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Options are declared with a long and an optional short option. A descriptionmust be provided. The third argument is the value, if omitted it is boolean.Any type can be given as long as it can be parsed, with operator>>.
To parse the command line do:
auto result = options.parse(argc, argv);
To retrieve an option useresult.count("option") to get the number of timesit appeared, and
result["opt"].as<type>()
to get its value. If "opt" doesn't exist, or isn't of the right type, then anexception will be thrown.
Boolean values
Boolean options have a default implicit value of"true", which can beoverridden. The effect is that writing-o by itself will set optiono totrue. However, they can also be written with various strings using=value.There is no way to disambiguate positional arguments from the value followinga boolean, so we have chosen that they will be positional arguments, andtherefore,-o false does not work.
Vector values
Parsing of list of values in form of anstd::vector<T> is also supported, as long asTcan be parsed. To separate single values in a list the ',' is used by default.Ensure that you use no whitespaces between values because those would be interpretedas the next command-line option. Example for a command-line optionthat can be parsed as astd::vector<double>:
--my_list=1,-2.1,3,4.5
To set up custom separation character (semicolon, for example) use:
An implicit value is the value that an option takes when it is given on thecommand line without an argument. The following specifies an implicit value:
If an option had both, then not specifying it would give the value"value",writing it on the command line as--option would give the value"implicit",and writing--option=another would give it the value"another".
Note that the default and implicit value is always stored as a string,regardless of the type that you want to store it in. It will be parsed asthough it was given on the command line.
Options specified multiple times
The same option can be specified several times, with different arguments, which will allbe recorded in order of appearance. An example:
--use train --use bus --use ferry
this is supported through the use of a vector of value for the option:
options.add_options() ("use", "Usable means of transport", cxxopts::value<std::vector<std::string>>())
Positional Arguments
Positional arguments are those given without a preceding flag and can be usedalongside non-positional arguments. There may be multiple positional arguments,and the final positional argument may be a container type to hold a list of allremaining positionals.
To set up positional arguments, first declare the options, then configure aset of those arguments as positional like:
options.add_options() ("script","The script file to execute", cxxopts::value<std::string>()) ("server","The server to execute on", cxxopts::value<std::string>()) ("filenames","The filename(s) to process", cxxopts::value<std::vector<std::string>>());options.parse_positional("script","server","filenames");// Parse options the usual wayoptions.parse(argc, argv);
will result in parsed arguments like the following table:
Field
Value
"script"
"my_script.py"
"server"
"my_server.com"
"filenames"
{"file1.txt", "file2.txt", "file3.txt"}
Unrecognised arguments
You can allow unrecognised arguments to be skipped. This applies to bothpositional arguments that are not parsed into another option, and--arguments that do not match an argument that you specify. This is done bycalling:
options.allow_unrecognised_options();
and in the result object they are retrieved with:
result.unmatched()
Exceptions
Exceptional situations throw C++ exceptions. There are two types ofexceptions: errors defining the options and errors when parsing a list ofarguments. All exceptions derive fromcxxopts::option_error. Errorsdefining options derive fromcxxopts::spec_error and errorsparsing arguments derive fromcxxopts::parse_error.
All exceptions define awhat() function to get a printable stringexplaining the error.
Help groups
Options can be placed into groups for the purposes of displaying help messages.To place options in a group, pass the group as a string toadd_options. Then,when displaying the help, pass the groups that you would like displayed as avector to thehelp function.
Custom help
The string after the program name on the first line of the help can becompletely replaced by callingoptions.custom_help. Note that you mightalso want to override the positional help by callingoptions.positional_help.
Custom parsers
template<>structcxxopts::value_parser<custom_type> {using value_type = custom_type;/// Is this a container type?staticconstexprbool is_container = <true> | <false>;voidparse(const cxxopts::parse_context& ctx,const std::string& text, custom_type& value) {// parse value from text here }};
Command / subcommand pattern
In case then a program has some global options and specific sets of optionsfor each subcommand, the following technique can be used.