Please explain std::launder

[jgot...@gmail.com]

  A few posts here have mentioned a function called std::launder.  I know it has something to do with pointer lifetime, but I don't really understand what.  Could someone tell me in simple terms what it does and when I might need to call it?

Thank you,

Joe Gottman

[sean.mid...@gmail.com]

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1776

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4430.html

The lifetime rules in C++ let a compiler make assumptions about the relationship between addresses and instances. Reinitializing an object with constant members (e.g. via placement new) does not interact well with those rules. std::launder is a hint to the compiler that an address can only refer to a new object and not to any object that previously existed at the same address, independent of the lifetime rules.

At least, that's what I'm gathering after a few minutes of glancing through the above links. N4430 is a little dense to read. :)

[Richard Smith]

On Mon, Jun 20, 2016 at 1:56 PM, <sean.mid...@gmail.com> wrote:

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1776

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4430.html

The lifetime rules in C++ let a compiler make assumptions about the relationship between addresses and instances. Reinitializing an object with constant members (e.g. via placement new) does not interact well with those rules. std::launder is a hint to the compiler that an address can only refer to a new object and not to any object that previously existed at the same address, independent of the lifetime rules.

At least, that's what I'm gathering after a few minutes of glancing through the above links. N4430 is a little dense to read. :)

That's essentially right. From the object model perspective, it maps from a pointer to some (potentially) out-of-lifetime object at a given address to a pointer to a currently-live object at that address. From the implementation perspective, it's an optimization barrier that prevents the compiler from assuming that it knows anything about the identity of the object pointed to by the returned version of the pointer.

On Friday, May 20, 2016 at 6:28:50 PM UTC-7, jgot...@gmail.com wrote:

  A few posts here have mentioned a function called std::launder.  I know it has something to do with pointer lifetime, but I don't really understand what.  Could someone tell me in simple terms what it does and when I might need to call it?

Thank you,

Joe Gottman

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