[boost] yield statement proposal

2 Jun 2005

      Hi all,

As of version 2.0 C# will have yield statement. It intended for 
simplifying iterator development. Yield suspends execution of iterator 
code and resumes it on next increment. Thus complex traversal iterators 
are much easier to write in natural loop-like form instead of unroll them.

Consider following example (C#):

public IEnumerator< int > GetEnumerator()
{
     for ( int i = 1; i < 4; ++i )
         for ( int j = 1; j < 4; ++i )
             yield return i * j;
}

Such iterator will generate following sequence: 1 2 3 2 4 6 3 6 9.

The same code in C++ (using my library) will look like:

class simple_generator
{
public:
     YIELD_BEGIN_WITH_CTOR( simple_generator, int )
         YIELD_FOR_0( i = 1, i < 4, ++i )
             YIELD_FOR_1( j = 1, j < 4, ++j )
                 YIELD_RETURN( i * j )
             YIELD_END_FOR_1
         YIELD_END_FOR_0
     YIELD_END

private:
     int i;

     int j;
};

int main( int, char ** )
{
     for ( simple_generator g; !g.at_end(); ++g )
         cout << *g << " ";
     return 0;
}

Have a look at few more examples:

Example 1:
     YIELD_BEGIN_WITH_CTOR( simple_generator, string )
         YIELD_RETURN( "Hello," )
         YIELD_RETURN( "world!" )
     YIELD_END

Will print "Hello, world!" in two iterations with the above main() function.

Example 2:
     YIELD_BEGIN_WITH_CTOR( simple_generator, int )
         YIELD_FOR( i = 0, i < 10, ++i )
             YIELD_IF( i % 2 == 0 )
                 YIELD_CONTINUE
             YIELD_END_IF
             YIELD_RETURN( i )
         YIELD_END_FOR
     YIELD_END

Will print "1 3 5 7 9 ".

If anyone interested in such thing?

Sincerely yours,
Maksym Motornyy.