Frustration-free command line processing
So many different command line processing libraries out there and none of them just work!
Some bring their whole extended family of related and unrelated external dependencies (yes, I'm looking at you Boost).
Some require quirky syntax and/or very verbose setups that sacrifice simplicity for the generation of a cute usage message and validation. Many come to dominate your main() file and yet others do not build on multiple plaforms - for some even their own tests and trivial usage cause crashes on some systems. Argh!
If you're writing a highly-sophisticated command line tool, then Boost.Program_options and its kind might give you many advanced options. However, if you need to get up and running quickly, effectively and with minimal fuss, give the single header-file argh a try.
It doesn't get much simpler than this:
#include <iostream>
#include "argh.h"
int main(int, char* argv[])
{
argh::parser cmdl(argv);
if (cmdl[{ "-v", "--verbose" }])
std::cout << "Verbose, I am.\n";
return EXIT_SUCCESS;
}- Arguments over Arguments - Adi Shavit - Core C++ 2019
- Arguments over Arguments, but you already know this... - Adi Shavit - CppCon 2019
Contrary to many alternatives, argh takes a minimalist laissez-faire approach, very suitable for fuss-less prototyping with the following rules:
The API is:
- Minimalistic but expressive:
- No getters nor binders
- Just the
[]and()operators - Easy iteration (range-
fortoo)
- You don't pay for what you don't use
- Conversion to typed variables happens (via
std::istream >>) on the user side after the parsing phase - No exceptions thrown for failures
- Liberal BSD license
- Single header file
- No non-
stddependencies
argh does not care about:
- How many '
-' preceded your option - Which flags and options you support - that is your responsibility
- Syntax validation: any command line is a valid (not necessarily unique) combination of positional parameters, flags and options
- Automatically producing a usage message
Create parser:
auto cmdl = argh::parser(argc, argv);In fact, you can even drop argc. This will also work:
argh::parser cmdl(argv);Positional argument access by (integer) index with [<size_t>]:
cout << "Exe name is: " << cmdl[0] << '\n';
^^^
assert(cmdl[10000].empty()); // out-of-bound index returns empty string
^^^^^Boolean flag argument access by (string) name with [<std::string>]:
cout << "Verbose mode is " << ( cmdl["verbose"] ? "ON" : "OFF" ) << '\n';
^^^^^^^^^^^Any dashes are trimmed so are not required.
Your flag can have several alternatives, just list them with [{ "<name-1>", "<name-2>", ... }]:
cout << "Verbose mode is " << ( cmdl[{ "-v", "--verbose" }] ? "ON" : "OFF" ) << '\n';
^^^^^^^^^^^^^^^^^^^^^^^Beyond bool and std::string access with [], as shown above, we can also access the argument values as an std::istream. This is very useful for type conversions.
std::istream positional argument access by (integer) index with (<size_t>):
std::string my_app_name;
cmdl(0) >> my_app_name; // streaming into a string
^^^
cout << "Exe name is: " << my_app_name << '\n';We can also check if a particular positional arg was given or not (this is like using [<std::string>] above):
if (!cmdl(10))
cerr << "Must provide at least 10 arguments!" << '\n';
else if (cmdl(11))
cout << "11th argument is: " << cmdl[11] << '\n';But we can also set default values for positional arguments. These are passed as the second argument:
float scale_factor;
cmdl(2, 1.0f) >> scale_factor;
^^^^^^^If the position argument was not given or the streaming conversion failed, the default value will be used.
Similarly, parameters can be accessed by name(s) (i.e. by string or list of string literals) with:
(<std::string> [, <default value>]) or ({ "<name-1>", "<name-2>", ... } [, <default value>]):
float scale_factor;
cmdl("scale", 1.0f) >> scale_factor; // Use 1.0f as default value
^^^^^^^^^^^^^
float threshold;
if (!(cmdl({ "-t", "--threshold"}) >> threshold)) // Check for missing param and/or bad (inconvertible) param value
cerr << "Must provide a valid threshold value! Got '" << cmdl("threshold").str() << "'" << endl;
else ^^^^^^
cout << "Threshold set to: " << threshold << '\n';As shown above, use std::istream::str() to get the param value as a std:string or just stream the value into a variable of a suitable type. Standard stream state indicates failure, including when the argument was not given.
