What do I need to make this work?

#include <iostream>
#include <fstream>

using namespace std;

struct LoggedCout {
  ofstream file;
  LoggedCout(const char* name) : file(name, ios::app)  {}
  template<class T>
  LoggedCout& operator<<(const T& t)
  { cout << t;  file << t;  return *this; }
};

LoggedCout lout("flushtest.txt");

int main() {
  lout << "output";
  lout << flush;
  // no match for 'operator<<' in 'lout << std::flush'
  { char c;  cin >> c; }
  return 0;
}


Martin

-- 
Quidquid latine scriptum sit, altum viditur.

Martin Eisenberg wrote:
> What do I need to make this work?
>
> #include <iostream>
> #include <fstream>
>
> using namespace std;
>
> struct LoggedCout {
>  ofstream file;
>  LoggedCout(const char* name) : file(name, ios::app)  {}
>  template<class T>
>  LoggedCout& operator<<(const T& t)
>  { cout << t;  file << t;  return *this; }
> };
>
> LoggedCout lout("flushtest.txt");
>
> int main() {
>  lout << "output";
>  lout << flush;
>  // no match for 'operator<<' in 'lout << std::flush'
>  { char c;  cin >> c; }
>  return 0;
> }

Was it you who asked a similar question about 'std::endl' yesterday?
If it was, why do you think it's different with 'flush'?  It's just
another function pointer when used liek that in an expression.
If it wasn't, look in the archives for yesterday's postings here.
Please don't use this newsgroup as a write-only medium.  Read it
before posting.  Use Google Groups to read it beyond what your ISP's
news server retains.

V
-- 
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask 

Martin Eisenberg wrote:
> What do I need to make this work?
>
> #include <iostream>
> #include <fstream>
>
> using namespace std;
>
> struct LoggedCout {
>   ofstream file;
>   LoggedCout(const char* name) : file(name, ios::app)  {}
>   template<class T>
>   LoggedCout& operator<<(const T& t)
>   { cout << t;  file << t;  return *this; }
> };

Add this to your class:

 typedef ostream&(*IOManip)(ostream&);
 LoggedCout& operator<<( const IOManip iomanip )
 {
   cout << iomanip;
   file << iomanip;
   return *this;
 }

Or better, consider making it a non-member function since the whole
thing is public anyway.

>
> LoggedCout lout("flushtest.txt");
>
> int main() {
>   lout << "output";
>   lout << flush;
>   // no match for 'operator<<' in 'lout << std::flush'
>   { char c;  cin >> c; }
>   return 0;
> }
[...]
> --
> Quidquid latine scriptum sit, altum viditur.

Sic, sed nemo id intelleget.

Cheers! --M

mlimber wrote:
> Martin Eisenberg wrote:

>> struct LoggedCout {
>>   ofstream file;
>>   LoggedCout(const char* name) : file(name, ios::app)  {}
>>   template<class T>
>>   LoggedCout& operator<<(const T& t)
>>   { cout << t;  file << t;  return *this; }
>> };
> 
> Add this to your class:
> 
>  typedef ostream&(*IOManip)(ostream&);
>  LoggedCout& operator<<( const IOManip iomanip )
>  {
>    cout << iomanip;
>    file << iomanip;
>    return *this;
>  }

Thanks! You're right, Victor, I should have seen that endl thread. 
Let me change the question: why is T in my code not deduced as 
something like IOManip above? Shouldn't it work as per 14.8.2/3?


Martin

-- 
In the Wacky Protestor's return, he hijacks the popular
children's TV show, "Mr. Funky's Wild Time", and uses it
to control the hearts and minds of every child watching.
-- http://www.bibleman.com/bibleman/store.jsp

Martin Eisenberg wrote:
> [..]
> Let me change the question: why is T in my code not deduced as
> something like IOManip above? Shouldn't it work as per 14.8.2/3?

You supply a pointer to function.  What would you expect T to be?

Have you tried changing the argument to 'T' instead of 'const T&'?

Notice that 'IOManip' is not passed by reference.

V
-- 
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask 

Victor Bazarov wrote:
> Martin Eisenberg wrote:

>> Let me change the question: why is T in my code not deduced as
>> something like IOManip above? Shouldn't it work as per 14.8.2/3?
> 
> You supply a pointer to function.  What would you expect T to be?

Actually, I just realized that "flush" does not name a single entity 
so there's no way to deduce anything at all in the context in 
question. Right?


Martin

-- 
Sphinx of black quartz, judge my vow!
--David Lemon

Martin Eisenberg wrote:
> Victor Bazarov wrote:
>> Martin Eisenberg wrote:
>
>>> Let me change the question: why is T in my code not deduced as
>>> something like IOManip above? Shouldn't it work as per 14.8.2/3?
>>
>> You supply a pointer to function.  What would you expect T to be?
>
> Actually, I just realized that "flush" does not name a single entity
> so there's no way to deduce anything at all in the context in
> question. Right?

Probably.

'std::flush' is a template.  'flush' also is an 'std::basic_ostream'
member function.  'std::basic_ostream' declares at least three overloaded
operator << functions that take function pointers.  As soon as you write
'flush' there, the compiler tries to understand which 'flush' you mean.
Most likely it finds the 'std::flush' template and tries to see if it
can figure out that template's arguments.  In fact, it has to try the
same template arguments as the 'basic_ostream' has.  It succeeds, AFAICT.

I am not sure my explanation makes sense.  Josuttis probably has a better
one in his Standard Library book.

V
-- 
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask