String Literal Overloading

Question

Is it possible to make an overload of a constructor which will accept only a blank string when used in an initializer list?

struct null_ptr_type;

struct str
{
  str(null_ptr_type*) {}
  str(const char(&)[1]) {}
};

struct config
{
  str s;
};

int main()
{
  config c1 = {0}; // Works, implicit conversion to a null pointer
  config c2 = {str("")}; // Works
  config cx = {str("abc")}; // Fails (as desired)
  config c3 = {""}; // Fails with no conversion possible
}

Is there a way to make the syntax for c3 work without also accepting non-empty strings? I don't see why it doesn't, given that c1 works. Is there some rule I'm missing here that forbids this?

FWIW, VS 2013 invokes the implicit conversion as you would expect. Not the best reference I know but it's something. — Captain Obvlious, Apr 17 '14 at 17:40
Thanks, maybe I should specify: `g++ (Ubuntu/Linaro 4.7.3-2ubuntu1~12.04) 4.7.3`, but good to hear it does work somewhere. :) — Dark Falcon, Apr 17 '14 at 17:41
Could you not give it a default value, so you'd do config(string fred = ""); ? — , Apr 17 '14 at 17:44
The real code is much more complex, with members before and after that are initialized. There are also thousands of strings in a format not easily batch-modified. (Lots of legacy code) — Dark Falcon, Apr 17 '14 at 17:45
@Cheersandhth.-Alf, internationalization. This string is displayed to the user, and I want to enforce that all non-empty strings are wrapped in `N_()`, but a lot of those strings are blank strings and I don't want to modify each and every one. — Dark Falcon, Apr 17 '14 at 17:51
If N_ is a macro, then you have the option of doing things like `#define N_( s ) tutti_frutti( s, sizeof( s ) )`. In C++03 you could even determine whether `s` was a literal. In C++011 that's only possible with a suitable assumption (clearly communicated to users). — Cheers and hth. - Alf, Apr 17 '14 at 17:55
On further reflection that doesn't solve your problem of "avoid fixing those existing empty strings". Not sure I grok the problem though. It seems strange to have so many empty string arguments around. — Cheers and hth. - Alf, Apr 17 '14 at 18:01

score 0 · Answer 1 · answered May 04 '14 at 15:05

Using C++11 and "uniform initialization syntax" you can make this work, on assumption that you are able to modify interface of struct config:

struct config
{
  str s;

  config(str s) : s(s) {}
};

int main()
{
  config c1 = {0}; // Works, implicit conversion to a null pointer
  config c2 = {str("")}; // Works
  config cx = {str("abc")}; // Fails (as desired)
  config c3 = {""}; // Works
}

String Literal Overloading

1 Answers1