What is `std::launder` and How Does it Address Memory Management Issues in C ?

std::launder: The Significance of Memory Laundering

The introduction of std::launder in P0137 has raised questions regarding its purpose and the subsequent changes it entails in the C standard.

Purpose of std::launder

std::launder's primary function is "memory laundering," as its name suggests. It addresses a specific problem that arises when initializing unions with aggregate initialization while containing const members, as highlighted in the paper:

U u = {{ 1 }};

In this scenario, the compiler can optimize based on the assumption that the const member u.x.n will remain unchanged, even after subsequent assignments to the same memory location through a different pointer:

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

Accessing u.x.n after this assignment would incorrectly return the initial value of 1, violating the compiler's assumption that const members remain constant.

std::launder solves this problem by forcing the compiler to invalidate its optimization assumptions concerning the memory location.

Changes to the Standard

The introduction of std::launder has necessitated numerous changes to the standard in sections related to unions, lifetime, and pointers. These changes include:

• Modifying [basic.life]/8 to allow accessing a new object created in the storage of the old one after using std::launder.
• Adding examples of std::launder's usage to the standard text.
• Extending the prohibition on accessing old objects through pointers to the new one to cases where the types of the objects differ, which can be circumvented using std::launder.

Understanding the purpose and implications of std::launder is essential for programmers working with unions and managing memory usage efficiently.

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