References

GFlags使用文档, http://www.yeolar.com/note/2014/12/14/gflags/
How To Use gflags (formerly Google Commandline Flags), https://gflags.github.io/gflags/

Install

git clone https://github.com/gflags/gflags.git

CMake

Find gflags installation. The gflags_DIR variable must be set to the <prefix>/lib/cmake/gflags directory containing the gflags-config.cmake file if <prefix> is a non-standard location. Otherwise, CMake should find the gflags installation automatically.

   find_package(gflags REQUIRED)

   add_executable(foo main.cc)
   target_link_libraries(foo gflags::gflags)

DEFINE: Defining Flags In Program

Defining a flag is easy: just use the appropriate macro for the type you want the flag to be, as defined at the bottom of gflags/gflags.h. Here's an example file, foo.cc:

   #include <gflags/gflags.h>

   DEFINE_bool(big_menu, true, "Include 'advanced' options in the menu listing");
   DEFINE_string(languages, "english,french,german",
                 "comma-separated list of languages to offer in the 'lang' menu");

DEFINE_bool defines a boolean flag. Here are the types supported:

DEFINE_bool: boolean
DEFINE_int32: 32-bit integer
DEFINE_int64: 64-bit integer
DEFINE_uint64: unsigned 64-bit integer
DEFINE_double: double
DEFINE_string: C++ string

Accessing the Flag

All defined flags are available to the program as just a normal variable, with the prefix FLAGS_ prepended. In the above example, the macros define two variables, FLAGS_big_menu (a bool), and FLAGS_languages (a C++ string).

You can read and write to the flag just like any other variable:

   if (FLAGS_consider_made_up_languages)
     FLAGS_languages += ",klingon";   // implied by --consider_made_up_languages
   if (FLAGS_languages.find("finnish") != string::npos)
     HandleFinnish();

You can also get and set flag values via special functions in gflags.h. That's a rarer use case, though.

Miscellaneous Notes

If your application has code like this:

   #define STRIP_FLAG_HELP 1    // this must go before the #include!
   #include <gflags/gflags.h>

Gflags