问题
Is it possible to use parameters of this kind with boost::program_options?
program -p1 123 -p2 234 -p3 345 -p12 678
i.e., is it possible to specify the parameter name with a first token (e.g. -p) followed by a number, dynamically?
I would like to avoid this:
program -p 1 123 -p 2 234 -p 3 345 -p 12 678
回答1:
Boost.ProgramOptions does not provide direct support for this. Nevertheless, there are two general solutions that each have their trade-offs:
- Wildcard options.
- Custom parser.
Wildcard Options
If it is an acceptable to use --p instead of -p, then a wildcard option can be used. This requires iterating through the variables_map during extraction, as Boost.ProgramOptions does not provide support receiving both the key and value in an overloaded validate() function.
#include <iostream>
#include <map>
#include <string>
#include <utility>
#include <vector>
#include <boost/algorithm/string.hpp>
#include <boost/foreach.hpp>
#include <boost/lexical_cast.hpp>
#include <boost/program_options.hpp>
typedef std::map<int, int> p_options_type;
/// @brief Extract options from variable map with the a key of
/// <prefix>#*.
p_options_type get_p_options(
const boost::program_options::variables_map& vm,
const std::string prefix)
{
p_options_type p_options;
const std::size_t prefix_size = prefix.size();
boost::iterator_range<std::string::const_iterator> range;
namespace po = boost::program_options;
BOOST_FOREACH(const po::variables_map::value_type& pair, vm)
{
const std::string& key = pair.first;
// Key is too small to contain prefix and a value, continue to next.
if (key.size() < (1 + prefix.size())) continue;
// Create range that partitions key into two parts. Given a key
// of "p12" the resulting partitions would be:
//
// ,--------- key.begin -., prefix = "p"
// / ,-------- result.begin -:, post-prefix = "12"
// / / ,----- key.end, result.end -'
// |p|1|2|
range = boost::make_iterator_range(key.begin() + prefix_size,
key.end());
// Iterate to next key if the key:
// - does not start with prefix
// - contains a non-digit after prefix
if (!boost::starts_with(key, prefix) ||
!boost::all(range, boost::is_digit()))
continue;
// Create pair and insert into map.
p_options.insert(
std::make_pair(
boost::lexical_cast<int>(boost::copy_range<std::string>(range)),
pair.second.as<int>()));
}
return p_options;
}
int main(int ac, char* av[])
{
namespace po = boost::program_options;
po::options_description desc;
desc.add_options()
("p*", po::value<int>())
;
po::variables_map vm;
store(po::command_line_parser(ac, av).options(desc).run(), vm);
BOOST_FOREACH(const p_options_type::value_type& p, get_p_options(vm, "p"))
{
std::cout << "p" << p.first << "=" << p.second << std::endl;
}
}
And its usage:
./a.out --p1 123 --p2 234 --p3=345 --p12=678 p1=123 p2=234 p3=345 p12=678
This approach requires iterating over the entire map to identify wildcard matches, resulting in a complexity of O(n). Additionally, it requires a modification to the desired syntax, where --p1 123 needs to be use instead of -p1 123. This limitation is the result Boost.ProgramOptions's default parser behavior, where a single hyphen is expected to be followed by a single character.
Custom Parser
The alternative approach is to add a custom parser to the command_line_parser. A custom parser will allow -p1 syntax, as well as other common forms, such as --p1 123 and -p1=123. There are a few behaviors that need to be handled:
- A parser will receive a single token at a time. Thus, it will receive
p1and123on individual invocations. It is the parsers responsibility to pairp1to123. - Boost.ProgramOptions expects at least one parser to handle a token. Otherwise boost::program_options::unknown_option will be thrown.
To account for these behaviors, the custom parser will manage state and perform encoding/decoding:
- When the parser receives
p1, it extracts1, storing state in the parser. Additionally, it encodes a no operation value forp. - When the parser receives
123, it encodes it alongside the stored state as value forp.
Thus, if the parser receives -p1 and 123, 2 values are inserted into the variables_map for p: the no operation value and 1:123.
{ "p" : [ "no operation",
"1:123" ] }
This encoding can be transparent to the user by providing a helper function to transform the encoded p vector into a map. The result of decoding would be:
{ 1 : 123 }
Here is the example code:
#include <iostream>
#include <map>
#include <string>
#include <utility> // std::pair, std::make_pair
#include <vector>
#include <boost/algorithm/string.hpp>
#include <boost/foreach.hpp>
#include <boost/lexical_cast.hpp>
#include <boost/program_options.hpp>
typedef std::map<int, int> p_options_type;
/// @brief Parser that provides the ability to parse "-p# #" options.
///
/// @note The keys and values are passed in separately to the parser.
