Having trouble figuring out how ostream functors work

This is a discussion on Having trouble figuring out how ostream functors work within the C++ Programming forums, part of the General Programming Boards category; Clearly I'm not understanding the difference between "ostream << endl" and "ostream << type". I want a class that behaves ...

  1. #1
    Kiss the monkey. CodeMonkey's Avatar
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    Having trouble figuring out how ostream functors work

    Clearly I'm not understanding the difference between "ostream << endl" and "ostream << type". I want a class that behaves exactly like an ostream (in regards to <<) except that it is just a wrapper for two ostreams. I am having problems with the endl, flush, etc.
    Code:
    #include <ostream>
    #include <iostream>
    #include <fstream>
    
    using std::ostream;
    
    class dubbstream
    {
    public:
    	ostream & s1;
    	ostream & s2;
    
    	explicit dubbstream(ostream & stream1, ostream & stream2) 
    		: s1(stream1), s2(stream2) {}
    };
    
    template<typename T> 
    dubbstream & operator << (dubbstream & dubb, T & arg)
    {
    	dubb.s1 << arg;
    	dubb.s2 << arg;
    	return dubb;
    }
    
    template<typename T> 
    dubbstream & operator << (dubbstream & dubb, const T & arg)
    {
    	dubb.s1 << arg;
    	dubb.s2 << arg;
    	return dubb;
    }
    
    template <class charT, class traits>
    dubbstream & operator << (dubbstream & dubb, std::basic_ostream<charT,traits> & arg)
    {
    	dubb.s1 << arg;
    	dubb.s2 << arg;
    	return dubb;
    }
    
    int main(int argc, char* argv[])
    {
    	using namespace std;
    	ofstream fout("mer.txt");
    	dubbstream say(cout, fout);
    	say << "Hi" << (24 * 4.112) << 'l' << "ala\n"; //okay
    	say << endl; //error
    	return 0;
    }
    Code:
    dubbstream.cpp
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2914: 'operator <<' : cannot deduce template argument as function argument is ambiguous
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2914: 'operator <<' : cannot deduce template argument as function argument is ambiguous
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2914: 'operator <<' : cannot deduce template argument as function argument is ambiguous
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<_Elem,_Traits> &std::operator <<(std::basic_ostream<_Elem,_Traits> &,const std::error_code &)' : could not deduce template argument for 'std::basic_ostream<_Elem,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(1085) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<_Elem,_Traits> &std::operator <<(std::basic_ostream<_Elem,_Traits> &&,_Ty)' : could not deduce template argument for 'std::basic_ostream<_Elem,_Traits> &&' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(968) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<char,_Traits> &std::operator <<(std::basic_ostream<char,_Traits> &,unsigned char)' : could not deduce template argument for 'std::basic_ostream<char,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(958) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<char,_Traits> &std::operator <<(std::basic_ostream<char,_Traits> &,const unsigned char *)' : could not deduce template argument for 'std::basic_ostream<char,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(951) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<char,_Traits> &std::operator <<(std::basic_ostream<char,_Traits> &,signed char)' : could not deduce template argument for 'std::basic_ostream<char,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(944) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<char,_Traits> &std::operator <<(std::basic_ostream<char,_Traits> &,const signed char *)' : could not deduce template argument for 'std::basic_ostream<char,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(937) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<_Elem,_Traits> &std::operator <<(std::basic_ostream<_Elem,_Traits> &,_Elem)' : could not deduce template argument for 'std::basic_ostream<_Elem,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(898) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<_Elem,_Traits> &std::operator <<(std::basic_ostream<_Elem,_Traits> &,const _Elem *)' : could not deduce template argument for 'std::basic_ostream<_Elem,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(851) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<char,_Traits> &std::operator <<(std::basic_ostream<char,_Traits> &,char)' : could not deduce template argument for 'std::basic_ostream<char,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(811) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<char,_Traits> &std::operator <<(std::basic_ostream<char,_Traits> &,const char *)' : could not deduce template argument for 'std::basic_ostream<char,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(764) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<_Elem,_Traits> &std::operator <<(std::basic_ostream<_Elem,_Traits> &,char)' : could not deduce template argument for 'std::basic_ostream<_Elem,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(726) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2784: 'std::basic_ostream<_Elem,_Traits> &std::operator <<(std::basic_ostream<_Elem,_Traits> &,const char *)' : could not deduce template argument for 'std::basic_ostream<_Elem,_Traits> &' from 'dubbstream'
    1>          c:\program files (x86)\microsoft visual studio 10.0\vc\include\ostream(679) : see declaration of 'std::operator <<'
    1>c:\users\david\documents\visual studio 2010\projects\dubbstream\dubbstream\dubbstream.cpp(48): error C2676: binary '<<' : 'dubbstream' does not define this operator or a conversion to a type acceptable to the predefined operator
    1>
    1>Build FAILED.
    What is going on with the definition of operator<<(ostream, endl-type-thing) ? I don't have my Stroustrup book on me anymore.

    Thanks.
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

  2. #2
    C++ Witch laserlight's Avatar
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    std::endl is actually a function. The idea is that the operator<< that reads it in calls it.
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  3. #3
    Registered User
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    std::endl is a function, not a reference to a stream.

    Replace the last template function with
    Code:
    dubbstream & operator << (dubbstream & dubb, std::ostream & (*arg)(std::ostream &))
    {
    	dubb.s1 << arg;
    	dubb.s2 << arg;
    	return dubb;
    }
    Right 98% of the time, and don't care about the other 3%.

    If I seem grumpy or unhelpful in reply to you, or tell you you need to demonstrate more effort before you can expect help, it is likely you deserve it. Suck it up, Sunshine, and read this, this, and this before posting again.

  4. #4
    Kiss the monkey. CodeMonkey's Avatar
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    Thanks
    "If you tell the truth, you don't have to remember anything"
    -Mark Twain

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