Overloading std::string constructor

Question

Can I overload the std::string constructor?

I want to create a constructor which takes std::wstring and return a std::string. is it possible and how?

Thanks.

Answer 1

Can I overload the std::string constructor?

Nope, it would require changing std::string declaration.

I want to create a constructor which takes std::wstring and return a std::string. is it possible and how?

You can have a conversion function instead, like:

std::string to_string(std::wstring const& src);

However, you need to decide what to do with symbols that can't be represented using 8-bit encoding of std::string: whether to convert them to multi-byte symbols or throw an exception. See wcsrtombs function.

Answer 2

Rather define a free function:

std::string func(const std::wstring &)
{
}

Answer 3

No, you cannot add any new constructors to std::string. What you can do is create a standalone conversion function:

std::string wstring_to_string(const wstring& input)
{
    // Your logic to throw away data here.
}

If you (think you) want this to happen automatically, I strongly suggest re-evaluating that idea. You'll cause yourself a significant amount of headaches as wstrings are automatically treated as string when you least expect it.

Answer 4

This is not the Right True Way of doing it, and I think that I had smoked something when I coded this, but it solves the problem. Check the last function, `convert_str'.

#pragma once    

#include <memory>
#include <string>
#include <vector>

#include <boost/utility/enable_if.hpp>
#include <boost/type_traits/remove_const.hpp>
#include <boost/type_traits/remove_pointer.hpp>
#include <boost/type_traits/is_same.hpp>
#include <boost/mpl/logical.hpp>

template <typename Target, typename Source, typename cond>
struct uni_convert {
};

template <typename Target, typename Source > 
struct uni_convert<Target,Source,
    typename boost::enable_if< boost::is_same<Target, Source>, int >::type > {
    static Target doit(Source const& src) 
    {

        return src;
    }
};

template <typename Cond > 
struct uni_convert<std::string,std::wstring,
    Cond > {
    static std::string doit(std::wstring const& src) 
    {
        std::vector<char> space(src.size()*2, 0);
        wcstombs( &(*( space.begin() )), src.c_str(), src.size()*2 );
        std::string result( &(*( space.begin() )) );
        return result;
    }
};

template <typename Cond > 
struct uni_convert<std::wstring,std::string,
    Cond > {
    static std::wstring doit(std::string const& src) 
    {
        std::vector<wchar_t> space(src.size()*2, 0);
        mbstowcs( &(*( space.begin() )), src.c_str(), src.size()*2 );
        std::wstring result( &(*( space.begin() )) );
        return result;
    }
};

template< typename TargetChar >
std::basic_string< TargetChar > convert_str( std::string const& arg)
{
    typedef std::basic_string< TargetChar > result_t;
    typedef uni_convert< result_t, std::string, int > convertor_t;
    return convertor_t::doit( arg );
}