Terms & Definitions¶
Definitions¶
Variables:¶
A variable is a named memory location that stores a value. The variable always has a type associated with it, which is a fixed size, determined by the cumulative size of its attributes. It serves as a reference point for accessing, modifying or performing operations on the data att hat location. A variable stores a value.
Values:¶
A value is the data that is stored in a variable. Values can represent entities such as numbers, booleans, or more complex structures, such as vectors. Values can be moved, copied, and borrowed. Note that whilst a value can be borrowed, its associated variable is said to be borrowed from, where it lends its value to another variable, but retains ownership of the value.
Owned value:¶
An owned value is a value that exists in the current scope as in a non-borrowed state. The variable it is tied to explicitly owns the value. An owned value was either created in, or moved into, the variable that contains it in the current context, and is not borrowed from another variable.
Borrowed value:¶
A borrowed value is a value borrowed from another variable. The owned value is accessed through a guaranteed valid reference, allowing temporary usage of memory managed by another variable. Borrowing enables access to a value without transferring ownership, facilitating shared or exclusive usage under controlled conditions. Borrowed values will always be valid, and cannot outlive the lifetime of the owned value that is being borrowed.
Move:¶
A “move” operation is a memory-management operation that transfers the value from one variable into another variable. After the move, the original variable no longer retains access to the value, and any attempt to use the variable results in a compile-time memory error. Typically, a move operation will occur when passing a value into a function, returning from a function, or assigning a value to a variable.
Partial Move:¶
A “partial move” operation is where ownership of a subset of an owned value is transferred to another variable, such as a field of an object. The remaining value is now in a partially-moved state, and its variable considered partially initialized. Partially initialized variables conform to strict memory rules, similar to that of non-initialized variables.
Fully initialized:¶
A fully initialized variable is a variable whose value doesn’t contain any partial moves. The entire section of memory allocated for the variable contains initialized bytes, and the variable is considered fully initialized.
Partially initialized:¶
A variable is partially initialized if its value has been partially moved. This means that whilst the entire part of memory for the size of the variable’s type is allocated, a subset of it contains no bytes, where some fields have been moved from. These empty sections are non-initialized memory, but as the collective section of memory representing the entire variable now contains a mix of initialized and non-initialized bytes, the variable is considered partially initialized.
Non-initialized:¶
A non-initialized variable is a variable that has either been defined without a value, or has been moved from. Therefore, all the bytes in the memory allocated for the variable are non-initialized, and the variable is considered non-initialized. However, a partially-initialized variable that has had all its fields moved from is still considered partially-initialized, because the rules of memory management are slightly different.
Immutable¶
An immutable value is a value whose bytes in memory cannot be changed. Once a variable is defined as immutable, there is no way for the value inside it to be modified. The section on mutability goes into more detail.