Re: [Boost-users] Converting loop to using boost::lambda where containers has boost::shared_ptrs

From: Stephen Torri (storri_at_[hidden])
Date: 2007-05-21 10:58:00

Thanks for the help. Roman's idea of using std::string constructor is
better than a handcrafted loop. I just got my mind into a trap thinking
I had to create a loop. I don't see the need to change the base class
to_String to a function. My intention is to leave it as it is since vc8
and gcc both compile it fine.

Stephen

On Sun, 2007-05-20 at 22:30 -0400, Christian Henning wrote:
> Alright, I solved it. Two small changes in the code did it. Hope they
> are not too bad for you.
> 
> First I needed to remove the =0 statement for the to_string in your
> base class. If I don't do that I can the following error:
> 
> c:\boost_1_34_0\boost\tuple\detail\tuple_basic.hpp(328) : error C2259:
> 'Base' : cannot instantiate abstract class
> [snip]
> 
> So my base class looks like as follows:
> 
> class Base {
> public:
> 
>    virtual std::string to_String ( boost::uint32_t indent )
>    { return "" ;}
> };
> 
> 
> Second change was to remove the boost::bind lib and to use lambda's
> bind. I also needed to add an asterisk to _1.
> 
> Here is the complete code that works on my machine.
> 
> #include <string>
> #include <sstream>
> #include <boost/cstdint.hpp>
> #include <vector>
> #include <boost/shared_ptr.hpp>
> #include <boost/lambda/lambda.hpp>
> #include <boost/lambda/bind.hpp>
> #include <iostream>
> 
> using namespace boost::lambda;
> 
> class Base {
> public:
> 
>    virtual std::string to_String ( boost::uint32_t indent )
>    { return "" ;}
> };
> 
> class A : public Base {
> public:
> 
>    A ( boost::uint32_t value )
>        : m_value ( value )
>    {}
> 
>    virtual std::string to_String ( boost::uint32_t indent )
>    {
>        std::stringstream output;
> 
>        for ( boost::uint32_t i = 0; i < indent; i++ )
>            {
>                output << " ";
>            }
> 
>        output << m_value;
> 
>        return output.str();
>    }
> 
> private:
> 
>    boost::uint32_t m_value;
> };
> 
> class D : public Base {
> public:
> 
>    D ( boost::uint32_t value )
>        : m_value ( value )
>    {}
> 
>    virtual std::string to_String ( boost::uint32_t indent )
>    {
>        std::stringstream output;
> 
>        for ( boost::uint32_t i = 0; i < indent; i++ )
>            {
>                output << " ";
>            }
> 
>        output << m_value;
> 
>        return output.str();
>    }
> 
> private:
> 
>    boost::uint32_t m_value;
> };
> 
> const char* print( const std::string& s )
> {
>    return s.c_str();
> }
> 
> int main ( int, char** )
> {
>    typedef boost::shared_ptr<A> A_ptr_t;
>    typedef boost::shared_ptr<D> D_ptr_t;
> 
>    typedef std::vector < boost::shared_ptr<Base> > Data_t;
>    Data_t m_data;
> 
>    A_ptr_t a ( new A ( 50 ) );
>    D_ptr_t d ( new D ( 30 ) );
> 
>    m_data.push_back ( a );
>    m_data.push_back ( d );
> 
>    boost::uint32_t indent_value = 4;
> 
>    std::for_each ( m_data.begin(),
>                    m_data.end(),
>                    std::cout << bind( &Base::to_String,
>                                               *_1,
>                                               indent_value ) );
>    return 0;
> }
> 
> Regards,
> Christian
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