Question

Basically it is the following code, which cannot pass compiler (g++)

#include <boost/program_options.hpp>
#include <iostream>
using std::cout;
using std::endl;
namespace po = boost::program_options;

class static_class {
public:
  static po::options_description cmd_opt;    // here is the definition
};

po::options_description static_class::cmd_opt("dummy");
// the line below cannot pass the compiler !!!
static_class::cmd_opt.add_options()
("help", "show usage info.")
;

main() {
  cout << static_class::cmd_opt << endl;
}

The error message:

test.cpp:16:1: error: ‘cmd_opt’ in class ‘static_class’ does not name a type

Any idea?

P.S.
I am trying to define a separated options_description for each command I need to handle in a small command line environment. I am using bison and flex to parse the command line environment. All arguments of a command will be sent to this static class for argument parsing.

As the argument definition is static, I do not want to make them some sort of data structures in stack (just in my mind may be this is fast and clean). I think these code will be ok if they are not static but what happen if they are?

Answer 1

You’re not allowed to have standalone statements at global or namespace scope.

You could get around the issue by using the return value of the extended add_options expression to declare and initialize a dummy variable, as below.

po::options_description static_class::cmd_opt("dummy");

auto const dummy = cmd_opt.add_options()
  ("help", "show usage info.")
  ;

If your version of C++ doesn’t support auto like that, then you can use the type’s full name, po::options_description_easy_init, instead.