Golang const不安全。

Question

Don't understand why I can do

const OK = uint64(0)

const OK = int(unsafe.Sizeof(uint64(0)))

but not this?

const NOK = binary.Size(uint64(0))

Answer 1

It's explained in the specification.

Package unsafe is implemented in the compiler. The expression unsafe.Sizeof(uint64(0)) can be evaluated at compile time. It is a constant expression.

The function plain function call binary.Size(uint64(0)) cannot be evaluated at compile time. It is not constant expression.

Constant declarations require a constant expression.