Re: Template functions and avoiding implicit conversions

From:

cbarron3@ix.netcom.com (Carl Barron)

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 27 Feb 2007 23:09:08 CST

Message-ID:

<1hu7cuu.avz83onjz94wN%cbarron3@ix.netcom.com>

tarjei.knapstad@gmail.com <tarjei.knapstad@gmail.com> wrote:

Consider the following:

class CommandLineOptions
{
public:
  template <typename ValueType>
  void
  Add(std::string& arg, bool required);

  template <typename ValueType>
  void
  Add(std::string& arg, ValueType default);
};

The problem I have is that if I write something like:

CommandLineOptions clo;
clo.Add<std::string>("method", "invert");

    I assume that you really mean the first argument to be a const
std::string &, or the conversion from char * to string should cause the
complier to complain about possing a temp as a non const reference.

  boost or tr1's type_traits are your friend.

  class Options
  {
     template <class T>
     void do_it(const std::string &s,T x,std::tr1::true_type)
     {
       // convert x to bool and process.
     }
     template <class T>
     void do_it(const std::string &s,T x,std::tr1::false_type)
     {
         // handle non bool's
     }
public:
     template <class T>
     void Add(const std::string &,T x)
     {
        typedef typedef std::tr1::is_same<T,bool>::type choice;
        do_it(s,t,choice());
     }
};

using boost instead std::tr1:: becomse boost::

std::tr1::true_type becomse boost::mpl::true_;
std::tr1::false_type becomse boost::mpl::false_;
same logic.

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