boost::lexical_cast and stringification of non-builtin types

Question

I have a (maybe) simple problem about boost::lexical_cast with composite types (in my case std::vector.

My first version of a templatized stringification function was the following

template <typename T>
std::string stringiy(const T &t)
{
std::ostringstream o;
o<< t;
return o.str();
}

and a working example was the following:

vector<int> x(10,-3;
cout << stringify<vector<int> >(x) << endl;

with the output "-3-3-3-3-3-3-3-3"~ but for performance reasons I want to exploit the boost::lexical_cast

Now I changed the function implementation:

template <typename T>
std::string stringiy(const T &t)
{
   return boost::lexical_cast<string>(t);
}

while this method works for built-in types, it stops working for cases like the previous one, when it's used for std::vector

The problem remains (it doesn't compile) if a create a specialized template for vectors

template <typename T>
std::string stringiy(const std::vector<T> &t)
{
     vector<string> strret = num2str(t);
     string r;
     for ( vector<string>::iterator iter = strret.begin(); iter!=strret.end(); ++iter )
    r.append(*iter);
     return r;
}

Any suggestion?

g++-4.5, ubuntu 11.10 amd64

In file included from Util.h:41:0,
                 from testLexicalCast.cpp:49:
/usr/include/boost/lexical_cast.hpp: In member function ‘bool boost::detail::lexical_stream<Target, Source, Traits>::operator<<(const Source&) [with Target = std::basic_string<char>, Source = std::vector<double>, Traits = std::char_traits<char>]’:
/usr/include/boost/lexical_cast.hpp:1151:13:   instantiated from ‘Target boost::detail::lexical_cast(typename boost::call_traits<Source>::param_type, CharT*, size_t) [with Target = std::basic_string<char>, Source = std::vector<double>, bool Unlimited = true, CharT = char, typename boost::call_traits<Source>::param_type = const std::vector<double>&, size_t = long unsigned int]’
/usr/include/boost/lexical_cast.hpp:1174:77:   instantiated from ‘Target boost::lexical_cast(const Source&) [with Target = std::basic_string<char>, Source = std::vector<double>]’
Util.h:211:43:   instantiated from ‘std::string util::stringify(const T&) [with T = std::vector<double>, std::string = std::basic_string<char>]’
testLexicalCast.cpp:72:53:   instantiated from here
/usr/include/boost/lexical_cast.hpp:595:48: error: no match for ‘operator<<’ in ‘((boost::detail::lexical_stream<std::basic_string<char>, std::vector<double>, std::char_traits<char> >*)this)->boost::detail::lexical_stream<std::basic_string<char>, std::vector<double>, std::char_traits<char> >::stream << input’
/usr/include/c++/4.5/ostream:108:7: note: candidates are: std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ostream_type& (*)(std::basic_ostream<_CharT, _Traits>::__ostream_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:117:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__ios_type& (*)(std::basic_ostream<_CharT, _Traits>::__ios_type&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>, std::basic_ostream<_CharT, _Traits>::__ios_type = std::basic_ios<char>]
/usr/include/c++/4.5/ostream:127:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(std::ios_base& (*)(std::ios_base&)) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:165:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:169:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:173:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(bool) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/bits/ostream.tcc:91:5: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(short int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.5/ostream:180:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(short unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/bits/ostream.tcc:105:5: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(int) [with _CharT = char, _Traits = std::char_traits<char>]
/usr/include/c++/4.5/ostream:191:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:200:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:204:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long long unsigned int) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:209:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(double) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:213:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(float) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:221:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(long double) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/ostream:225:7: note:                 std::basic_ostream<_CharT, _Traits>::__ostream_type& std::basic_ostream<_CharT, _Traits>::operator<<(const void*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__ostream_type = std::basic_ostream<char>]
/usr/include/c++/4.5/bits/ostream.tcc:119:5: note:                 std::basic_ostream<_CharT, _Traits>& std::basic_ostream<_CharT, _Traits>::operator<<(std::basic_ostream<_CharT, _Traits>::__streambuf_type*) [with _CharT = char, _Traits = std::char_traits<char>, std::basic_ostream<_CharT, _Traits>::__streambuf_type = std::basic_streambuf<char>]
make: *** [testLexicalCast.o] Error 1

Answer 1

You have probably declared your operator<< for vectors in the global namespace, and boost::lexical_cast will only look in the boost:: namespace and in the std:: namespace, where ostream is defined. Try adding your overload to the std:: namespace by wrapping your operator<< 's declaration and definition in namespace std { ... }.

Answer 2

I maybe way off here, but personally I think you would be better off instead of trying to stringify a container that you make a 'serialize' method for all containers.

EDIT: FIXED

template<typename Container>
   std::string serialize_container(const Container& container)
{
     std::stringstream ss;
     std::copy(container.begin(), container.end(),    
              std::ostream_iterator<typename Container::value_type>(ss,"-"));
     return ss.str();    
}

EDIT: that said if you want to use a vector with stringify then you do it like this.

template <typename T, typename A=void>
std::string stringiy(const T& t)
{
    //...
} 

template <typename T, typename A>
std::string stringiy(const std::vector<T, A>& container)
{
     std::stringstream ss;

     //if << is overloaded for vector
     ss << container;

     /*//else
     std::copy(container.begin(), container.end(),
               std::ostream_iterator<T>(ss,"-"));    
     //        std::ostream_iterator<typename Container::value_type>(ss,"-"));
     */
     return ss.str();    
}

N.B untested but that is the gist.