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Gotta love these C++ FUD arguments! Can you tell us an example where a library’s internal usage of placement new results in bugs unless the consumers use std::l

• tsimionescu 5y ago
  Isn't this exactly what this argument shows? In older versions of C++, if you have an object with a non-const sub-object with a const member, and you give this object to a library, and then that library uses placement new to replace the sub-object within your object, you had to use std::launder to keep referring to the fields of your object, or you'd invoke UB. In newer C++, thankfully, this is no longer required. Edit: found a committee paper discussing exactly such cases: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0532r0.pdf http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p053... Obj x; x.use(); // ok mutate(x); // does this call placement new? x.use(); // may have undefined behavior std::launder(&x)->use(); // may be necessary x.use(); // may still have undefined behavior