Question

What’s a good way to replace single-line // input number comments with multi-line /* input number */ comments?

I don’t have any preference for the language used to accomplish the task; I was thinking of Perl or sed. The source language will be C (ANSI X3.159-1989).

Simple scripts like

while(<>) {
  if (m#^(.*?)//#) {
    print $1;
  } else {
    print $_;
  }
}

would be fooled by strings containing // and are not OK. Similarly, // inside a multi-line comment should be left alone.

Edit: Code can assume that there are no trigraphs.

---

This is the opposite of replace C style comments by C++ style comments. It is similar to Replacing // comments with /* comments */ in PHP (though the accepted answer there cannot handle the special cases I mentioned and so is arguably wrong).

Answer 1

You can use boost::wave lexer’s output to replace all the c++ style comments to C style comments. without getting bothered about the edge cases.

 #include <iostream>
 #include <fstream>

 #include <boost/wave/cpplexer/cpp_lex_token.hpp>
 #include <boost/wave/cpplexer/cpp_lex_iterator.hpp>

 typedef boost::wave::cpplexer::lex_token<> token_type;
 typedef boost::wave::cpplexer::lex_iterator<token_type> token_iterator;
 typedef token_type::position_type position_type;

 int main()
 {
       const char* infile = "infile.h";
     const char* outfile = "outfile.h";
       std::string instr;
       std::stringstream outstrm;
       std::string cmt_str;
       std::ifstream instream(infile);
       std::ofstream outstream(outfile);

       if(!instream.is_open()) {
               std::cerr << "Could not open file: "<< infile<<"\n";
           }
     if(!outstream.is_open()) {
         std::cerr << "Could not open file: "<< outfile<<"\n";
     }

           instream.unsetf(std::ios::skipws);
           instr = std::string(std::istreambuf_iterator<char>(instream.rdbuf()),
                                        std::istreambuf_iterator<char>());

           position_type pos(infile);
           token_iterator  it = token_iterator(instr.begin(), instr.end(), pos,
            boost::wave::language_support(boost::wave::support_cpp|boost::wave::support_option_long_long));
           token_iterator end = token_iterator();

           boost::wave::token_id id = *it;

      while(it!=end) {
         //here you check the c++ style comments 
         if(id == boost::wave:: T_CPPCOMMENT) {
            std::cout<<"Found CPP COMMENT";
            cmt_str = it->get_value();
            cmt_str[0] = '/';
            cmt_str[1] = '*';
            //since the last token is the new_line token so replace the new line
            cmt_str[cmt_str.size()-1] = '*';
            cmt_str.push_back('/');
            //and then append the newline at the end of the string 
            cmt_str.push_back('\n');
            outstrm<<cmt_str;
         }
         else {
           outstrm<<it->get_value(); 
         }
         ++it;
         id = *it;
     }
     outstream<<outstrm;

     return 0;
}

For further documentation please see:
http://www.boost.org/doc/libs/1_47_0/libs/wave/index.html