Apps similar to SQLAPI++ include Abstract Database Connector, which is free and open source. Other options are CppDB - SQL Connectivity Library. Between them they cover Windows and Linux.
Abstract Database Connector is a C/C++ library for making connections to several databases (MySQL, mSQL, PostgreSQL, Interbase, Informix, BDE, ODBC). It runs on Linux, UNIX, BeOS, and Windows, and a dynamic driver loader for ELF OSes is under development.
CppDB is an SQL connectivity library that is designed to provide platform and Database independent connectivity API similarly to what JDBC, ODBC and other connectivity libraries do. http://cppcms.com/sql/cppdb/
