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Message: [QUOTE="James Kanze, post: 3972398"] [...] In practice, with most compilers, yes. The current standard doesn't talk about "layout-compatible", and no C++ standard can impose anything on the C compiler. I didn't see anything in N2914 (the latest draft that I happen to have handy) that guaranteed that a "standard-layout" class is in anyway compatible with C. On the other hand, the whole concept of "layout-compatible" seems to have been lifted from the C standard, and from a QoI point of view, I would expect that anything with "standard-layout" be compatible with C if the system defines a standard C ABI (as most do). Per se, they don't have much use. They do restrict the implementation in some ways, however, that typically allows them to be used in mixed language headers (headers which are used by both C and C++). Nothing. Otherwise, the constructor wouldn't be trivial. I'm not sure what you're trying to ask there, but if it is "why do we need a special syntax for this?", it's because the current standard says that if a class has any constructor, the compiler won't provide a default constructor implicitly, and a lot of code counts on this. [/QUOTE]