Re: Generic ostream operator<<?

From:

Victor Bazarov <v.bazarov@comcast.invalid>

Newsgroups:

comp.lang.c++

Date:

Mon, 25 Jun 2012 07:48:48 -0400

Message-ID:

<js9j71$qu7$1@dont-email.me>

On 6/24/2012 11:52 PM, Gerhard Fiedler wrote:

#include<iostream>
#include<type_traits>

template<typename T>
class is_printable
{
     typedef char yes[1];
     typedef char no[2];

     template<typename C> static yes& test( decltype(&C::print) );
     template<typename C> static no& test(...);

public:
     static const bool value = (sizeof(test<T>(0)) == sizeof(yes));
};

template< class Printable>
typename
std::enable_if<is_printable<Printable>::value,std::ostream&>::type
operator<<( std::ostream&stream, Printable const&printable )
{
    printable.print( stream );
    return stream;
}

class MyClass
{
public:
    MyClass( char const *val ) : str( val ) {}
    void print( std::ostream&stream ) const { stream<< str; }
private:
    char const *str;
};

int main(int argc, char* argv[])
{
    std::cout<< MyClass( "bar" )<< std::endl;
}

template<typename T>
void GTestStreamToHelper(std::ostream* os, const T& val) {
    *os<< val;
}

int main(int argc, char* argv[])
{
    MyClass foo( "bar" );
    std::cout<< "test "<< foo<< std::endl;

    std::string const gtest( "gtest" );
    GTestStreamToHelper(&std::cout, gtest );
    std::cout<< std::endl;
}

This:
---------------------------------------------->8 cut here
    #include <iostream>
    #include <type_traits>
    #include <string>

    template <typename T>
    class is_printable
    {
       typedef char yes[1];
       typedef char no[2];

       template <typename C> static yes& test( decltype(&C::print) );
       template <typename C> static no& test(...);

    public:
       static const bool value = (sizeof(test<T>(0)) == sizeof(yes));
    };

    template<> class is_printable<std::string> {
       public: static const bool value = false;
    };

    template < class Printable >
    typename
       std::enable_if<is_printable<Printable>::value,std::ostream&>::type
       operator<<( std::ostream &stream, Printable const &printable )
    {
       printable.print( stream );
       return stream;
    }

    class MyClass
    {
    public:
       MyClass( char const *val ) : str( val ) {}
       void print( std::ostream &stream ) const { stream << str; }
    private:
       char const *str;
    };

    template <typename T>
    void GTestStreamToHelper(std::ostream* os, const T& val) {
       *os << val;
    }

    int main()
    {
       MyClass foo( "bar" );
       std::cout << "test " << foo << std::endl;

       std::string const gtest( "gtest" );
       GTestStreamToHelper( &std::cout, gtest );
       std::cout << std::endl;
    }
---------------------------------------------->8 cut here
compiled for me fine with Visual C++ 2010. And the output was
test bar
gtest

Does that solve your problem?

V
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