[boost] [Boost Review] Do really property_tree and program_options overlap?

From: Marcin Kalicinski (kalita_at_[hidden])
Date: 2006-04-20 14:53:35

Hi Everybody,

I see there is some confusion about possible overlap of program_options 
library and property_tree libraries. I will try to present my view on the 
problem here.

First of all, my opinion is there is very little overlap. The biggest 
difference between the libraries is that property_tree is hierarchical and 
program_options is linear - i.e. options are accessible through a map-like 
interface.

Structure of file formats, like XML, JSON etc. is inherently hierarchical. 
The presence of hierarchy is their greatest strength, and most important 
feature. Large amount of information is conveyed by structure of the tree 
alone, not only by values of options it contains. One option in two 
different branches of the tree can have completely different meaning. 
Contrary to that, in program_options library there is only at best a notion 
of "positional options" which is completely different from hierarchy.

Briefly, PO library operates on a flat strings of options, property_tree is 
a DOM. Think of property_tree like a replacement for Microsoft XML parser, 
or JSON API, or Windows INI API, or registry API. Do you imagine using 
program_options to XML trees, like you get from XML parser? It is not suited 
for that, its problem domain lies in completely different place, namely 
parsing a linear string of (command-line) options, possibly supplied as a 
config file or files.

In addition to that, PO library does not support writing the structures back 
to config files. It is not a flaw with the library, it just does not need to 
do it, because options for programs are meant to be "one shot", or "read and 
forget", like command line arguments. There's not need to store them back 
where they came from.

On the other hand, PO library supports many things that do not belong to 
property_tree. These include, for example, options descriptions and notify() 
mechanisms.

Taking all that into account I do not see why anybody insists there might 
exist 1:1 correspondence between the libraries. I agree both can parse 
command lines, but that's about it. Other functionality is very different.

Best regards,
Marcin