dotcomma.org: Introduction to C++ Classes for C Programmers

Introduces the key concepts of C++ classes. Designed for
experienced C programmers who wish to learn the fundamentals of
object-oriented programming.

“C is what is known as a procedure-oriented language in which
the programmer starts by describing the data and then writes
procedures to manipulate the data. Eventually, however, programmers
discovered that it made a program much clearer and easier to
understand if they were able to group data with the operations
(functions) that worked on that data. These groupings are known as
objects, and languages such as C++ that use objects are known as
object-oriented languages.”