Sane way to use boost::program_options with INI and command line? - c++

Question

I want to write a program that parses a config file, and allows the command line to override what's written there. So I can have a config file that says:
[section1]
opt1=42
[section2]
opt2=17
And then, I can run the command with:
./so --opt2=3
And the program will get opt1 as 42 and opt2 and 3. I use the following program to try and do it:
#include <fstream>
#include <boost/program_options.hpp>
namespace po = boost::program_options;
int main(int argc, char *argv[]) {
po::options_description options1("section1");
options1.add_options()
("opt1", po::value<int>(), "Option 1");
po::options_description options2("section2");
options2.add_options()
("opt2", po::value<int>(), "Option 2");
po::options_description options;
options.add(options1);
options.add(options2);
po::variables_map values;
po::store( po::command_line_parser( argc, argv ).options(options).run(), values );
std::ifstream iniFile( "options.ini" );
po::store(
parse_config_file( iniFile, options ),
values );
}
This, of course, doesn't work. Boost::program_options wants opt1 under section1 to be called section1.opt1. If I do that, however, my program becomes harder to maintain on two fronts:
I need to define two options_descriptions, one for the INI and one for the command line.
Since the options' keys are now different, I need to manually merge the two.
Is there a way achieve this without doing the work manually?

Answer 1

The trivial solution is, to not use the sections. There might be some confusion around "sections" in the options descriptions vs. sections in ini-files.
The sections in ini-files refer only to options named with embedded periods: "section1.opt1". So you can simply write the config file as:
opt1=42
# perhaps with a comment
opt2=17
See it Live On Coliru
#include <boost/program_options.hpp>
#include <fstream>
#include <iostream>
namespace po = boost::program_options;
int main(int argc, char* argv[]) {
po::options_description options;
options.add_options()
("opt1", po::value<int>(), "Option 1")
("section2.opt2", po::value<int>(), "Option 2");
std::cout << options << "\n";
po::variables_map values;
po::store(po::parse_command_line(argc, argv, options), values);
std::ifstream iniFile("options.ini");
po::store(parse_config_file(iniFile, options), values);
auto report = [&values](char const* name) {
if (auto opt = values[name]; !opt.empty())
std::cout << name << ": " << opt.as<int>() << "\n";
};
report("opt1");
report("opt2");
report("section1.opt1");
report("section2.opt2");
}
Prints
echo "opt1=42" >> options.ini; ./a.out --section2.opt2 99
--opt1 arg Option 1
--section2.opt2 arg Option 2
opt1: 42
section2.opt2: 99