Perhaps another nice utility to also generate (not just consume) cmdlines

[xparq]

• Just dump the current state... Well, and also quote args with spaces... and other scary chars...

  • Umm... Except map doesn't preserve order, so we're fucked with the named ones!... :-o (And no, ordered_map doesn't mean the order of insertion either!... :-/ )
  • Ummmm... And even if some rudimentary quoting is kinda easy, there's still the open-ended problem of command-lines being consumed by shells, so such a generator must actually speak the (quoting-globbing-escaping) language of a certain particular target shell that it prepares the command-line to!... A hard NO to that!
    • Mmm, but actually... ;) OK, well, just that some generic features which are mostly OK for most shells, or are a good baseline for further custom app-level processing, would still be nice.
      (A quoting example is done below, and some escaping callback lambda could also be added, too.)

• And then listvals() that's used in the tests could be added, too, as that could just the very same mechanics. The tests use a stream as an output:

  auto listvals(auto const& container, const char* tail = "\n", const char* sep = ", ")
  {
      for (auto v = container.begin(); v != container.end(); ++v)
  	    cout << (v == container.begin() ? "":sep)
  	         << *v
  	         << (v+1 == container.end() ? tail:"");
  }
  

  but it could just as well write to a string (and an improved one could even be nice and precalc. its length first -- and an even nicer one that also does auto-quoting, or an even nicer one that does that with multi-char quotes... :) ):

  #include <string>
  #include <string_view>
  #include <cstring>
  #include <cassert>
  
  #define FOUND(expr) ((expr) != std::string::npos)
  #define CONTAINS(str, chars) FOUND((str).find_first_of(chars))
  string listvals(auto const& container, const char prewrap[] = "", const char postwrap[] = "", const char sep[] = ", ",
      const char* quote = "\"", // not const char[], to hint that it accepts nullptr!
      const char* scary_chars = " \t\n")
  // The pre/post wrapping are optional parts that only get written if not empty,
  // to support cases where callers would otherwise have to add an annoying
  // `if (container.empty())` or two themselves.
  {
      string result;
      if (!container.empty()) {
  	    size_t QLEN = quote ? strlen(quote) : 0;
  	    // Precalc. size... (Note: we're processing cmd args. We got time.)
  	    size_t size = strlen(prewrap) + (container.size() - 1) * strlen(sep) + strlen(postwrap);
  	    for (auto& v : container)
  		    size += v.length()
  			    + (quote && *quote && CONTAINS(v, scary_chars) ? // add quotes...
  				    (QLEN>1 ? QLEN:2) : 0); // special case for 1 (-> pair)!
  	    result.reserve(size);
  	    // Write...
  	    result += prewrap;
  	    for (auto v = container.begin(); v != container.end(); ++v) {
  		    if (quote && *quote && CONTAINS(*v, scary_chars))
  			    { result += string_view(quote, quote + (QLEN/2 ? QLEN/2 : 1)); // special case for 1 quote!
  			      result += *v;
  			      result += string_view(quote + QLEN/2); }
  		    else    { result += *v; }
  		    result += (v+1 == container.end() ? postwrap : sep);
  	    }
  //cout << "\n\n["<<result<<"]: " << "result.length() =? size: " << dec << result.length() << " vs. " << size << "\n\n";
  	    assert(result.length() == size);
      }
      return result;
  }
  #undef FOUND
  #undef CONTAINS
  

• ...then the args "serializer" could be as simple as (well, but still needs to write to a string, as the other!):

  void dumpargs(Args& args, char prefixchar = '-', const char* longprefix = "--")
  {
      // Named...
      for (auto& [name, val] : args.named()) {
  	    if (name.length() == 1)
  		    cout << prefixchar << name << listvals(val, " ", "", " ");
  	    else
  		    cout << longprefix << name << listvals(val, "=", "", " ");
  	    cout << " ";
      }
      // Positional...
      cout << listvals(args.positional(), "", "", " ");
  }
  

• Could be extra useful if the named/positional accessors would drop their (pretty orthodox) const (Drop const from named() and positional() (or have it both ways?) #55)! Then you could manipulate the arg set, and then "render" it to a new command line!