When using multiple names, the first value found will be returned.
Positional arguments can be iterated upon directly using range-for:
cout << "Positional args:\n";
for (auto& pos_arg : cmdl)
cout << '\t' << pos_arg << '\n';Similarly, cmdl.size() will return the count of positional arguments.
Positional arguments, flags and parameters are accessible as "ranges":
cout << "Positional args:\n";
for (auto& pos_arg : cmdl.pos_args())
cout << '\t' << pos_arg << '\n';
cout << "\nFlags:\n";
for (auto& flag : cmdl.flags())
cout << '\t' << flag << '\n';
cout << "\nParameters:\n";
for (auto& param : cmdl.params())
cout << '\t' << param.first << " : " << param.second << '\n';If a parameter appears several times in the command line, all its duplicates may be accessed, in order, like so:
cout << "\nValues for all `--input` parameters:\n";
for (auto& param : cmdl.params("input")) // iterate on all params called "input"
cout << '\t' << param.first << " : " << param.second << '\n';By default, options are assumed to be boolean flags. When this is not what you want, there are several ways to specify when an option is a parameter with an associated value.
-
Specify
PREFER_PARAM_FOR_UNREG_OPTIONmode to interpret any<option> <non-option>as<parameter-name> <parameter-value>:using namespace argh; auto cmdl = parser(argc, argv, parser::PREFER_PARAM_FOR_UNREG_OPTION); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ cout << cmdl("--threshold").str() << '\n';
-
Pre-register an expected parameter name with
add_param()(before callingparse()):argh::parser cmdl; cmdl.add_param("threshold"); // pre-register "threshold" as a param: name + value cmdl.parse(argc, argv); cout << cmdl("threshold").str() << '\n';
You may also batch pre-register multiple options as parameters with
add_params({ ... }):argh::parser cmdl; cmdl.add_params({ "-t", "--threshold", "-s", "--scale" }); // batch pre-register multiple params: name + value cmdl.parse(argc, argv); cout << cmdl("threshold").str() << '\n';Unregistered options will default to boolean flags.
-
Since pre-registration has to be done before parsing, we might as well just use the ctor:
argh::parser cmdl({ "-t", "--threshold", "-s", "--scale" }); // batch pre-register multiple params: name + value cmdl.parse(argc, argv); cout << cmdl("threshold").str() << '\n';
-
Use a
=with no spaces around it within the option when calling the app:>> my_app --threshold=42 42
This will automatically be interpreted as a named parameter-value pair.
- By default, arguments of the form
--<name>=<value>(with no spaces, one or more dashes), e.g.--answer=42, will be parsed as<parameter-name> <parameter-value>. To disable this specify theNO_SPLIT_ON_EQUALSIGNmode. - Specifying the
SINGLE_DASH_IS_MULTIFLAGmode, a.k.a Compound Arguments, will split a single-hyphen argument into multiple single-character flags (as is common in various POSIX tools). - When using
SINGLE_DASH_IS_MULTIFLAG, you can still pre-register the last character as a param with the value, such that if we pre-registerfas a param,>> myapp -xvf 42will be parsed with two boolean flagsxandvand a one paramf=42. - When parsing parameter values as strings that may contain spaces (e.g.
--config="C:\Folder\With Space\Config.ini"), prefer using.str()instead of>>to avoid the default automatic whitespace input stream tokenization:
cout << cmdl({ "-c", "--config" }).str().
Any command line is composed of 2 types of Args:
- Positional Args:
Free standing, in-order, values
e.g.config.json - Options:
Args beginning with-(and that are not negative numbers).
