memory-asap-and-destructor-behaviours.md

🧹 ASAP: will call __del__ when last used

with v0.4.0

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It's wonderfull because mojo free memory as soon as possible.

Lifetimes is on the roadmap: Ownership and Lifetimes.

source: Behavior of destructors

using that struct as an example:

@value
struct ValuePointer:
    
    var pointer: Pointer[Int]

    #instantiate
    def __init__(inout self):
        #allocate memory
        self.pointer = Pointer[Int].alloc(1)
        #store 123
        self.pointer.store(0,123)

    #delete
    def __del__(owned self):
        #free memory
        self.pointer.free()

the challenge:

def main():

    var original = ValuePointer()

    #original is deleted here after last use/access:
    var copy_of_pointer = original.pointer

    #copy now point to freed memory
    print(copy_of_pointer.load(0))

1. original will get deleted after it's last use, the pointer inside will be freed by our __del__ function.
2. copy_of_pointer now point to freed memory and unknown data is printed

the solution:

def main():

    var original = ValuePointer()

    var copy_of_pointer = original.pointer

    print(copy_of_pointer.load(0))

    #original is deleted here after last use/access:
    _ = original

by assigning original to _ , it's last use will be there.

copy_of_pointer no will longer point to freed memory.

123 got printed.