How Does std::launder Solve C Memory Issues with Unions and Const Variables?

Understanding std::launder: Memory Laundering in C

P0137 introduces std::launder as a solution to a fundamental issue in C concerning unions, lifetime, and pointers. Let's delve into the problem and explore how std::launder addresses it.

The Problem: Invalid Assumptions and Const Variables

Aggregate initialization of union members assumes const members remain unchanged. However, this assumption can lead to erroneous results when a new object is created in the place of an existing one, potentially modifying the const member.

Enter std::launder: Memory Laundering

std::launder performs memory laundering, effectively obscuring the connection between the old and new objects. It forces the compiler to reevaluate the object's state, preventing it from making invalid assumptions based on const members.

Application Examples:

1. Fixing Const Member Changes:

   struct X { const int n; };
   union U { X x; float f; };
   ...
   U u = {{ 1 }};
   X *p = new (&u.x) X {2};
   assert(*std::launder(&u.x.n) == 2);

2. Accessing New Objects through Invalid Pointers:

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

Key Features of std::launder:

• Forces the compiler to re-evaluate object state, ensuring valid optimizations and access.
• Allows access to new objects created in place of old objects, even if the type changes.
• Facilitates interaction with objects initialized by aggregate initialization.

Conclusion:

std::launder serves as a powerful tool in C to address potential memory issues and ensure correct program execution by preventing the compiler from making erroneous assumptions based on const variables or object type changes.

The above is the detailed content of How Does std::launder Solve C Memory Issues with Unions and Const Variables?. For more information, please follow other related articles on the PHP Chinese website!