uPg: Mapping a C++ type to PostgreSQL user type

In PosgtgreSQL the following kinds of user types are available:

• composite (row) types, see uPg: Composite user types
• enumerations, see uPg: Mapping a C++ enum to PostgreSQL enum type.
• ranges, see uPg: Range types
• domains

Domains are essentially some data types with database constraints applied to them, they map to their base data types' C++ counterparts.

Other user types can be mapped to C++ types, more information on defining a mapped C++ type can be found on respective pages. After a C++ type is defined, it must be mapped to it's PostgreSQL counterpart by specialising CppToUserPg template for the type. C++ types are mapped to PostgreSQL types by their names, so the specialization for CppToUserPg template must have a static constexpr member of type DBTypeName named postgres_name.

C++ type

namespace pgtest {
struct FooBar {
  pg::Integer i;
  std::string s;
  double d;
};
}  // namespace pgtest

Declaring C++ type to PostgreSQL type mapping

Warning

The type mapping specialization must be accessible at the points where parsing/formatting of the C++ type is instantiated. The header where the C++ type is declared is an appropriate place to do it.

namespace storages::postgres::io {
// This specialization MUST go to the header together with the mapped type
template <>
struct CppToUserPg<pgtest::FooBar> {
  static constexpr DBTypeName postgres_name = "__pgtest.foobar";
};
}  // namespace storages::postgres::io

A connection gets the data types' definitions after connect and uses the definitions to map C++ types to PostgreSQL type oids.

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