Let's start directly from the two Go language applet: * * program 1** ' gopackage mainimport "FMT" var (a int = b + 1b int = 1) func main () {FMT. Println (a) fmt. Println (b)} "* * program 2**" "Gopackage mainimport" FMT "Func Main () {var (a int = b + 1b int = 1) fmt. Println (a) fmt. Println (b)} "if these two pieces of code output the same result, then they are not good material, fortunately, their results are different: * * program 1** ' 21" * * Program 2** This program can not be compiled, and even in the 7th line to report a compile-time error " Undefined:b ". What is it that has brought about this discrepancy? The "normal" initialization expression in the variable declaration, initialized from left to right and top to bottom as you expect: ' ' Gofunc f () int {fmt. Println ("F"); Return 1}func g () int {fmt. Println ("G"); Return 2}func h () int {fmt. Println ("H"); Return 3}func Main () {var (a int = f () b int = g () c int = h ()) Fmt. Println (A, B, c)} "output:" FGH1 2 3 "" normal "means that it is initialized in its own function. When these initialization code is placed in the top-level declaration of a package as in Program 1, it becomes more and more interesting: "' Gopackage mainimport" FMT "var (a = C-2B = 2c = f ()) func f () int {fmt. PRINTF ("inside f and B =%d\n", b) return B + 1}func main () = FMT. Println (a) fmt. Println (b) fmt. Println (c)} "' These variables are declared in the following order: * B is the first because it does not depend on other uninitialized variables * C is the second, after initialization of the variable B required by the ' F ' function, followed by initialization * A is the output of the handler in the third round initialization loop after C is initialized For: "' Inside F andb = 2123 "in the order of declaration, each initialization process selects the first variable that can be initialized. The entire process continues until all variables are initialized or the compiler finds a loop similar to the following: "' Gopackage mainimport" FMT "var (a = BB = CC = f ()) func f () int {return A}func main () {FMT. The code above PRINTLN (A, B, c)} "will cause the compile-time error" initialization loop ". The initialization dependency mechanism works based on the package level: **sandbox.go** "Gopackage mainimport" FMT "var (a = C-2B = 2) func main () {FMT. Println (a) fmt. Println (b) fmt. Println (c)} "**utils.go**" "gopackage mainvar C = f () func f () int {return B + 1} ' ' Compile and output: ' ' 123 ' if you like the above, please follow me, To accelerate the development of future stories.
via:https://medium.com/golangspec/initialization-dependencies-in-go-51ae7b53f24c
Author: Michałłowicki Translator: Rxcai proofreading: polaris1119
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