An (In-)Complete Guide to C++ Object Lifetimes

When are objects created and when are they destroyed? How does temporary lifetime extension come into play and what changed there recently? What happens when you std::malloc memory and just pretend objects are there without creating anything? Or worse: You use mmap() to read shared memory. How do unions interact with constructors, strict aliasing, or the "common initial sequence"? What when you explicitly call the destructor and later re-use the same storage? What's the deal with std::launder, std::bit_cast, and std::start_lifetime_as?

We'll answer all of those questions and much more. We'll do that by looking at the C++ standard, old and new proposals, and compiler optimizations.