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String conversion

The PostgreSQL server accepts and represents data in string form. It has its own formats for various data types. The string conversions define how various C++ types translate to and from their respective PostgreSQL text representations.

Each conversion is defined by a specialisations of `string_traits`. It gets complicated if you want top performance, but until you do, all you really need to care about when converting values between C++ in-memory representations such as `int` and the postgres string representations is the `pqxx::to_string` and `pqxx::from_string` functions.

If you need to convert a type which is not supported out of the box, you'll need to define your own specialisations for these templates, similar to the ones defined here and in `pqxx/conversions.hxx`. Any conversion code which "sees" your specialisation will now support your conversion. In particular, you'll be able to read result fields into a variable of the new type.

There is a macro to help you define conversions for individual enumeration types. The conversion will represent enumeration values as numeric strings.