How Does C 's `std::launder` Solve Memory Aliasing and Lifetime Issues?

Memory Laundering: The Introduction of std::launder in C

P0137 introduces std::launder to address certain issues related to unions, lifetime, and pointers in C . This function template allows for memory laundering, a process that prevents the compiler from making assumptions about the modified contents of memory.

Memory Laundering

std::launder performs memory laundering, which removes any assumptions the compiler may have made about the contents of a memory location. This is particularly relevant when:

• A const member of a union is modified, leading to unexpected behavior when accessing the union's fields.
• The type of an object is changed, potentially violating lifetime rules.

Example: Laundering a const Union Field

Consider this example:

struct X { const int n; };
union U { X x; float f; };

U u = {{ 1 }};
X *p = new (&u.x) X {2};

Initializing u.x with {1} assumes the const member n will always be 1. However, assigning p to a new X object with n set to 2 violates this assumption.

To correctly access u.x.n after this modification, we must launder the memory:

assert(*std::launder(&u.x.n) == 2); // Will be true

Other Applications

std::launder can also be used in situations where lifetimes are potentially violated, such as when allocating a new object in the storage of an old object without using placement new:

alignas(int) char data[sizeof(int)];
new(&data) int;
int *p = std::launder(reinterpret_cast<int*>(&amp;data));

By laundering the pointer, we bypass the lifetime rules that would otherwise prevent accessing the new object.

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