C#•2y ago

❔ Pointer Issue.

My pathetic code:

c#
unsafe class Test
{
    static void Main()
    {
        int a = 1; int b = 2;
        int* x = &a, y = &b;
        Console.WriteLine($"{*x} and {*y}");
        Swap(out x, out y);
        Console.WriteLine($"{*x} and {*y}");
    }

    static void Swap(out int* a, out int* b)
    {
        long x = (long)a;
        long y = (long)b;
        x ^= y ^= x ^= y;
        a = (int*)x; b = (int*)y;
    }
}

c#
unsafe class Test
{
    static void Main()
    {
        int a = 1; int b = 2;
        int* x = &a, y = &b;
        Console.WriteLine($"{*x} and {*y}");
        Swap(out x, out y);
        Console.WriteLine($"{*x} and {*y}");
    }

    static void Swap(out int* a, out int* b)
    {
        long x = (long)a;
        long y = (long)b;
        x ^= y ^= x ^= y;
        a = (int*)x; b = (int*)y;
    }
}

Terrible error:

Error    CS0269    Use of unassigned out parameter 'a'    ConsoleApp1
Error    CS0269    Use of unassigned out parameter 'b'    ConsoleApp1

Error    CS0269    Use of unassigned out parameter 'a'    ConsoleApp1
Error    CS0269    Use of unassigned out parameter 'b'    ConsoleApp1

Why?

12 Replies

Angius•2y ago

Well, you're never using the out parameters You do Swap(out x, out y) but then Console.WriteLine($"{a} and {b}") So either just not have those be out, or use the x and y Also, as a side note, swapping two variables is just (a, b) = (b, a) in case you were actually intending to use this code lol

thirteenbinaryOP•2y ago

I did make some mistakes so I change the code. But it is not the point. The point is long x = (long)a; makes compiler unhappy. Fact, the x is &a. How can it say it is unassigned. I just wanna try to swap the address in the pointer.

Angius•2y ago

¯\_(ツ)_/¯ I never had any need to go even remotely close to pointers in C#, so I won't be of help there If nobody insane enough to use pointers in C# comes by this thread, you could try asking in #advanced

thirteenbinaryOP•2y ago

Thank u anyway

Jimmacle•2y ago

you really shouldn't need pointers for anything besides pinvoke stuff or you're micro-optimizing because you know better than the JIT

Aaron•2y ago

out parameters are uninitialized until you assign to them you have effectively done

int* a;
long x = (long)a;

int* a;
long x = (long)a;

as far as the compiler is concerned, you haven't given a a value yet so it won't let you use it you don't even need pointers to see this error

MODiX•2y ago

Windows10CE#8553

REPL Result: Failure

static void Test(out int i)
{
    uint u = (uint)i;
}

static void Test(out int i)
{
    uint u = (uint)i;
}

Exception: CompilationErrorException

- Use of unassigned out parameter 'i'
- The out parameter 'i' must be assigned to before control leaves the current method

- Use of unassigned out parameter 'i'
- The out parameter 'i' must be assigned to before control leaves the current method

Compile: 414.713ms | Execution: 0.000ms | React with ❌ to remove this embed.

Jimmacle•2y ago

should Swap's parameters even be out? it's already passing a pointer in, so it's turning into effectively a double pointer

Aaron•2y ago

yes thats the point its swapping pointers the sane way to write this is

static void Swap(ref int* a, ref int* b)
{
    (a, b) = (b, a);
}

static void Swap(ref int* a, ref int* b)
{
    (a, b) = (b, a);
}

Jimmacle•2y ago

seems unnecessary as a whole

Aaron•2y ago

correct!

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Pointer Issue.