Re: [boost] program options - default values question

From: Vladimir Prus (ghost_at_[hidden])
Date: 2003-05-07 01:05:59

Hi Neal,

Neal D. Becker wrote:
> It looks like when setting variables using
> options_description.add_options(), bool variables are set to false if no
> option is actually given, but variables of other types, such as int, are
> not initialized if not given on the command line?  This is my understanding
> based on the program behaviour - is this correct?

Yes, this is so. The behaviour is due to code in 
parameter_creator<bool>::operator(), in options_description.tcc:211
Two things are done: 
- the default value of bool parameters is "0"
- the value when options is present withouth a value (like in "--switch") is 
"1"

The rationale is that for boolean values default is quite obvious, and test 
for boolean option can be as simple as 

   if (vm["switch"].as<bool>())

For ints, there's no such obvious default, so if default value is desired it 
should be specified manually.

I've updated the docs to specify the semantics.

Thanks,
Volodya

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