/// Thus, the struct must be stateful.
class p_parser
{
public:
explicit
p_parser(const std::string& prefix)
: prefix_(prefix),
hyphen_prefix_("-" + prefix)
{}
std::pair<std::string, std::string> operator()(const std::string& token)
{
// To support "-p#=#" syntax, split the token.
std::vector<std::string> tokens(2);
boost::split(tokens, token, boost::is_any_of("="));
// If the split resulted in two tokens, then key and value were
// provided as a single token.
if (tokens.size() == 2)
parse(tokens.front()); // Parse key.
// Parse remaining token.
// - If tokens.size() == 2, then the token is the value.
// - Otherwise, it is a key.
return parse(tokens.back());
}
/// @brief Decode a single encoded value.
static p_options_type::value_type decode(const std::string& encoded)
{
// Decode.
std::vector<std::string> decoded(field_count_);
boost::split(decoded, encoded, boost::is_any_of(delimiter_));
// If size is not equal to the field count, then encoding failed.
if (field_count_ != decoded.size())
throw boost::program_options::invalid_option_value(encoded);
// Transform.
return std::make_pair(boost::lexical_cast<int>(decoded[0]),
boost::lexical_cast<int>(decoded[1]));
}
/// @brief Decode multiple encoded values.
static p_options_type decode(const std::vector<std::string>& encoded_values)
{
p_options_type p_options;
BOOST_FOREACH(const std::string& encoded, encoded_values)
{
// If value is a no-op, then continue to next.
if (boost::equals(encoded, noop_)) continue;
p_options.insert(decode(encoded));
}
return p_options;
}
private:
std::pair<std::string, std::string> parse(const std::string& token)
{
return key_.empty() ? parse_key(token)
: parse_value(token);
}
/// @brief Parse key portion of option: "p#"
std::pair<std::string, std::string> parse_key(const std::string& key)
{
// Search for the prefix to obtain a range that partitions the key into
// three parts. Given --p12, the partitions are:
//
// ,--------- key.begin -., pre-prefix = "-"
// / ,-------- result.begin -:, prefix = "-p"
// / / ,----- result.end -:, post-prefix = "12"
// / / / ,-- key.end -'
// |-|-|p|1|2|
//
boost::iterator_range<std::string::const_iterator> result =
boost::find_first(key, prefix_);
// Do not handle the key if:
// - Key end is the same as the result end. This occurs when either
// either key not found or nothing exists beyond the key (--a or --p)
// - The distance from start to prefix start is greater than 2 (---p)
// - Non-hyphens exists before prefix (a--p)
// - Non-numeric values are after result.
if (result.end() == key.end() ||
distance(key.begin(), result.begin()) > 2 ||
!boost::all(
boost::make_iterator_range(key.begin(), result.begin()),
boost::is_any_of("-")) ||
!boost::all(
boost::make_iterator_range(result.end(), key.end()),
boost::is_digit()))
{
// A different parser will handle this token.
return make_pair(std::string(), std::string());
}
// Otherwise, key contains expected format.
key_.assign(result.end(), key.end());
// Return non-empty pair, otherwise Boost.ProgramOptions will
// consume treat the next value as the complete value. The
// noop entries will be stripped in the decoding process.
return make_pair(prefix_, noop_);
}
/// @brief Parse value portion of option: "#"
std::pair<std::string, std::string> parse_value(const std::string& value)
{
std::pair<std::string, std::string> encoded =
make_pair(prefix_, key_ + delimiter_ + value);
key_.clear();
return encoded;
}
private:
static const int field_count_ = 2;
static const std::string delimiter_;
static const std::string noop_;
private:
const std::string prefix_;
const std::string hyphen_prefix_;
std::string key_;
};
const std::string p_parser::delimiter_ = ":";
const std::string p_parser::noop_ = "noop";
/// @brief Extract and decode options from variable map.
p_options_type get_p_options(
const boost::program_options::variables_map& vm,
const std::string prefix)
{
return p_parser::decode(vm[prefix].as<std::vector<std::string> >());
}
int main(int ac, char* av[])
{
const char* p_prefix = "p";
namespace po = boost::program_options;
// Define options.
po::options_description desc;
desc.add_options()
(p_prefix, po::value<std::vector<std::string> >()->multitoken())
;
po::variables_map vm;
store(po::command_line_parser(ac, av).options(desc)
.extra_parser(p_parser(p_prefix)).run()
, vm);
// Extract -p options.
if (vm.count(p_prefix))
{
// Print -p options.
BOOST_FOREACH(const p_options_type::value_type& p,
get_p_options(vm, p_prefix))
{
std::cout << "p" << p.first << "=" << p.second << std::endl;
}
}
}
And its usage:
./a.out -p1 123 --p2 234 -p3=345 --p12=678 p1=123 p2=234 p3=345 p12=678
Aside from the being a larger solution, one drawback is the requirement to go through the decoding process to obtain the desired values. One cannot simply iterate over the results of vm["p"] in a meaningful way.
来源：https://stackoverflow.com/questions/15554842/boostprogram-options-parameters-with-a-fixed-and-a-variable-token