Adding Flags

The most basic addition to a command line program is a flag. This is simply something that does not take any arguments. Adding a flag in CLI11 is done in one of three ways.

Boolean flags

The simplest way to add a flag is probably a boolean flag:

bool my_flag{false};
app.add_flag("-f", my_flag, "Optional description");

This will bind the flag -f to the boolean my_flag. After the parsing step, my_flag will be false if the flag was not found on the command line, or true if it was. By default, it will be allowed any number of times, but if you explicitly1 request ->take_last(false), it will only be allowed once; passing something like ./my_app -f -f or ./my_app -ff will throw a ParseError with a nice help description.

Integer flags

If you want to allow multiple flags, simply use any integer-like instead of a bool:

int my_flag{0};
app.add_flag("-f", my_flag, "Optional description");

After the parsing step, my_flag will contain the number of times this flag was found on the command line, including 0 if not found.

Arbitrary type flags

CLI11 allows the type of the variable to assign to in the add_flag function to be any supported type. This is particularly useful in combination with specifying default values for flags. The allowed types include bool, int, float, vector, enum, or string-like.

Default Flag Values

Flag options specified through the add_flag* functions allow a syntax for the option names to default particular options to a false value or any other value if some flags are passed. For example:

app.add_flag("--flag,!--no-flag",result,"help for flag");

specifies that if --flag is passed on the command line result will be true or contain a value of 1. If --no-flag is passed result will contain false or -1 if result is a signed integer type, or 0 if it is an unsigned type. An alternative form of the syntax is more explicit: "--flag,--no-flag{false}"; this is equivalent to the previous example. This also works for short form options "-f,!-n" or "-f,-n{false}". If variable_to_bind_to is anything but an integer value the default behavior is to take the last value given, while if variable_to_bind_to is an integer type the behavior will be to sum all the given arguments and return the result. This can be modified if needed by changing the multi_option_policy on each flag (this is not inherited). The default value can be any value. For example if you wished to define a numerical flag:

app.add_flag("-1{1},-2{2},-3{3}",result,"numerical flag")

using any of those flags on the command line will result in the specified number in the output. Similar things can be done for string values, and enumerations, as long as the default value can be converted to the given type.

Pure flags

Every command that starts with add_, such as the flag commands, return a pointer to the internally stored CLI::Option that describes your addition. If you prefer, you can capture this pointer and use it, and that allows you to skip adding a variable to bind to entirely:

CLI::Option* my_flag = app.add_flag("-f", "Optional description");

After parsing, you can use my_flag->count() to count the number of times this was found. You can also directly use the value (*my_flag) as a bool. CLI::Option will be discussed in more detail later.

Callback flags

If you want to define a callback that runs when you make a flag, you can use add_flag_function (C++11 or newer) or add_flag (C++14 or newer only) to add a callback function. The function should have the signature void(std::size_t). This could be useful for a version printout, etc.

auto callback = [](int count){std::cout << "This was called " << count << " times";};
app.add_flag_function("-c", callback, "Optional description");


The name string, the first item of every add_ method, can contain as many short and long names as you want, separated by commas. For example, "-a,--alpha,-b,--beta" would allow any of those to be recognized on the command line. If you use the same name twice, or if you use the same name in multiple flags, CLI11 will immediately throw a CLI::ConstructionError describing your problem (it will not wait until the parsing step).

If you want to make an option case insensitive, you can use the ->ignore_case() method on the CLI::Option to do that. For example,

bool flag{false};
app.add_flag("--flag", flag)

would allow the following to count as passing the flag:

gitbook $ ./my_app --fLaG


The following program will take several flags:

bool flag_bool;
app.add_flag("--bool,-b", flag_bool, "This is a bool flag");

int flag_int;
app.add_flag("-i,--int", flag_int, "This is an int flag");

CLI::Option *flag_plain = app.add_flag("--plain,-p", "This is a plain flag");

The values would be used like this:

cout << "The flags program" << endl;
    cout << "Bool flag passed" << endl;
if(flag_int > 0)
    cout << "Flag int: " << flag_int << endl;
    cout << "Flag plain: " << flag_plain->count() << endl;

Source code

If you compile and run:

gitbook:examples $ g++ -std=c++11 flags.cpp
gitbook:examples $ ./a.out -h
Flag example program
Usage: ./a.out [OPTIONS]

  -h,--help                   Print this help message and exit
  -b,--bool                   This is a bool flag
  -i,--int                    This is an int flag
  -p,--plain                  This is a plain flag

gitbook:examples $ ./a.out -bii --plain -i
The flags program
Bool flag passed
Flag int: 3
Flag plain: 1

1 It will not inherit this from the parent defaults, since this is often useful even if you don't want all options to allow multiple passed options.

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