🚀 go-pugleaf

Roman Lechtchinsky wrote:

> ... when declaring templates, why isn't it required to declare what 
> one expects of the template's parameters? 
> ... has [this] been considered and rejected ( and why ) ?

[This is from a paper by Koenig and Stroustrup, "Foundations for
Native C++ Styles" that appeared in Software Practice and Experience
vol. 25, December '95 (a Wiley pub.).  I encourage readers of this
newsgroup to look it up.] 

[I hope my excerpting hasn't done too much damage.]

----------------

What does C++ offer no such facility?   There are three main reasons:

[1] Any such facility would have to take into account not only inheritance
  but also built-in types and operations on types not defined as members.
  ... Ordinary pointers meet the requirements for random-access iterators
  when they are used to point to elements of (built-in) arrays.  That means
  we would need some way of saying that for any type T, T* is a random-
  access iterator, ... [or] forego the ability to use ... [template] 
  functions on arrays. ...

[2] The facility would offer little additional safety, if any.  The main
  benefit would be that errors would be detected when a template function
  ... is called, rather than when code is generated for it... [this is]
  not enough to justify a whole new type-checking facility.

[3] Even if such a facility existed ... that would still not guarantee 
  safety. ... Specifying "the expected operations" can be messy and 
  constraining; specifying "the expected semantics" can be surprisingly 
  difficult. ...

In general, we know of no way of expressing constraints on template
parameters that wouldn't be either too cumbersome or too constraining.
...

-----------------

[In short, any such apparatus (in C++, at least) would be complicated,
over-restrictive, and would fail to do the job required of it.  Some 
incompatible successor to C++ might be designed with such an apparatus
in mind, but today that's a research topic.]

Nathan Myers
ncm@cantrip.org  http://www.cantrip.org/
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