将空字符串作为参数传递给 boost::program_options 的最佳方法是什么？

我有一个程序使用升压::程序选项解析命令行。其中一个参数是 AMQP 交换的名称，并提供默认值。出于测试目的，我想用空字符串覆盖此 AMQP 交换名称（以使用默认交换）。

我不知道如何将空字符串传递给升压::程序选项。那可能吗？不修改源码？如果没有，你有什么推荐？

这是一个最小的工作示例代码：

#include <boost/program_options.hpp>

namespace po = boost::program_options;

constexpr auto EXECUTABLE_DESCRIPTION = 
"Minimum working example showing my difficulties while trying to " 
"pass an empty string as an argument to boost::program_options.";

int main(int argc, char **argv) {

  std::string elkExchange;
  po::options_description config("");
  config.add_options()
          ("amqp.exchange",
          po::value<std::string>(&elkExchange)  
          ->default_value("default-exchange"),  // default value used in prod
          "Exchange on which to send the events");

  // do the parsing 
  po::options_description cmdline_options(std::string{EXECUTABLE_DESCRIPTION});
  cmdline_options.add(config);
  po::variables_map args;
  store(po::command_line_parser(argc, argv).
          options(cmdline_options).run(), args);
  notify(args);

  // debug display 
  std::cout << "Send event on elk.exchange: {" << elkExchange << "}" << std::endl;

  // real application code here ...

  return EXIT_SUCCESS;
}

这是我想做的：

# default value is used as expected
$ ./boost/empty-string-in-cli/exec
Send event on elk.exchange: {default-exchange}

# override with non-empty value work as expected
$ ./boost/empty-string-in-cli/exec --amqp.exchange='custom'
Send event on elk.exchange: {custom}

# here is where the troubles begin
$ ./boost/empty-string-in-cli/exec --amqp.exchange=''
terminate called after throwing an instance of 'boost::wrapexcept<boost::program_options::invalid_command_line_syntax>'
  what():  the argument for option '--amqp.exchange' should follow immediately after the equal sign
Aborted (core dumped)

提前致谢 ： ）

令我惊讶的是，相邻的长选项解析器绝对禁止=使用零长度值进行符号，请参阅上的代码https://github.com/boostorg/program_options/blob/develop/src/cmdline.cpp#L520 https://github.com/boostorg/program_options/blob/develop/src/cmdline.cpp#L520.

因此，您唯一的办法似乎是避免使用相邻值语法：

for EXCHANGE in foo bar ''; do ./build/sotest --amqp.exchange "$EXCHANGE"; done

Prints

Send event on elk.exchange: {foo}
Send event on elk.exchange: {bar}
Send event on elk.exchange: {}