Re: Correct swapping

=?iso-8859-1?q?Daniel_Kr=FCgler?= <>
Thu, 19 Apr 2007 05:10:50 CST
On 19 Apr., 10:50, Carl Barron <> wrote:

In article <>,

Daniel Kr?gler <> wrote:

Another point to consider is, that it's usually a bad idea, to
provide operator<< on some arbitrary std::pair, because neither
std::pair nor std::ostream are *your* types. To fix your problem
I recommend in this special situation to use one of the following:

(a) Replace your std::copy invocation by a std::for_each call
(which uses a special print functor and *not* operator<< on
std::pair) or an explicit loop.

(b) Use a better customizable iterator like boost::transform_iterator
which uses the in (a) mentioned print-functor. This ansatz is
nearly identical to your std::copy approach, where you simply

    Depends if I have a complicated function using an output iterator
I would just create a small class and the needed operator << ().

struct Derived:std::pair<double,double>
    Derived(double a,double b):std::pair<double,double>(a,b){}
    Derived(std::pair<double,double> const
    Derived & operator == (const std::pair<double,double> &x)

Should probably be:

Derived & operator = (const std::pair<double,double> &x)



inline std::ostream & operator << (std::istream &s,const Derived &x)

Should be:

inline std::ostream & operator << (std::ostream &s,const Derived &x)

I see several problems with this approach:

1) You derive publicly from class which is not designed to be a base
2) You put up with writing a complete new class (which does not bring
actual advantages compared to its base class) just to reuse
although transform_iterator (or something similar) would be the better
3) Your solution does not work with the OP's use-case, which uses
std::map<?1, ?2>::value_type, which is std::pair<const ?1, ?2>. So
have to provide one further class Derived2 to realize this issue - all
of the
implementation is basically the same as in Derived. That is, this
scales badly for related types or problems.
4) You willingly accept the fact, that every std::pair argument from
original container is *copied* into a temporary Derived instance
5) The above design does not provide more security/encapsulation
compared to defining the criticized

operator<<(std::ostream&,const std::pair<double, double&),

in the first place because the following *will* compile:

void foo(std::ostream& os) {
   std::pair<double, double> p; // *not* Derived!

   os << p; // Finds and uses operator<<(std::ostream&,const Derived&)
                // with one implicit conversion std::pair<double,
double> -> Derived

Taking all these points I see no advantage compared to the natural

void save(const char * filename, const Histogram::Map & data)
   std::ofstream file(filename);
boost::make_transform_iterator(data.begin(), MyPrint(file,"\n"));

where MyPrint is the functor in fact replacing

ostream& operator<<(ostream & os, const

There exists practically no difference in effort in writing MyPrint
compared to
above operator<< in the most simple case. In contrast, it is very easy
to create
a reusable and *non-intrusive* component class (template) from that
one without
polluting the namespace scope with needless/dangerous operator

Greetings from Bremen,

Daniel Kr?gler

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