Re: [Boost-users] basic_string literals

29 Jul 2006

      Daryle Walker wrote:
...
On 7/28/06 10:04 AM, "Johan Råde"  wrote:
...
Does Boost offer any facility for declaring basic_string literals in
templates, i.e. something like
basic_string<CharType> s = BOOST_STRING("foo");
No, it doesn't AFAIK.  Would this provide anything useful if it did exist?
It wouldn't be a literal, since "basic_string" isn't a built-in type.  That
means there is no compile-time savings; a string object is still created and
it happens at run-time.  What advantage would it have over something like:
std::string  s( "foo" );
If you really meant any "CharType" and not just "char", then a
"BOOST_STRING" macro would have to reference a locale (probably the default
global one) to perform a conversion.
Daryle,

I ran into the problem when I wrote test cases for my facets for 
nonfinite numbers.

I wanted to do something like:

     template<class CharType> void test()
     {
	basic_string<CharType> s1;

	... some test code ...

         basic_string<CharType> s2 = "inf";
	BOOST_ASSERT(s1 == s2);
     }

This will of course only compile if CharType is char.

How would you fix this code, so that it compiles both for char and wchar_t?

This is what I eventually came up with:

     #define S_(a) string_literal_helper(a, L##a)

     class string_literal_helper {
     public:
         string_literal_helper(const char* s, const wchar_t* ws) : 
s_(s), ws_(ws) {}
         operator std::string() { return s_; }
         operator std::wstring() { return ws_; }
     private:
         const char* s_;
         const wchar_t* ws_;
     };

     template<class CharType> void test()
     {
         basic_string<CharType> s1;

         ... some test code ...

         basic_string<CharType> s2 = S_("inf");
         BOOST_ASSERT(s1 == s2);
     }

Now the code compiles both for CharType = char and CharType = wchar_t.
The idea is not mine. I have seen it somewhere on the web.

--Johan Råde