libpqxx
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Reference to a field in a result set. More...
#include <field.hxx>
Public Types | |
using | size_type = field_size_type |
Public Member Functions | |
field (const row &R, row_size_type C) noexcept | |
Constructor. More... | |
template<> | |
bool | to (const char *&Obj) const |
Specialization: to(const char *&) . More... | |
Comparison | |
bool | operator== (const field &) const |
Byte-by-byte comparison of two fields (all nulls are considered equal) More... | |
bool | operator!= (const field &rhs) const |
Byte-by-byte comparison (all nulls are considered equal) More... | |
Column information | |
const char * | 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 | |
const char * | 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, const char *>::value), bool >::type |
Read value into Obj; or leave Obj untouched and return false if null. 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, const T &Default) const -> typename std::enable_if<(not std::is_pointer< T >::value or std::is_same< T, const char *>::value), bool >::type |
Read value into Obj; or use Default & return false if null. More... | |
template<typename T > | |
T | as (const T &Default) const |
Return value as object of given type, or Default if null. More... | |
template<typename T > | |
T | as () const |
Return value as object of given type, or throw exception if null. More... | |
template<typename T , template< typename > class O> | |
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 | |
const result & | home () const noexcept |
size_t | idx () const noexcept |
row_size_type | col () const noexcept |
Protected Attributes | |
long | m_col |
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.
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noexcept |
Constructor.
Create field as reference to a field in a result set.
R | Row that this field is part of. |
C | Column number of this field. |
Implementation of the pqxx::field class.
pqxx::field refers to a field in a query result.
Copyright (c) 2000-2019, 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.
T pqxx::field::as | ( | const T & | Default | ) | const |
Return value as object of given type, or Default if null.
Note that unless the function is instantiated with an explicit template argument, the Default value's type also determines the result type.
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
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.
const char * 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::from_string(), pqxx::icursorstream::icursorstream(), pqxx::operator<<(), operator==(), pqxx::pipeline::resume(), pqxx::to_string(), and pqxx::field_streambuf< CHAR, TRAITS >::underflow().
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protectednoexcept |
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 (i.e. only usable when std::optional
or std::experimental::optional
are available), or specify container type with get<int, std::optional>()
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protectednoexcept |
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noexcept |
const char * pqxx::field::name | ( | ) | const |
Column name.
References col(), pqxx::result::column_name(), and home().
row_size_type pqxx::field::num | ( | ) | const |
Referenced by pqxx::const_row_iterator::operator-().
bool pqxx::field::operator!= | ( | const field & | rhs | ) | const |
Byte-by-byte comparison (all nulls are considered equal)
bool pqxx::field::operator== | ( | const field & | rhs | ) | const |
Byte-by-byte comparison of two fields (all nulls are considered equal)
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.
bool pqxx::field::operator>> | ( | T & | Obj | ) | const |
Read value into Obj; or leave Obj untouched and return false
if null.
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noexcept |
Return number of bytes taken up by the field's value.
Includes the terminating zero byte.
References col(), home(), and idx().
Referenced by pqxx::from_string(), pqxx::operator<<(), operator==(), pqxx::to_string(), and pqxx::field_streambuf< CHAR, TRAITS >::underflow().
pqxx::oid pqxx::field::table | ( | ) | const |
What table did this column come from?
References col(), pqxx::result::column_table(), and home().
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().
auto pqxx::field::to | ( | T & | Obj | ) | const -> typename std::enable_if<( not std::is_pointer<T>::value or std::is_same<T, const char*>::value ), bool>::type |
Read value into Obj; or leave Obj untouched and return false
if null.
Note this can be used with optional types (except pointers other than C-strings)
References pqxx::from_string().
auto pqxx::field::to | ( | T & | Obj, |
const T & | Default | ||
) | const -> typename std::enable_if<( not std::is_pointer<T>::value or std::is_same<T, const char*>::value ), bool>::type |
Read value into Obj; or use Default & return false
if null.
Note this can be used with optional types (except pointers other than C-strings)
bool pqxx::field::to | ( | const char *& | Obj | ) | const |
Specialization: to(const char *&)
.
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.
pqxx::oid pqxx::field::type | ( | ) | const |
Column type.
References col(), pqxx::result::column_type(), and home().
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protected |
You'd expect this to be a size_t, but due to the way reverse iterators are related to regular iterators, it must be allowed to underflow to -1.