Re: Corrected: Proposal: Increasing type safety with a keyword

From:

Jeff Schwab <jeff@schwabcenter.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 30 Mar 2009 18:46:09 -0400

Message-ID:

<2Z-dnSAI6eos1kzUnZ2dnUVZ_gOdnZ2d@giganews.com>

Alf P. Steinbach wrote:

* Ioannis Vranos:

Proposal:

We can increase type safety in C++ by adding a single keyword. The
current proposal uses the keyword "only".

Example 1:
only int x= 4;

x= 5; // Ok
x= 5.0; // Error
x= 5U // Error

Example 2:
int i= 5;

only unsigned x= 4U;
x= 5; // Error
x= 5LU; // Error
x= i; // Error

Example 3:
only float f= 4.0F;

f= 4; // Error
f= 5.0; // Error
f= 4.0F; // OK

// It accepts any built in type
void somefunc(const int &x);

// It accepts only an int and a const int object
void somefunc(only const int &x);

It is simple like that, and the concept is backwards compatible.

What do you think?

As I recall someone proposed a similar class template a while back.

I proposed it in c.l.c.moderated, as a response to Andrei Alexandrescu.
Dave Abrahams was also on the thread. I'm too lazy to look for the
thread just this moment.

But even if nobody did, I do that now, so, it's done:

  template< typename T >
  class Only
  {
  private:
      T myValue;

      template< typename U > Only( U v ); // No constr. from other
types.

  public:
      // STATIC_ASSERT( that T is built-in type or something like that )
      Only( T v ): myValue( v ) {}

      T value() const { return myValue; }
      operator T () const { return value(); }
  };

Disclaimer: above code is off-the-cuff, not fondled by any dirty compiler.

Hm, thinking of it I have actually proposed something very similar
earlier, many years ago, namely, for the purpose of ensuring 'bool'
conditional expressions in novice code.

However, that proposal was shot down by (1) others' ideas about how
unsuitable this was for novices, and (2) actually trying it, which
showed that the standard's formal UB for redefining a keyword when using
standard library headers, is not just formal UB but, with at least one
compiler, very real...

I don't recall (1) being suggested, and I'm not sure what you mean by
(2). What keyword is being "redefined?"