Dave Moore wrote:
[color=blue]
> As with std::ostream, std::valarray is not a simple class type,
> therefore, an attempt to forward declare it like:
>
> using namespace std;
> class valarray;[/color]
I didn't even want to introduce the namespace at that point. I find
the /using/ directive for namespaces to be contrary to my sense of order.
I much prefer introducing specific names as needed with /using/
declaration.
[color=blue]
> will fail. Unfortunately there is AFAIK no mechanism like <iosfwd>
> providing forward declarations of other STL classes. OTOH, you can
> probably roll your own by looking at the examples in <iosfwd> ...
> without checking myself, I expect something like:[/color]
I used something that I'm pretty sure is either explicitly undefined, or
non-conforming by introducing my own names into std:: This is the whole
ugly solution I cam up with:
/*************
Makefile
/*************
forward : forward.hpp forward.cc main.cc
g++ -o forward forward.cc main.cc
clean:
rm *.o forward
/*************
forward.cc
/*************
#include "forward.hpp"
#include <iostream>
#include <valarray>
V::V(const size_t& size){
va = new std::valarray<double>(1.1,size);
}
std::ostream& V::stringify(std::ostream& out){
for(size_t i = 0; i < va->size(); i++)
{
out << (*va)[i] << " ";
}
return out;
}
/*************
forward.hpp
/*************
#ifndef FORWARD_HPP
#define FORWARD_HPP
#include <iosfwd>
#include "stringable.hpp"
namespace std{
template<class> class valarray;
}
struct V: public Stringable {
V(const size_t& size);
std::valarray<double>* va;
std::ostream& stringify(std::ostream& out);
};
#endif
/*************
main.cc
/*************
#include "forward.hpp"
#include <iostream>
int main(int argc, char* argv[]) {
V v(10);
std::cout << v << "\n";
}
/*************
stringable.hpp
/*************
#ifndef STRINGABLE_H
#define STRINGABLE_H
#include <iosfwd>
using std::ostream;
/**
An un-universal base class
*/
class Stringable {
public:
virtual ostream& stringify(ostream& out)=0;
};
inline ostream& operator<<(ostream& out, Stringable& s){
s.stringify(out);
return out;
}
#endif
[color=blue]
> using namespace std;
> template <class T> class valarray;
>
> should remove the need to include <valarray> in your header file.[/color]
That is, if I have std:: available from some other #include. As You see
from the code above, I actually opened the std namespace up, and put my own
forwar declaration in there.
I have to wonder how many problems exist with the 'solution' I presented
above.
--
STH
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