Re: [Boost-users] basic_string literals

29 Jul 2006

      Johan Råde wrote:
...
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.
I once brought up this problem on comp.std.c++. Someone suggested, along 
the lines I was thinking, that C++ have a literal_cast<type>(literal 
expression) to cast a literal from one type to another using type 
notation. This would parallel the other C++ casts, such as static_cast 
and dynamic_cast.

The notation L'char' ( and L"char-string" ), to cast from a narrow 
character ( character string ) to a wide character ( wide character 
string ) does not work nicely with templates as you have discovered, 
whereas a literal_cast<type>(expression) would do so, ie 
literal_cast('char') ( or literal_cast<wstring>("char-string") 
). In the literal_cast case, of course, one could use a character ( or 
string ) template type as the type passed to literal_cast and an 
equivalent narrow character or narrow string value, and always be 
guaranteed of the correct result.

Of course literal_cast, if it existed, would work with any types which 
are currently represented as a literal in C++, but needless to say its 
main use would be to go from the C language L'' ( L"" ) notation to a 
notation that worked correctly and effortlessly in template code.

However I have never written up such a suggestion to the C++ standard 
comittee. I still find the idea correct, however.

Re: [Boost-users] basic_string literals

Edward Diener