使用StructOpt时如何判断选项值来自用户输入还是默认值？

Problem Description #

I'm using the structopt crate and defined the following struct:

#[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();

When options.length is 50, how can I tell if this value comes from the preset default or was actively provided by the user?

Solution #

StructOpt itself doesn't track whether a value came from a default or user input directly in the parsed struct, but since it's built on top of clap, we can use clap's ArgMatches to check if the parameter was explicitly provided.

Here's a straightforward implementation:

1. Retrieve the raw ArgMatches from the StructOpt-generated CLI app alongside parsing your options.
2. Use the is_present method on ArgMatches to verify if the parameter was explicitly set by the user.

Modified code example:

use structopt::StructOpt;

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

fn main() {
    // Get raw argument matches from clap
    let matches = CommandlineOptions::clap().get_matches();
    // Parse options as usual
    let options = CommandlineOptions::from_args();
    
    // Check if "length" was explicitly provided by the user
    let is_length_provided = matches.is_present("length");
    
    if is_length_provided {
        println!("Length was explicitly set to {} by the user", options.length);
    } else {
        println!("Length uses the default value of {}", options.length);
    }
}

Key Notes: #

• matches.is_present("length") returns true in both scenarios:
  • The user runs test --length 50 (explicit flag with value)
  • The user runs test 50 (since length is a positional argument with index=1)
• This works because is_present checks if the parameter was encountered during parsing, regardless of the value. Even if the user inputs the exact same value as the default, it's still counted as explicitly provided.

内容的提问来源于stack exchange，提问作者Just a learner