Hello all,
It is my understanding that there is not a portable way to write a
user-defined manipulator that takes arguments. Please consider below which,
for purposes of illustration, implements a manipulator similar to setw().
Presumably, I have broken a rule here and this program is non-conforming.
However, I cannot see what rule that would be. May I request those who are
so inclined to try and poke holes in it?
Thank!
Dave
#include <iostream>
#include <ostream>
#include <string>
using namespace std;
template<typename T, typename C>
class ManipInfra
{
public:
ManipInfra(
basic_ostream<C> &(*pFun)(basic_ostream<C> &, T),
T val
): manipFun_(pFun), val_(val)
{
}
void operator()(basic_ostream<C> &os) const
{
manipFun_(os, val_);
}
private:
T val_;
basic_ostream<C> &(*manipFun_)(basic_ostream<C> &, T);
};
template<typename T, typename C>
basic_ostream<C> &operator<<(
basic_ostream<C> &os,
const ManipInfra<T, C> &manip
)
{
manip(os);
return os;
}
ostream &setTheWidth(ostream &os, int n)
{
os.width(n);
return os;
}
ManipInfra<int, char> setWidth(int n)
{
return ManipInfra<int, char>(setTheWidth, n);
}
int main()
{
cout << setWidth(10) << right << "foo" << endl;
} 2 2144
* Dave: It is my understanding that there is not a portable way to write a user-defined manipulator that takes arguments.
Where on Earth did you get that idea?
But it does seem to be difficult and perhaps impossible to provide
a solution that is both efficient and totally type-safe for arbitrary
argument type.
That is a general problem in C++.
Please consider below which, for purposes of illustration, implements a manipulator similar to setw(). Presumably, I have broken a rule here and this program is non-conforming.
Yes, you use three spaces for indentation. That was just something I
suggested tongue-in-cheek in a meeting. And what do you know, suddenly
they all started using it, and now, years after, there seems to be no
way to stop 'em... :-(
Seriously, you have _inconsistent_ indentation.
That's Very Bad, although not a language rule violation.
[snip] May I request those who are so inclined to try and poke holes in it?
OK.
#include <iostream> #include <ostream> #include <string>
using namespace std;
template<typename T, typename C> class ManipInfra { public: ManipInfra( basic_ostream<C> &(*pFun)(basic_ostream<C> &, T), T val ): manipFun_(pFun), val_(val) { }
Use typedefs.
You don't support non-copyable T objects.
For a generic ManipInfra class don't copy the T objects: pass by
reference to const, all the way (that means that the final iostream
manipulator function cannot be used as a standalone function, add a
comment to that effect).
void operator()(basic_ostream<C> &os) const { manipFun_(os, val_); }
To support the client code return the stream object.
private: T val_; basic_ostream<C> &(*manipFun_)(basic_ostream<C> &, T); };
template<typename T, typename C> basic_ostream<C> &operator<<( basic_ostream<C> &os, const ManipInfra<T, C> &manip ) { manip(os);
return os; }
E.g. this would be simplified.
--
A: Because it messes up the order in which people normally read text.
Q: Why is it such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?
"Alf P. Steinbach" <al***@start.no> wrote in message
news:41****************@news.individual.net... * Dave: It is my understanding that there is not a portable way to write a user-defined manipulator that takes arguments.
Where on Earth did you get that idea?
Josuttis, "The C++ Standard Library", page 613.
Thanks for the reply; I'll study it more tomorrow when my wits are about me! This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics
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