std::launder intentionally obfuscates the origin of a pointer for the abstract machine / the compiler so that source and result may have different lifetimes and types. When used for e.g. (static) v...

This question results from thinking about my answer in Given two objects of different types and their relative location in memory, can I derive a pointer to one object from a pointer to the other?....

The following code example is from cppreference on std::launder: alignas(Y) std::byte s[sizeof(Y)]; Y* q = new(&s) Y{2}; const int f = reinterpret_cast<Y*>(&s)->z; // Class member ...

std::launder has a precondition that all bytes reachable from the would-be-returned pointer are reachable through the passed pointer. My understanding is that this is meant to allow compiler optimi...

P0137 introduces the function template std::launder and makes many, many changes to the standard in the sections concerning unions, lifetime, and pointers. What is the problem this paper is solvin...

Basic.life/8 tells us that we can use the storage occupied by an object to create a new one after its lifetime has ended and use its original name to refer to it unless: the type of the original ...

One of the preconditions on std::launder requires that object is within its lifetime. I assume that it is a necessary condition for being able to dereference the element. Does it mean, that if I ob...

I've just read What is the purpose of std::launder? and frankly, I am left scratching my head. Let's start with the second example in @NicolBolas' accepted answer: aligned_storage<sizeof(int), ...

This is a code example from the C++20 spec ([basic.life]/8): struct C { int i; void f(); const C& operator=( const C& ); }; const C& C::operator=( const C& other) { if ( this...

[1] Are there any cases in which the addition of p0593r6 into C++20 (§ 6.7.2.11 Object model [intro.object]) made std::launder not necessary, where the same use case in C++17 required std::launder...

This is the part of the example that I don't understand: struct Y { int z; }; int main() { alignas(Y) std::byte s[sizeof(Y)]; Y *q = new (&s) Y{2}; const int f = reinterpret_cast<Y *&gt...

The std::launder function requires that every byte that would be reachable through the result is reachable through the argument. "Reachable" is defined as follows: A byte of storage is reachable...

This question is a follow-up to: Is adding to a "char *" pointer UB, when it doesn't actually point to a char array? In CWG 1314, CWG affirmed that it is legal to perform pointer ari...

I wanted to write my own "small vector" type, and the first hurdle has been figuring out how to implement the on-stack storage. I stumbled upon std::aligned_storage, which seems purpose-designed f...

It is like std::optional, but doesn't store an extra bool. User has to make sure to access only after initializing. template<class T> union FakeOptional { //Could be a normal struct in which...

The current draft standard (and presumably C++17) say in [basic.compound/4]: [ Note: An array object and its first element are not pointer-interconvertible, even though they have the same address....

According to this answer, since C++17, even if a pointer has the right address and the right type dereferencing it can cause undefined behaviour. alignas(int) unsigned char buffer[2*sizeof(int)];...