Re: Returning Nulls in Templates

"Paul" <>
Tue, 22 Mar 2011 06:48:01 -0000
"James Kanze" <> wrote in message

On Mar 21, 3:39 pm, Ruben Safir <> wrote:

On Sun, 20 Mar 2011 11:37:37 -0700, James Kanze wrote:

On Mar 20, 3:39 pm, Ruben Safir <> wrote:

On Sun, 20 Mar 2011 12:03:16 +0000, Paul wrote:

You'd be safer to use '\0' to null-initialise an unkown type.

That can be used for an number type, and pointer as well? That might
be just what I'm looking for.

The correct way to initialize an unknown type is to use T(). That works
for numbers, pointers and smart pointers. Beyond that, as far as the
compiler is concerned, 0, NULL, '\0' and false are all integral
constants evaluating to 0. In code, I've seen two different
conventions: use 0 systematically, for everything (except usually
false), and use 0 for numeric values, NULL for pointers, '\0' for the
nul character, and false for boolean values---in other words, behave as
if the language were logically typed.

But it doesn't work with a string object,

What doesn't work with a string object. A string object doesn't
have a "nul" value, whatever that would mean. Regretfully,
string(0) (or string(NULL)) is a syntactically legal expression,
which results in undefined behavior.

 string could be initilased with string(0,'\0'). I don't think the character
really matters if its empty. Also string("") would intialise it to empty.

but strangely enough, Victor's
suggestion, which I had though about myself and rejected with out testing
because of the same facts that you stated here, works perfectly for all
the toyes I've tested now. It seems that '\0' is more universal and '0'

No. '\0' and 0 have the same value.

He didn't type 0 , he typed '0', Need to be carefull with that.

The first has type char,
the second type int. Both are null pointer constants (since
both are integral constant expressions evaluating to zero).
However, I would consider code using the first as a null point
defective, since it corresponds to no established convention.
As I said before, if you want a null pointer, use 0 or NULL
(depending on the local conventions). If you want a default
initialized value (e.g. in a template), use T(). If you want a
nullable value (of an arbitrary type), use Nullable<T> (or
Maybe<T>, or Fallible<T>, or boost::optional<T>). And if you
don't know what you want, figure that out first.

I'm not sure what you retruning from or where the strings are, but it should
be easy to implement a template specialisation to despatch this
initialisation to appropriate function.
For example:

/*** A couple of templates, add to your program someplace***/
template<class T>
struct null_value{operator T(){return T(0);}};
struct null_value<std::string>{operator std::string(){return
/**** Make sure the temple names dont clash if you put these inside another
template ****/
/*** use the string(0, '\0') form of intialisation or anything you want for
strings. *****/

/**Then use it like this in your class**/
template<typename T>
struct yourClass{
 null_value<T> nv; /*create a null_value object of suitable type for your
class instanciation.*/
 null_value<T> get_null_value(){return nv;} /*use it like this to return it
from a function*/

/**Test program**/
int main(){
 D<std::string> d1;
 D<int> d2;

 std::string s("not empty");
 std::cout<< s << std::endl;
 s= d1.get_null_value();
 std::cout<< s << std::endl;

 int x = 5;

It seems to work but I haven't thoroughly tested it.


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