING for copyright license. If you did not receive a file called COPYING with this source code, please notify the distributor of this mistake, or contact the author.

Member Function Documentation

◆ as() [1/2]

template<typename T >
T pqxx::field::as ( T const &  default_value) const

Return value as object of given type, or default value if null.

Note that unless the function is instantiated with an explicit template argument, the Default value's type also determines the result type.

◆ as() [2/2]

template<typename T >
T pqxx::field::as ( ) const

Return value as object of given type, or throw exception if null.

Use as as<std::optional<int>>() or as<my_untemplated_optional_t>() as an alternative to get<int>(); this is disabled for use with raw pointers (other than C-strings) because storage for the value can't safely be allocated here

References pqxx::nullness< TYPE, ENABLE >::null(), and pqxx::internal::throw_null_conversion().

◆ as_array()

array_parser pqxx::field::as_array ( ) const

Parse the field as an SQL array.

Call the parser to retrieve values (and structure) from the array.

Make sure the result object stays alive until parsing is finished. If you keep the row of field object alive, it will keep the result object alive as well.

◆ c_str()

char const * pqxx::field::c_str ( ) const

Read as plain C string.

Since the field's data is stored internally in the form of a zero-terminated C string, this is the fastest way to read it. Use the to() or as() functions to convert the string to other types such as int, or to C++ strings.

References col(), home(), and idx().

Referenced by pqxx::binarystring::binarystring(), pqxx::icursorstream::icursorstream(), operator==(), pqxx::to_string(), and pqxx::field_streambuf< CHAR, TRAITS >::underflow().

◆ col()

◆ get()

template<typename T , template< typename > class O = std::optional>
constexpr O<T> pqxx::field::get ( ) const

Return value wrapped in some optional type (empty for nulls).

Use as get<int>() as before to obtain previous behavior, or specify container type with get<int, std::optional>()

◆ home()

result const& pqxx::field::home ( ) const
protectednoexcept

◆ idx()

result::size_type pqxx::field::idx ( ) const
protectednoexcept

Referenced by c_str(), is_null(), and size().

◆ is_null()

bool pqxx::field::is_null ( ) const
noexcept

Is this field's value null?

References col(), home(), and idx().

Referenced by operator==().

◆ name()

char const * pqxx::field::name ( ) const

Column name.

References col(), pqxx::result::column_name(), and home().

◆ num()

row_size_type pqxx::field::num ( ) const

◆ operator!=()

bool pqxx::field::operator!= ( field const &  rhs) const

Byte-by-byte comparison (all nulls are considered equal)

Warning
See operator==() for important information about this operator

◆ operator==()

bool pqxx::field::operator== ( field const &  rhs) const

Byte-by-byte comparison of two fields (all nulls are considered equal)

Warning
null handling is still open to discussion and change!

Handling of null values differs from that in SQL where a comparison involving a null value yields null, so nulls are never considered equal to one another or even to themselves.

Null handling also probably differs from the closest equivalent in C++, which is the NaN (Not-a-Number) value, a singularity comparable to SQL's null. This is because the builtin == operator demands that a == a.

The usefulness of this operator is questionable. No interpretation whatsoever is imposed on the data; 0 and 0.0 are considered different, as are null vs. the empty string, or even different (but possibly equivalent and equally valid) encodings of the same Unicode character etc.

References c_str(), is_null(), and size().

◆ operator>>()

template<typename T >
bool pqxx::field::operator>> ( T &  obj) const

Read value into obj; or leave obj untouched and return false if null.

◆ size()

pqxx::field::size_type pqxx::field::size ( ) const
noexcept

Return number of bytes taken up by the field's value.

Includes the terminating zero byte.

References col(), home(), and idx().

Referenced by operator==(), pqxx::to_string(), and pqxx::field_streambuf< CHAR, TRAITS >::underflow().

◆ table()

pqxx::oid pqxx::field::table ( ) const

What table did this column come from?

References col(), pqxx::result::column_table(), and home().

◆ table_column()

pqxx::row::size_type pqxx::field::table_column ( ) const

What column number in its originating table did this column come from?

References col(), home(), and pqxx::result::table_column().

◆ to() [1/3]

template<typename T >
auto pqxx::field::to ( T &  obj) const -> typename std::enable_if< (not std::is_pointer<T>::value or std::is_same<T, char const *>::value), bool>::type

Read value into obj; or if null, leave obj untouched and return false.

This can be used with optional types (except pointers other than C-style strings).

References pqxx::from_string(), and pqxx::is_null().

◆ to() [2/3]

template<typename T >
auto pqxx::field::to ( T &  obj,
T const &  default_value 
) const -> typename std::enable_if< (not std::is_pointer<T>::value or std::is_same<T, char const *>::value), bool>::type

Read value into obj; or if null, use default value and return false.

Note this can be used with optional types (except pointers other than C-strings)

◆ to() [3/3]

template<>
bool pqxx::field::to ( char const *&  obj) const

Specialization: to(char const *&).

The buffer has the same lifetime as the data in this result (i.e. of this result object, or the last remaining one copied from it etc.), so take care not to use it after the last result object referring to this query result is destroyed.

References pqxx::is_null().

◆ type()

pqxx::oid pqxx::field::type ( ) const

Column type.

References col(), pqxx::result::column_type(), and home().

◆ view()

std::string_view pqxx::field::view ( ) const

Read as string_view.

Referenced by pqxx::from_string().

Member Data Documentation

◆ m_col

row_size_type pqxx::field::m_col
protected

You'd expect this to be unsigned, but due to the way reverse iterators are related to regular iterators, it must be allowed to underflow to -1.


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