std::launder
summary
C++17 adds std::launder to allow reusing an old pointer after placement new when the new object has const or reference members by acting as a compiler-optimization barrier rather than mutating pointers. It requires calling launder each time an old pointer is used for the new object and does not make some previously-UB patterns safe (certain aliasing cases remain UB). Empirical note: a study found 40% of placement uses in one codebase ignore placement-new’s returned pointer.
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