Is type casting a costly operation in Golang?

Question

I am working with mongo and many of the mongo-driver methods return a slice or a map of interface{}. While processing these results I sometimes need the actual value of the returned results and have to do something like result.(string)

I am concerned whether this can turn out to be a costly operation if there are too many type casts (inside for loops) being done in my code

Also what is the difference between type assertions and type casting

Sumit Sharma · Accepted Answer

Go doesn't have type casts the way C does; instead, we have type conversions and type assertions, and you need to choose the one that's right for the situation. (Both would be a cast in C) Conversions ask the compiler to change the representation of one value into another. For example, truncating a float32 into an int. This is required whenever the underlying types are different, or if the types have different names but the same underlying type. This is generally pretty performant, with the exception of converting between byte slice and string, which incurs an allocation and copy, which can be quite costly. What you are using here, though, is a type assertion, which is required when you have an interface but need to pull out the concrete type from it (or check if it matches a different / broader interface). For a concrete type check, this is pretty fast, because an interface is basically a type pointer and a value pointer, so you can think of it like a conditional... If the type pointer matches, extract the value. So, could this be slow? Yes, but generally your latency to your user and/or the database will be much larger, so unless you are doing it millions or billions of times, there are probably other things which can be optimized first if you find it is getting slow.

Aditya Agrawal · Answer

I’d like to mention that converting between byte slices and strings does not always require an allocation. See, eg, the bytes package for Go 1.13 or map[(string(x)]. The compiler is getting smarter.

Owen Williams · Answer

So, could this be slow? Yes, but (...) there are probably other things which can be optimized first Yes, in other words: do not optimize prematurely. If you are curious or paranoid and an alternative way to get your data from the mongo driver exists, you could benchmark both and see which one performs better.

Go Language

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