How Does `std::launder` Address Compiler Assumptions Regarding Memory Initialization and Lifetime in C ?

std::launder: Memory Laundering for C Optimization

The recently introduced function template std::launder aims to address a fundamental issue in C related to memory initialization and lifetime. To fully grasp its purpose, let's delve into the intricacies of memory management in the language.

The Problem: Persistent Compiler Assumptions

Consider the following code:

struct X { const int n; };
union U { X x; float f; };
...
U u = {{ 1 }};

Aggregate initialization initializes the first member of U with {1}. Since n is constant, the compiler assumes that u.x.n will always be 1. However, consider the following:

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

This code legally creates a new object in the storage of u.x. Its n member is set to 2, violating the previous assumption made by the compiler.

The Issue: Lifetime and Optimization

According to the C standard, accessing a newly created object through variables/pointers/references to the old object is prohibited if the old object has a constant member or the type of the new object is different.

This restriction allows the compiler to make optimizations based on assumptions about the contents of memory. However, when these assumptions are broken, undefined behavior can occur.

std::launder: Breaking Compiler Assumptions

std::launder provides a solution to this problem by "laundering" memory. It effectively tells the compiler to ignore previous assumptions about the memory location, forcing it to treat it as if it has been freshly allocated.

In the previous example, this would allow us to access u.x.n properly:

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

Additionally, std::launder can facilitate accessing a newly created object through pointers to the old when the types differ:

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

Conclusion

std::launder is a powerful tool that allows programmers to break persistent compiler assumptions, enabling optimizations that would otherwise be prevented by lifetime and typing restrictions. By leveraging memory laundering, std::launder ensures that critical memory contents are treated in a flexible and well-defined manner, enhancing the safety and efficiency of C code.

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