Initializing variables at runtime in Rust can seem challenging, especially when dealing with global or static data that depends on values not known at compile time. Unlike simple constants or statics that can be initialized with literal values or constant expressions, many real-world scenarios require setup based on configuration files, environment variables, or the results of complex functions executed during program execution.
The standard Rust static declaration requires an initializer that can be evaluated at compile time. This restriction prevents direct assignment of values that are computed during the program’s startup phase or later. However, there are effective patterns and tools to handle this common requirement.
One primary approach to achieving runtime initialization for static or global data involves deferring the actual creation of the value until it’s first needed. This is often referred to as lazy initialization.
A powerful solution is provided by the once_cell crate, which offers two key types: Once and Lazy. Once<T> guarantees that a computation will be performed exactly once to initialize a shared value of type T. It’s ideal when you have a value that needs to be computed once, and subsequent accesses should return the same value without recomputing. Lazy<T> is a more general form of lazy static initialization. You provide a closure that computes the value, and the Lazy type handles executing that closure only on the first access, storing the result for all future uses. This is thread-safe and generally considered the modern, idiomatic way to handle many such scenarios in Rust.
Before once_cell became the de facto standard, the lazy_static crate was widely used. It provides a declarative macro that simplifies the creation of lazily initialized statics. While still functional, once_cell is often preferred now as it’s a more fundamental building block and is being integrated into the standard library.
For scenarios requiring thread safety and potential mutability after initialization, combining an Option with a synchronization primitive like std::sync::Mutex is a fundamental pattern. You can declare a static Mutex wrapping an Option<T>. The first time you need the value, you acquire the lock on the mutex, check if the Option is None, compute the value if necessary, store it in the Option (making it Some), and then release the lock. Subsequent accesses would find the value in the Option. This approach gives fine-grained control but is more verbose than using once_cell.
Choosing the right method depends on the specific needs: is the value truly global and needed throughout the program? Does it need to be mutable after initialization? Is performance critical on the first access? For most cases involving static or global values initialized at runtime, the once_cell crate, particularly Lazy, offers a clean, safe, and efficient solution. Mastering these techniques is crucial for writing robust Rust applications that require flexible setup.
Source: https://itnext.io/runtime-initialized-variables-in-rust-4ec98afe8dcd?source=rss—-5b301f10ddcd—4
