On the richness of C++

[Edward Diener]

Walter Bright wrote:
> Edward Diener wrote:
>> Thanks for the examples above. I do realize that D has some built-in 
>> features which enable template manipulation which Boost MPL has to 
>> emulate through some very clever C++ template metaprogramming. In 
>> particular, as you have shown above, D has the tuple concept, the 
>> static if...else, and also something I like quite a bit, which is that 
>> there is no necessity for a base template upon which specializations 
>> are built but that each template can be defined with its own 
>> specializations. That last feature makes D templates really much 
>> easier to write.
> 
> One aspect of C++ templates is rendered completely irrelevant in D is 
> D's ability to do compile time function execution. Any C++ 
> metaprogramming that computes a value can be replaced in D with an 
> ordinary function, that is then executed at compile time.
> 
> Other things that help are D's ability to pass strings as template 
> arguments, and parse those strings at compile time, and D's ability to 
> access local variables (not just globals) from template expansions.


I assume the first means function execution which produces a constant 
expression. I believe C++ has that on the plate for C++0x, but of course 
you have evidently have it now.

I never realized that a string, by which I believe you mean a string 
literal, could not be passed as a template argument which is a value and 
not a type. It does seem an arcane area.

The last point I do not follow. Surely a template can access its own 
member variables, but you seem to be saying that it should access local 
variables also where it is being instantiated. I can't imagine the usage 
for that since the template creator can not possibly know where his 
template is being used.

Nonetheless, I will look at these features in order to to understand 
them better. Templates are fun, and I imagine that D templates will be 
more fun than C++ templates because they are richer and easier to use.