Understanding Rust Error E0745: Address of Temporary Value

Overview of Rust Error E0745

Rust Error E0745 occurs when the address of a temporary value is taken. This is not a valid operation because the temporary value is destroyed immediately after the assignment, causing the pointer to reference an unavailable memory location.

Erroneous Code Example

#![feature(raw_ref_op)]
fn temp_address() {
    let ptr = &raw const 2; // error!
}

In this example, the address of the temporary value '2' is taken using the raw reference operator. However, since '2' is a temporary value, it is destroyed immediately after the assignment, which results in 'ptr' pointing to an invalid memory location.

Solution

To fix Rust Error E0745, bind the temporary value to a named local variable before taking its address. This will ensure that the value stays in scope and the address remains valid.

Corrected Code Example

#![feature(raw_ref_op)]
fn temp_address() {
    let val = 2;
    let ptr = &raw const val; // ok!
}

In this corrected example, we create a named local variable 'val' and assign it the value '2'. We then take the address of 'val' using the raw reference operator, which is a valid operation since 'val' is now in scope and the address is still available.

Deeper Dive into Temporary Values

Rust enforces strict memory safety rules to prevent common programming errors, such as use-after-free and dangling references. As a result, the language does not allow taking the address of a temporary value, since it may lead to invalid memory access.

Rust's 'Drop' trait is automatically called on temporary values when they go out of scope, effectively deallocating their memory.

In cases where you need to capture the address of an object, ensure it is assigned to a named local variable that has a longer lifetime to avoid running into Rust Error E0745.