Modular•5mo ago

UnsafePointer & Structs

would anyone be able to help me understand what is happening here? why are the outputs of the two print statements different?

@value
struct Test:
    
    var prev: UnsafePointer[Test]
    var label: String

    fn __init__(inout self, label: String):
        self.label = label
        self.prev = UnsafePointer[Test]()

    fn __init__(inout self, label: String, prev: Test):
        self.label = label
        self.prev = UnsafePointer[Test].address_of(prev)

    fn __str__(self) -> String:
        return "Test: " + self.label

    fn next(self, label: String) -> Test:
        return Test(label, self)

fn main():

    var x = Test("x")

    print(x.next("y").prev[])
    var y = x.next("y")
    print(y.prev[])

@value
struct Test:
    
    var prev: UnsafePointer[Test]
    var label: String

    fn __init__(inout self, label: String):
        self.label = label
        self.prev = UnsafePointer[Test]()

    fn __init__(inout self, label: String, prev: Test):
        self.label = label
        self.prev = UnsafePointer[Test].address_of(prev)

    fn __str__(self) -> String:
        return "Test: " + self.label

    fn next(self, label: String) -> Test:
        return Test(label, self)

fn main():

    var x = Test("x")

    print(x.next("y").prev[])
    var y = x.next("y")
    print(y.prev[])

The output is:

Test: x
Test:

Test: x
Test:

3 Replies

tcorpseOP•5mo ago

if I change label to be an Int, it works fine

Frank Saez•5mo ago

I've tried this

@value
struct Test(Stringable):
    
    var prev: UnsafePointer[Test]
    var label: String

    fn __init__(inout self, label: String):
        self.label = label
        self.prev = UnsafePointer[Test]()

    fn __init__(inout self, label: String, prev: Self):
        self.label = label
        self.prev = UnsafePointer[Test](prev)


    fn __str__(self) -> String:
        return "Test: " + self.label

    fn next(self, label: String) -> Self:
        return Test(label, self)

fn main():

    var x = Test("x")
    var y = x.next("y")
    var z = y.prev[]
    print(x)
    print(y)
    print(z)

@value
struct Test(Stringable):
    
    var prev: UnsafePointer[Test]
    var label: String

    fn __init__(inout self, label: String):
        self.label = label
        self.prev = UnsafePointer[Test]()

    fn __init__(inout self, label: String, prev: Self):
        self.label = label
        self.prev = UnsafePointer[Test](prev)


    fn __str__(self) -> String:
        return "Test: " + self.label

    fn next(self, label: String) -> Self:
        return Test(label, self)

fn main():

    var x = Test("x")
    var y = x.next("y")
    var z = y.prev[]
    print(x)
    print(y)
    print(z)

and it gives me :

Test: x
Test: y
Test: x

Test: x
Test: y
Test: x

I guess the difference is in :

self.prev = UnsafePointer[Test](prev)

self.prev = UnsafePointer[Test](prev)

sora•4mo ago

I think you hit a bug of Mojo's ASAP destruction: x is deleted on the line var y = x.next("y") Try adding _ = x; _ = y to the end of your file

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UnsafePointer & Structs