Swift optional inout parameters and nil

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It won’t compile because the function expecting a reference but you passed nil. The problem have nothing to do with optional.

By declaring parameter with inout means that you will assign some value to it inside the function body. How can it assign value to nil?

You need to call it like

var a : MyClass? = nil
testFunc(&a) // value of a can be changed inside the function

If you know C++, this is C++ version of your code without optional

struct MyClass {};    
void testFunc(MyClass &p) {}
int main () { testFunc(nullptr); }

and you have this error message

main.cpp:6:6: note: candidate function not viable: no known conversion from 'nullptr_t' to 'MyClass &' for 1st argument

which is kind of equivalent to the on you got (but easier to understand)

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