Go bits: Interfaces and Nil Pointers
source link: https://medium.com/@TonyBologni/go-bits-interfaces-and-nil-pointers-7eeee006118e
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Go bits: Interfaces and Nil Pointers
Different types of nil in Go and how to check against them
Go has interfaces, pointer types, and sadly also nil pointers.
Trying to access properties of nil types results in a runtime panic, in other words, the program dies:
Therefore, sensible code needs to check for those nil pointers.
The situation gets a bit more complicated once interfaces are involved because now there are three possible types of nil:
- nil pointer types
- nil interfaces
- interfaces with an underlying nil pointer type
A simple example
We need an interface:
and a pointer type implementing this interface:
I’ll use tests to be able to run the code easily (real tests should have assertions for sure …):
As we can see those
blocks check for nil and only access the CommandType() getter method if the command is not nil.
The problem appears in the last test case with the interfaceWithUnderlyingNilPointer
The interface itself is not nil as it is a typed interface — hence we run into a runtime panic when we dereference the underlying nil pointer.
A more realistic example
Let’s define another command type that fulfills the interface:
And a Command Handler which handles all those Commands:
As we can see the type switch turns the interface type into the underlying types — the implementations of the Command interface.
This is how we can check if the underlying type of an interface is a nil pointer — by casting it to the concrete type!
So far, so good. But we need to do the same nil pointer checks over and over, for each concrete Command.
We could use reflection to avoid those duplicate checks. But 1) reflection should be used sparingly and 2) each method should check its input instead of blindly trusting the caller (this is opinionated).
Finally, a test that proves we catch all types of nil here:
The second assertion in each test proves that we get back the expected error — either “command is nil interface” or “command is nil pointer”.
Additionally, the second test case shows that we can even pass untyped nil to a function. Go’s type inference turns it into a typed interface with a nil value.
I need to add that when the command arrives in the Handle() method we get the same type of thing in test cases three and four. That’s type inference in action again, which turns the nilCommand into an interfaceWithUnderlyingNilPointer. I just added test case four to show it explicitly.
I have stopped using pointer types for anything that is a data type and not a service (handler, repository, …). Not only but also to avoid this nil pointer trouble. No matter if you do the same or keep using pointer types for everything — I hope this article helps to avoid the nil pointer behind an interface pitfall. :-)
If you like this article — please don’t forget to clap or even follow me! :-)
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