How it works…

For the flag definition in code, the flag package defines two types of functions.

The first type is the simple name of the flag type such as Int. This function will return the pointer to the integer variable where the value of the parsed flag is.

The XXXVar functions are the second type. These provide the same functionality, but you need to provide the pointer to the variable. The parsed flag value will be stored in the given variable.

The Go library also supports a custom flag type. The custom type must implement the Value interface from the flag package.

As an example, let's say the flag retry defines the retry limit for reconnecting to the endpoint, the prefix flag defines the prefix of each row in a log, and the array is the array flag that will be send as an payload to server. The program call from the Terminal will look like ./util -retry 2 -prefix=example array=1,2.

The important part of the preceding code is the Parse() function which parses the defined flags from Args[1:]. The function must be called after all flags are defined and before the values are accessed.

The preceding code shows how to parse some data types from the command-line flags. Analogously, the other built-in types are parsed. 

The last flag, array, demonstrates the definition of the custom type flag. Note that the ArrayType implements the Value interface from the flag package.