Howard Hinnant wrote:
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>.
But the standard doesn't say so explicitly. So how do we know? Section 27.3 just says that "The header <iostream> declares objects that...". It doesn't say that any types or operators are defined.
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).
It *is* possible to just include <iosfwd> to be able to declare (not define) the extern stream objects. Isn't that what <iosfwd> was designed for? Bo Persson