How are Deferred Function Arguments Evaluated in Go?

Understanding Deferred Function Argument Evaluation

In the context of Go's defer statement, it's crucial to grasp how function arguments are handled. The statement reads as follows: "The deferred call's arguments are evaluated immediately, but the function call is not executed until the surrounding function returns."

Immediate Evaluation of Deferred Arguments

The "arguments" in this statement refer to the parameters passed to the deferred function. When a defer statement is encountered, these arguments are evaluated immediately, regardless of when the deferred function is actually executed. This implies that the following operations occur:

• The function expression used in the defer call is evaluated to obtain a function value.
• The arguments passed to the deferred function are evaluated using the current context and values.

Example

Consider the following code snippet:

func def(s string) func() {
    fmt.Println("tier up")
    fmt.Println(s)
    return func() { fmt.Println("clean up") }
}

func main() {
    defer def("defered line")()
    fmt.Println("main")
}

//Output:
//tier up
//defered line
//main
//clean up

In this example, the defer def("defered line")() statement immediately evaluates the arguments, which are passed to the def function. This means that the string defered line is immediately evaluated and stored for later use.

Execution of Deferred Functions

Once the arguments are evaluated, the execution of the deferred function is postponed until the surrounding function returns. This is where the second part of the quote comes into play: "the function call is not executed until the surrounding function returns."

In the example above, the def function is deferred within the main function. When the main function returns, the deferred function is executed, printing "clean up."

Conclusion

By understanding the immediate evaluation of deferred function arguments, developers can effectively manage the order of execution and data availability in their Go code. This allows for flexible control over program flow and resource management.

The above is the detailed content of How are Deferred Function Arguments Evaluated in Go?. For more information, please follow other related articles on the PHP Chinese website!