The legality of initializing a `thread_local` variable in the destructor of a global variable in C++ is a nuanced topic that touches on several aspects of the language's standard concerning the order of initialization and destruction. In this article, we'll explore these technical details, provide examples, and summarize the implications in a tabular format.

Understanding `thread_local` Storage Duration

In C++, `thread_local` variables have thread storage duration. This means they are initialized once per thread and are destroyed when the thread exits. Notably, `thread_local` variables:

• Can exist as global or local.
• Have their initialization and destruction tied to the lifetime of the thread.

Initialization Order of Global Variables

Global variables in C++ are initialized in the order they are defined within a translation unit; however, across translation units, the initialization order is undefined. The destruction order is the reverse of initialization.

`thread_local` Interaction with Global Destructors

When a `thread_local` variable is initialized in a destructor of a global variable, we run the risk of entering undefined behavior territory. This is because the C++ standard does not guarantee the order of thread exit and the completion of global destructors.

Example of Potential Issues

Let's explore a simple code example:

• The standard specifies that `thread_local` objects are destroyed after their respective threads exit.
• Global destructors may be executed after main thread completion, but before other thread exits, leading to possible contradictions.
• Avoid initializing `thread_local` variables in global destructors.
• If the use of `thread_local` is necessary in a destructor context, design the software to ensure these destructors are executed after all related threads have completed.