We identify 2 kinds of Options:- Flags:
Boolean options => (appear ? true : false)
e.g.-v,--verbose - Parameters:
A named value followed by a non-option value
e.g.--gamma 2.2
- Flags:
Thus, any command line can always be broken into some combination of (1) positional args (2) flags and (3) parameters.
Parse the command line using either
- The
parse()method:parser::parse([argc,] argv [, mode]); or - The shorter form using the ctor directly:
argh::parser([argc,] argv [, mode]); - The shortest form does not even require
argc, so in defaultmodejust use:
parser(argv);
Extra flexibility can be added by specifying parsing modes:
NO_SPLIT_ON_EQUALSIGN: By default, an option of the form--pi=22/7will be parsed as a parameterpiwith an associated value"22/7". By setting this mode, it will be not be broken at the=.PREFER_FLAG_FOR_UNREG_OPTION: Split<option> <non-option>into<flag>and<pos_arg>. e.g.myapp -v config.jsonwill havevas a lit flag andconfig.jsonas a positional arg. This is the default mode.PREFER_PARAM_FOR_UNREG_OPTION: Interpret<option> <non-option>as<parameter-name> <parameter-value>. e.g.myapp --gamma 2.2will havegammaas a parameter with the value "2.2".SINGLE_DASH_IS_MULTIFLAG: Splits an option with a single dash into separate boolean flags, one for each letter (a.k.a Compound Arguments). e.g. in this mode,-xvfwill be parsed as 3 separate flags:x,v,f.
-
Use bracket operators to access flags and positional args:
- Use
operator[index]to access position arg strings by index:- e.g.
assert(cmdl[0] == argv[0]), the app name.
- e.g.
- Use
operator[string]to access boolean flags by name:- e.g.
if (cmdl["v"]) make_verbose();
- e.g.
- Use
operator[{...}]to access boolean flags by multiple names:- e.g.
if (cmdl[{ "v", "verbose" }]) make_verbose();
- e.g.
- Use
-
Use the parenthesis operators to get an
std::istreamto stream values from parameters and positional args:- Use
operator(index)to access position argistreamby index:- e.g.
cmdl(0) >> my_app_name.
- e.g.
- Use
operator(string)to access parameter values by name:- e.g.
cmdl("scale") >> scale_factor;
- e.g.
- Use
operator({...})to access parameter values by multiple names:- e.g.
cmdl({ "-s", "--scale" }) >> scale_factor;
- e.g.
- Use
operator(index, <default>)andoperator(string/{list}, <default>)to stream a default value if the arg did not appear on the command line:- e.g.
cmdl("scale", 1.0f) >> scale_factor;
- e.g.
- Use
The streaming happens at the user's side, so conversion failure can be checked there: e.g
if (!(cmdl("scale") >> scale_factor))
cerr << "Must provide valid scale factor!" << '\n';Use the .str() method to get the parameter value as a string: e.g. cmdl("name").str();
- Use
parser::add_param(),parser::add_params()or theparser({...})constructor to optionally pre-register a parameter name when inPREFER_FLAG_FOR_UNREG_OPTIONmode. - Use
parser,parser::pos_args(),parser::flags()andparser::params()to access and iterate over the Arg containers directly.
- copy
argh.hsomewhere into your projects directories - or include the repository as a submodule
- or use CMake!
The provided CMakeLists.txt generates targets for tests, a demo application and an install target to install argh system-wide and make it known to CMake. You can control generation of test and example targets using the options BUILD_TESTS and BUILD_EXAMPLES. Only argh alongside its license and readme will be installed - not tests and demo!
Add argh to your CMake-project by using
find_package(argh)The package exports argh INTERFACE library target and argh_INCLUDE_DIR variable. Make argh.h known to your compiler by using one of the following methods; both will make the location of argh.h known to the compiler, not link in a precompiled library - even when using target_link_libraries().
target_include_directories(${MY_TARGET_NAME} PRIVATE "${argh_INCLUDE_DIR}")
#OR
target_link_libraries(${MY_TARGET_NAME} argh)
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