Value Lifecycle

Key Concepts and Terminology

1. High-Performance Code & Memory Safety:

Mojo aims to enable the development of high-performance code while ensuring memory safety. This is significant because it addresses a common challenge in systems programming, where manual memory management can often lead to errors such as memory leaks or invalid memory access.

2. Ownership Model:

The ownership model in Mojo is crucial for understanding how the language manages memory without a garbage collector or reference counting. This model dictates how data is owned and transferred throughout a program, ensuring memory safety by enforcing rules on how data is accessed and modified.

3. Manual Memory Management:

Despite its safe defaults, Mojo allows for manual memory management. This is particularly useful in systems programming where fine-grained control over memory allocation and deallocation is often required for efficiency.

4. Data Types as Structs:

In Mojo, all data types, including primitives like Bool, Int, and String, are implemented as structs. This means there are no built-in types with special privileges, offering a uniform way to interact with all kinds of data.

5. MLIR Dialects:

Mojo provides low-level primitives through MLIR (Multi-Level Intermediate Representation) dialects, allowing developers to implement custom data types and manage memory at a lower level.

6. Value Semantics & Ownership:

Value semantics and value ownership are central to ensuring that code is safe and predictable. By adhering to these principles, even when writing "unsafe" code, the resulting behavior remains secure from the perspective of the code that uses these custom types.

7. Lifecycle Methods:

Mojo requires the explicit definition of lifecycle methods for managing the memory and resources of value types. These include constructors, copy constructors, move constructors, and destructors.

Lifecycle and Lifetimes

Lifecycle: Defined by dunder methods in a struct, it encompasses the creation, copying, moving, and destruction of values. Each type has its own set of lifecycle methods, ensuring consistent behavior across all instances of that type.
Lifetime: Refers to the period during which a value is valid within the program, from initialization to destruction. Unlike the lifecycle, the lifetime of each value is unique, as it depends on the program's execution flow.
ASAP Destruction Policy: Mojo employs an aggressive strategy for value destruction, deallocating memory as soon as a value is no longer needed. This approach minimizes memory usage but requires careful tracking of value lifetimes.