当使用structopt时，如何测试用户是否提供了选项，或者它是否来自默认值？

Question

我使用的是structopt板条箱，我有以下结构：

#[derive(Clone, StructOpt, Debug)]
#[structopt(name = "test")]
pub struct CommandlineOptions {
    #[structopt(
    long = "length",
    help = "The length of the the string to generate",
    default_value = "50",
    index = 1
    )]
    pub length: usize,
}


let options = CommandlineOptions::from_args();

如果options.length是50，我怎么知道它来自默认值50，或者用户提供的值是50？

Answer 1

我不认为用structopt可以做到这一点。解决此问题的惯用方法是使用Option<usize>而不是usize (如文档所述的here)：

use structopt::StructOpt;

#[derive(Clone, StructOpt, Debug)]
#[structopt(name = "test")]
pub struct CommandlineOptions {
    #[structopt(
    long = "length",
    help = "The length of the the string to generate",
    index = 1
    )]
    pub length: Option<usize>,
}

fn main() {
    let options = CommandlineOptions::from_args();
    println!("Length parameter was supplied: {}, length (with respect to default): {}", options.length.is_some(), options.length.unwrap_or(50));
}

如果这在您的例子中不起作用，您也可以直接使用clap::ArgMatches结构(structopt只不过是clap的宏魔术)来检查length在ArgMatches::occurrences_of中出现的次数。然而，这并不是非常惯用的。

use structopt::StructOpt;

#[derive(Clone, StructOpt, Debug)]
#[structopt(name = "test")]
pub struct CommandlineOptions {
    #[structopt(
    long = "length",
    help = "The length of the the string to generate",
    default_value = "50",
    index = 1
    )]
    pub length: usize,
}

fn main() {
    let matches = CommandlineOptions::clap().get_matches();
    let options = CommandlineOptions::from_clap(&matches);
    
    let length_was_supplied = match matches.occurrences_of("length") {
        0 => false,
        1 => true,
        other => panic!("Number of occurrences is neither 0 nor 1, but {}. This should never happen.", other)
    };
    
    println!("Length parameter was supplied: {}, length (with respect to default): {}", length_was_supplied, options.length);
}