Unsafe Pointers

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unsafe_pointers.fl

// Build: build/flecha build examples/unsafe_pointers.fl -o build/unsafe_pointers
// Prints 42 and exits with 42. All pointers below refer to live local storage.

unsafe void write(constptr i32 destination, i32 value) {
    // The pointer binding is fixed; its target remains writable.
    *destination = value;
}

unsafe i32 read(*const i32 source) {
    // The target is read-only, including inside this unsafe function.
    return *source;
}

*i32 pass_pointer(*i32 pointer) {
    // Copying an address does not access its target and needs no unsafe block.
    return pointer;
}

i32 main() {
    i32 value = 0;
    *i32 pointer = none;
    if pointer != null { return 1; }

    unsafe {
        pointer = &raw value;
        write(pointer, 40);
        constptr i32 fixed = pointer;
        *fixed = *fixed + 2;

        // Read-only access may be added, but never removed implicitly.
        *const i32 readOnly = fixed;
        if read(readOnly) != 42 { return 2; }

        // Nested pointers preserve each layer's qualifiers.
        **i32 slot = &raw pointer;
        if **slot != 42 { return 3; }
    }

    pointer = pass_pointer(pointer);
    unsafe {
        // value still lives here. The unsafe block did not free anything.
        if *pointer != 42 { return 4; }
    }
    stdoutln(value);
    return value;
}