Re: [boost] mistaken uses of <iostream>

From: Howard Hinnant (hinnant_at_[hidden])
Date: 2006-11-20 11:08:33

On Nov 20, 2006, at 7:48 AM, John Maddock wrote:

> As an aside, I've always wanted a sort of "conceptual" std lib
> implementation, that included in each header nothing except what  
> absolutely
> has to be there.  Just the declarations would do, so we could do
> compile-time tests against it to verify inclusion of the right  
> headers: the
> only tricky bit is getting the compile time constants correct so  
> that it can
> still compile meta code.  Any volunteers? :->

One of the problems with this project is deciding exactly what should  
appear in the headers.  For example, imho, this is a valid C++ program:

#include <iostream>

int main()
{
     std::cout << "Hello World\n";
}

In order for that to be valid, <iostream> must declare and define the  
namepace scope:

template<class traits>
basic_ostream<char, traits>&
operator<< (basic_ostream<char, traits>& out, const char* s);

which lives in <ostream>.  In other words, I think it would be  
incorrect for <iostream> *not* to include <istream> and <ostream>.

I have heard people argue that HelloWorld should include <ostream>  
explicitly as well.  But imho that leads to a C++ language spec that  
is just too difficult and cumbersome to use (and we're already at  
risk for that reputation).

So to catch this particular bug (mistaken uses of <iostream>) we  
would need a tool that looked for std::ios_base::Init constructors in  
the translation unit, but no uses of one of the eight terminal  
streams.  Outside of a grep-like tool operating on the preprocessed  
translation unit (as an extra compilation phase), I'm not sure how to  
catch that with a std::lib implementation or prototype.  Seems like  
something the compiler could do no problem though. :-)

-Howard