Re: [Boost-users] basic_string literals

From: Johan Råde (rade_at_[hidden])
Date: 2006-07-29 05:21:24

Daryle Walker wrote:
> On 7/28/06 10:04 AM, "Johan Råde" <rade_at_[hidden]> 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