What is the history of struct/class in C++?

Question

I'm taking a class on Object Oriented Programming in C++. In a recent assignment I defined a member function within a struct. My instructor explained that, although it's compilable to use member functions within structs, he would prefer we didn't, for backward compatibility with C, and (especially in this beginner class) to practice good data encapsulation- we should use a struct for types that contain mostly data, and a class for applications that benefit from more procedural encapsulation. He indicated that this practice comes from the history of structs/classes in C++, which is what I'd like to know more about.

I know that structs are functionally the same as classes except for default member/inheritance access. My question is:

Why are structs AND classes included in C++? From my background in C#, where structs and classes have important differences, it seems like struct in C++ is just syntactic sugar for defining classes with default public-ness. Is it?

I'm not looking for opinions on when/why one should be used instead of the other- I was born well after these constructs were, and I'm looking for the history of them. Were they conceived together? If so, why? If not, which came first, and why was the second added? I realize that there are many venerable elders within this community who may have living memory of these features' origins, and links to standards publications, or examples of code, where both, one, or the other first appeared, will add to answers' helpfulness.

Please note, this question is not:

• What are the differences between struct and class in C++?
• C++ Structs with Member Functions vs. Classes with Public Variables
• Can C++ struct have member functions?
• nor Function for C++ struct

Answer 1

To ease development of polyglot headers.

C and C++ are separate languages, but the C++ language is designed to provide a large useful common subset with the C language. Several commonly used constructions in the C++ language have the same meaning (or nearly so) as they have in the C language. In the case of struct, the C++ language defines it as syntactic sugar for class that approximates the behavior of a struct in the C language, namely that members are public by default. Thus a program or part thereof can be written in this common subset. This allows, for example, a library to provide a single header file that declares the same API to both programs written in the C language and programs written in the C++ language.

Some differences between the C language and the C++ language are listed in a question that has since been closed as too broad. From the perspective of somebody programming in this common subset language, these differences can be treated as "implementation-defined behavior", where compilers for the C language produce one behavior and compilers for the C++ language produce the other.

In fact, the C++ standard provides mechanisms to aid development of polyglot programs that are valid in both C and C++ languages. The extern "C" linkage specifier allows a program to be written partly in C and partly in C++. And the __cplusplus symbol is used in #ifdef __cplusplus conditions to conditionally enable macros, linkage specifiers, and other specifics that only one of the two languages is supposed to see.