How Does `std::launder` Solve Compiler Optimization Issues with Const Members in Unions?

Unveiling the Essence of Memory Laundering: A Deeper Dive into std::launder

In the realm of C standardization, P0137 introduces std::launder, a function template that addresses a subtle issue concerning unions, lifetime, and pointers. To comprehend its purpose, let's delve into the specific problem this paper tackles and the subsequent language adjustments we need to acknowledge.

The Problem at Hand

Consider the following code snippet:

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

U u = {{ 1 }};

Here, aggregate initialization is performed, setting the first member of U (x) to the value 1. As n is a const variable, the compiler assumes that u.x.n will always remain 1.

The Optimization Trap

However, consider the following code:

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

Since X is trivial, we can create a new object in the same location as the old one, making this code syntactically valid. The new object will now have its n member set to 2.

Now, let's try to access u.x.n. What would you expect the result to be?

The Unfortunate Reality

Intuitively, one might think the result should be 2. However, this is not the case. The compiler, based on its assumption that const variables are immutable, optimizes the code, rendering the new value of u.x.n inaccessible.

Enter std::launder: Memory Laundering

To circumvent this optimization, we need to "launder" our memory using std::launder. Here's an illustrative example:

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

Memory laundering prevents the compiler from tracing the origin of our object, allowing us to access the new value despite the const member.

Additional Use Cases

std::launder can also aid in other situations where data type changes or storage allocation semantics hinder direct access.

In summary, std::launder is a powerful tool that allows us to bypass certain compiler optimizations that may hinder our ability to access memory correctly. By laundering memory, we prevent the compiler from making assumptions about its contents, ensuring that we have accurate and reliable data access.

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